Class 8: Solution of STARS AND SOLAR SYSTEM

1. What are celestial bodies?
Ans. The stars, the planets, the moon and many other objects in the sky are called celestial objects.

2. What is a natural satellite?

Ans. The natural satellite is a celestial body revolving around another celestial body. The moon is a natural satellite of the earth. The earth is a natural satellite of the Sun.

3. Write the name of first satellite of India.

Ans. ISRO built first satellite Aryabhata.

4. Which comet appears after every 76 years?
Ans. Halley’s comet appears after nearly every 76 years. It was last seen in 1986.

5. Name any two artificial satellites of our country.
Ans. Aryabhatta and INSAT are two artificial satellites of our country.

6. Name a star that always appear stationary from Earth's surface?
Ans. Pole star always appear stationary from earth's surface.

7. Why do we classify the Sun as a star?

Ans. Stars are celestial bodies that emit light of their own. and Sun is naturally luminous body, so Sun is a Star.

8. Why does the moon change its shape every day?
Ans. The moon does not produce its own light. We see the moon because the sunlight falling on it gets reflected towards us. We therefore, see only that part of the moon, from which the light of the Sun is reflected towards us.The size of the illuminated part of the moon visible from the Earth increases each day after the new moon day. After the full moon day, the sunlit part of the moon visible from the Earth decreases in size every day. It is due to relative position of earth, moon and sun.

9.Why the village sky is so different from the night sky in big cities?
Ans. Due to pollution the night sky in big cities different from the village sky.

10.The Sun rises in the east and sets in the west. Why?
Ans. Sun appears to rise in the east and set in the west as the Earth rotates from west to east on its axis.

11. What do you understand by light year? What is its value in km?
Ans. Light year is the distance travelled by light in one year.It is used to measure the large distance between stars and planets.
  
12. What is a Pole star?
Ans. The pole star is situated in the direction of the earth’s axis. It does not appear to move. The Pole star lies close to the axis of rotation of the Earth.

13. Give reason for
(i) Change in seasons on the earth 
(ii) Different phases of moon.
Ans. (i) The reason for changing in season on the earth is the revolution of earth around the Sun.
(ii) Different phases of moon is due to relative position of earth, moon and sun.

15. Why meteors are called shooting stars even though they are not stars?
Ans. A meteor is usually a small object that occasionally enters the earth’s atmosphere. At that time it has a very high speed. The friction due to the atmosphere heats it up. It glows and evaporates . A bright steak appears for a very short time and it is called shooting star.

16. Which constellation is called 'Hunter'?
Ans. Orion is called the Hunter. It can be seen during winter in the late evenings.It has seven or eight bright stars.The three middle stars represent the belt of the hunter.The four bright stars appear to be arranged in the form of a quadrilateral.

17. What is constellation? Name any three constellations?
Ans. The stars forming a group that has a recognisable shape is called a constellation.
Three constellations are Ursa major Ursa minor, Cassiopeia and Orion.

18. Can we hear any sound on the Moon? 
Ans. Sound cannot travel when there is no medium. So we can not hear any sound on the moon.

19. What are galaxies?
Ans. Galaxies are the huge collection of stars gas and dust.

20. Write the uses of artificial satellites.
Ans. Uses of artificial satellite:
(i) For weather forecasting
(ii) Communication 
(iii) Global positioning system.
(iv) Scientific investigation. 

21. Write short notes on Meteors and Meteorites.
Ans. A meteor is usually a small object of outer space that occasionally enters the earth’s atmosphere. At that time it has a very high speed. The friction due to the atmosphere heats it up. It glows and evaporates quickly and due to this we can see brite streak.These Streak of a Meteor are commonly known as shooting stars.
Some meteors are large so that they can reach the Earth before they evaporate completely. The body that reaches the Earth is called a meteorite. Meteorites help scientists in investigating the nature of the material from which the solar system was formed.

22. What is the difference between a star and a constellation?
Ans. Stars are celestial bodies that emit light of their own and the stars forming a group that has a recognisable shape is called a constellation.

23. Give one word for each

(a) Stars forming a recognizable pattern. 

(b) A solid body from outer space that is sometimes able to reach the earth‟s surface.

(c) A belt of small planetoids present mainly in between the orbits of Mars and Jupiter.
Ans. (a) Constellation.
(b) Meteorite.
(c) Asteroids.

24. What are asteroid?
Ans. There is a large gap in between the orbits of Mars and Jupiter. This gap is occupied by a large number of small objects that revolve around the Sun. These are called asteroids.

25. Give some details about Moon.
Ans. Moon is a natural satellite which revolves around the earth. It does not emit its won light. It reflects light of sun.It has no atmosphere. It has no water. Life cannot exist on the moon.

26. What do you mean by solar system?Give names of the planets in our solar system in increasing order of distance from the sun?      

Ans. The Sun and the celestial bodies which revolve around it form the solar system. It consists of large number of bodies such as planets, comets, asteroids and meteors. The gravitational attraction between the Sun and these objects keeps them revolving around it.
Names of the planets in our solar system in increasing order of distance from the sun :
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

27. In the solar system, name the following:   

(a) The largest planet,                                      

(b) The smallest planet,                                   

(c) The brightest planet ,                                

(d) The planet with largest number of satellites.
Ans. (a) The largest planet-  Jupiter.                                    

(b) The smallest planet - Mercury.                                   

(c) The brightest planet - Venus.                                 

(d) The planet with largest number of satellites - Saturn.

28. The planets do not collide while revolving around the Sun. Explain?
Ans. The planets do not collide while revolving around the Sun because all planets are revolved around the sun in a fixed orbit and orbits do not cross each other.

29. What do you mean by the term moon phase. Explain various phases of the moon?
Ans. The various shapes of the bright part of the moon as seen during a month are called the moon phase.
The day on which the whole disc of the moon is visible is known as the full moon day. Thereafter, every night the size of the bright part of the moon appears to become thinner and thinner. On the fifteenth day the moon is not visible. This day is known as the new moon day. The next day, only a small portion of the moon appears in the sky. This is known as the crescent moon. Then again the moon grows larger every day. On the fifteenth day once again we get a full view of the moon.

30. Explain the constellation Orion?
Ans. Orion is another well-known constellation that can be seen during winter in the late evenings. It is one of the most magnificent constellations in the sky. It also has seven or eight bright stars. Orion is also called the Hunter. The three middle stars represent the belt of the hunter. The four bright stars appear to be arranged in the form of a quadrilateral.

31. What do you understand by the terms, period of rotation and period of revolution of a planet?
Ans. The time taken by a planet to complete one revolution is called its period of revolution whereas, the time taken by a planet to complete one rotation is called its period of rotation.

32. If we imagine Saturn in a large pool of water then it will float. Why?What makes it unique in the solar system ?
Ans. Yes, it will float because the density of saturn is less than water.
 In the solar system its beautiful rings makes it unique.

33. What is a meteor shower?
Ans.When the Earth crosses the tail of a comet, swarms of meteors are seen. These are known as meteor showers. Some meteor showers occur at regular intervals each year. 

34. What are comets?
Ans. Comets are also members of our solar system. They revolve around the Sun in highly elliptical orbits. Their period of revolution round the Sun is usually very long. A Comet appears generally as a bright head with a long tail. The length of the tail grows in size as it approaches the sun. The tail of a comet is always directed away from the sun. comets are known to appear periodically. One such comet is Halley’s comet, which appears after nearly every 76 years. 

35. Differentiate with examples artificial and natural satellites?
Ans. i. The artificial satellites are man-made, they are launched from the Earth, Natural satellite are formed by nature.
ii. Artificial satellite revolves around the Earth much closer than earth’s natural satellite, the moon.
iii. Artificial satellite are used for forecasting weather, transmitting television and radio signals. They are also used for telecommunication and remote sensing, Whereas natural satellites are not.
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Solution of Periodic Classification

1.Write the number of vertical columns in the modern periodic table. What are these columns called?

Ans.The number of vertical columns in the modern periodic table is 18.These columns called groups.

2.Write the number of horizontal rows in the modern periodic table .What are these rows called?

Ans.The number of horizontal columns in the modern periodic table is 7.These columns called period.

3.Did Dobereiner’s triads also exist in the columns of Newlands’ Octaves? Compare and find out.

Ans. No, because the Law of Octaves was applicable only upto calcium.

4. Define valency and Atomic size.

Ans. Valency : As you know, the valency of an element is determined by the number of valence electrons present in the outermost shell of its atom.
Atomic size: The term atomic size refers to the radius of an atom. The atomic size is the distance between the centre of the nucleus and the outermost shell of an isolated atom. The atomic radius of hydrogen atom is 37 pm (Picometre, 1 pm = 10^-12m).

5.Write the atomic numbers of two elements X and Y having electronic configuration 2, 8, 2 and 2, 8, 6 respectively.

Ans. The atomic numbers of two elements X and Y having electronic configuration 2, 8, 2 and 2, 8, 6 respectively are 12 and 16.

6.Write any one difference between in the electronic configuration of group 1 and group 2 elements.

Ans. Group 1 element have 1 electron in their outermost shell and group 2 elements have 2 electrons.

7.The atomic number of three elements X,Y and Z are 3, 11 and 17 respectively. State giving reasons which two elements will show similar chemical properties.

Ans. X (E.C – 2, 1) and Y (E.C - 2, 8, 1) shows the similar chemical property as they belongs to same group in the periodic table, there valencies are same.

8.Write two reasons responsible for late discovery of noble gases.

Ans. Noble gases were discovered later than other elements, because they were extremely unreactive. Being inert they were difficult to isolate by the chemical reactions.

9. Why hydrogen should be placed in group 1.

Ans. Hydrogen has single electron, its atomic number is 1 and its properties match with alkali metals, so it is in the first group although it has properties similar to those of halogens too.

10.Name any two elements that have two electrons in their valence shells.

Ans.Two elements that have two electrons in their valence shells are Ca and Mg.

11. Name two elements of first period in Modern Periodic Table.

Ans. Two elements of first period in Modern Periodic Table are H and He.

12. On which side of the periodic table do you find metal and non-metal?

Ans. In the left side of the periodic table we find metal and in the right side non-metal.

13. What is the similarity in the valency of all the elements of group 1 in the modern periodic table?

Ans. The valency of all the elements is 1.

14. State the similarity in the electronic configuration of all the elements present in group 1 of the periodic table.

Ans. In the electronic configuration of all the elements present in group 1 of the periodic table have same valence electron and it is 1. 

15. How does the electronegativity changes as we move from left to right across a period?

Ans. The electronegativity increases as we move from left to right across a period because the number of charges on the nucleus increases. That attracts the electrons more strongly.

16. An element has atomic number 32 which period of the periodic table does it belong?

Ans. Atomic number – 32
Electronic configuration – 2, 8, 18, 4
Number of shell is 4 so it belongs to the 4^th period of the periodic table.

17. Write the formula on which the maximum number of electrons that can be accommodated in a shell depends.

Ans. The formula on which the maximum number of electrons that can be accommodated in a shell depends.on the formula 2n² where ‘n’ is the number of the given shell from the nucleus.

18.In the Modern Periodic Table, which are the metals among the first ten elements? 

Ans.Li and Be  (Lithium and beryllium)  are the metals among the first ten elements in the Modern Periodic Table.

19.Why do you think the noble gases are placed in a separate group?

Ans. Noble gases are inert in nature. Valency is zero.Due to these differences from other elements noble gases are placed in a separate group.

20. Besides gallium, which other elements have since been discovered that were left by Mendeléev in his Periodic Table? (any two) 

Ans. Scandium and germanium are the two elements have since been discovered that were left by Mendeléev in his Periodic Table. 

21.Arrange the following elements in the descending order of atomic size and give reasons for your answer .Mg, Cl, P and Ar ( Atomic number of the above elements are 12, 17, 15, 18 respectively.

Ans.Ar > Mg > P > Cl.
 In a period the size of a atom decreases from left to right due to increase in nuclear charge. Greater the atomic number ,greater is the nuclear charge. Ar is stable element, the effect of nuclear pull over the valence cell never be seen. So Ar is bigger however it has larger atomic number.

22. Newlands’ Law of Octaves worked well with lighter elements only. Explain it.

Ans.It was found that the Law of Octaves was applicable only upto calcium, as after calcium every eighth element did not possess properties similar to that of the first.
So Newlands’ Law of Octaves worked well with lighter elements only.

23. Mention any two trends exhibit by elements when we go from left to right across the period of periodic table.

Ans. Two trends exhibit by elements when we go from left to right across the period of periodic table are
(a) Atomic radius decreases.
(b) Metallic character decreases.

24. Define Dobereiner’s Triads with example.

Ans.In the year 1817, Johann Wolfgang Dobereiner, a German chemist grouped the elements in triads, which is a group of three elements in such a way that the atomic mass of the middle element was roughly the average of the atomic masses of the other two elements of tried. For example, Three elements- lithium (Li), sodium (Na) and potassium (K) with the respective atomic masses 6.9, 23.0 and 39.0.
Average atomic mass of Li and K  
= 6.9 + 39.0  = 45.9  
        2                   2             

= 22.95 nearly equal to 23 which is a mass of middle element Na.

25. Which groups are Dobereiner triads?                      

Group A Elements	At. Mass	Group B Elements	At. Mass	Group C Elements	At. mass Elements
N	14.0	Ca	40.1	Cl	35. 5
P	31.0	Sr	87.6	Br	79.9
As	74.9	Ba	137.3	I	126.9

Ans.  Groups B and C form Döbereiner triads .

Av. Mass of Sr = (Ca +Ba)/2 = (40.1 + 137.3)/2 = 88.7 and 
Av. Mass of Br = ( Cl + I) = 935.5 + 126.9)/2 = 81.2 . 
Which are nearly equal to the masses of Sr and Br.

26. What is the Newlands’ Law of Octaves?

Ans. Newlands an English scientist, observed that when elements are arranged in increasing order of their atomic mass, every element beginning from any element resembles the first element in its physical and chemical properties. This was known as law of octaves. In Newlands’ Octaves, the properties of lithium and sodium were found to be the same. Sodium is the eighth element after lithium. Similarly, beryllium and magnesium resemble each other.  

27. Elements in the periodic table show periodicity of properties. List any four such properties.

Ans. Four such properties are
i. Atomic radius decreases.
ii. Ionization Energy Increases.
iii. Electron Affinity increases.
iv. Electronegativity Increases.

28. The atomic number of three elements A, B and C are 12,18 and 20 respectively. State giving reasons, which two elements will show similar properties.

Ans. A and C show similar properties because both are in same group because they have same valence electrons 2.( E.C of A - 2,8,2 and E.C of C - 2.8,8,2).
Element B is an inert element having valency 0,that is Ar. E.C of Ar - 2,8,8)

29. List any two properties of the element belonging to the first group of the modern periodic table.

Ans. The element belonging to the first group of the modern periodic table 
(i) are alkali metals and (ii) valency is 1.

30. (a) Atomic number of Mg and Al are 12 and 13 respectively. Write down their electronic configuration.

(b) Mention the period of the modern periodic table to which the above two elements belong. Give reasons for your answer.
Ans. Electronic configuration of Mg - 2, 8, 2  (at.no.12)
Electronic configuration of Al - 2, 8, 3  (at.no.12)
Number of shell is 3 so they belongs to 3rd period. 

31. Three elements X , Y and Z belongs to 17th group but 2nd , 3rd and 4th period respectively. Number of valence electrons in Y is 7. Find the number of the valence electrons in X and Z.

Ans.  The number of the valence electrons in X and Z is also 7 because they belong to same group.

32. (a) Atomic radius of hydrogen in 37 pm. Express it in meters.

(b) How does atomic size vary in group and in a period?
Ans. (a) 1 pm = 10^-12m    37 pm =37 x 10^-12m= 3.7 x 10^-11m
(b) In group atomic size increases from top to bottom and in period atomic size decreases from left to right. 

33. How does the metallic character of the elements vary:

(i) in a group (ii) in a period of the modern periodic table ?
Ans.(i) In a group - Metallic character increases from top to bottom due increase in number of shell and decrease in nuclear pull over valence electrons.So it can lose electrons easily.
(ii) In a period - Metallic character decreases from left to right due to increase in atomic number and increase in nuclear pull  also. So it cannot lose electrons easily.

34. Na, Mg, Al and P belongs to 3rd period but are placed in 1st 2nd 13th and 15th group. Number of shells occupied in Mg is 3. What is the number of occupied shells in Na, Al and P?. Give reasons for your answer.

Ans.The number of occupied shells in Na, Al and P is 3 because all are present in same period.
Out of the elements X and Y which has bigger atomic radius? Give reasons to justify your answer.

35. (i) X has atomic number 18 and atomic mass 40.

(ii)Y has atomic number 20 and atomic mass 40.
Ans. Element X has bigger atomic radius because X is a stable element and outer shell is complete. As a result nuclear pull over valence electron cannot been seen.

36. Element Y with atomic number 3 combines with element X with atomic number 17.

(a) What is the electronic configuration of Y and X? What will be their valencies?
(b) What would be the formula of the compound? 
(c) What is the electronic configuration of an element Z with at. no 10. what will be its valency?
Ans. (a) Electronic configuration of Y=2, 1       valency = 1  Y is Li.
Electronic configuration of X = 2, 8, 7   and valency = 8 - 7=1   X is Cl.
(b) Molecular formula of compound= XY = LiCl

Formula of compound formed by combining Li and Cl is .

(c) Electronic configuration of element Z with atomic number 10 is 2, 8.

and valency is 0 because this element has their outermost orbit complete.

37. How does the tendency to lose electrons changes as we go from:

(a) left to right across a period? (b) Down a group?
Ans. (a)The tendency to lose electrons decreases from left to right across period.
(b) The tendency to lose electrons increases from top to down a group.

38. (a) What are metalloids?

(b) Name any four metalloids.
Ans. (i) The borderline  elements of metal and nonmetal elements – boron, silicon, germanium, arsenic, antimony, tellurium and polonium – are intermediate in properties and are called metalloids or semi-metals which show some properties of metals and some of nonmetals. 
(ii) Boron, Silicon, Germanium and Arsenic.

39. Study the following tables in which position of six elements A, B, C, D, E and F are shown as they are in the modern periodic table. 

Group-> Period	1	2	3-12	13	14	15	16	17	18
2 3	A			D	E	B			C F

On the basis of the above table answer the following questions?
(i) Name the element which forms only covalent compounds.
(ii) Name the element which is a metal with valency 3
(iii) Name the element which is nonmetal with valency 3
(iv) Out of D and E which is bigger in size and why? And write a common name of family C an F.
Ans. (i) E- Carbon which is present in 14th group and 2nd period of periodic table forms covalent bond by sharing electrons.Its valency is 4.

(ii) D - Aluminium belongs to 13th group and 3rd period with valency 3.

(iii) B - Nitrogen belongs to 15th group and 2nd period with valency 3.

(iv) Out of D and E , D is bigger in size.

Reason:  On moving across a period nuclear pull increases due to the increase in atomic number, and thus atomic size decreases. 

(v) C and F are noble or inert gases.

40. Given below is a part of the periodic table. How does the valency vary as we move vertically downwards from Li to Fr. Give reasons.

Li	Be
Na
K
Rb
Cs
Fr Ra	Ra

Ans. All have same valency 1 because they are in the same group.

41. What were the limitations of Newlands’ Law of Octaves?

Ans. (i) The Law of Octaves was applicable only upto calcium .It was not applicable to elements of higher atomic masses.

(ii) It was assumed by Newlands that only 56 elements existed in nature and no more elements would be discovered in the future.

(iii)  Newlands adjusted two elements in the same slot, but also put some unlike elements under the same note.Cobalt and nickel are in the same slot and these are placed in the same column as fluorine, chlorine and bromine which have very different properties than these elements.

(iv) Iron, which resembles cobalt and nickel in properties, has been placed far away from these elemen

42. What were the limitations of Dobereiner’s classification? 

Ans. According to Dobereiner classification, the elements in triads, which is a group of three elements arranged in such a way that the atomic mass of the middle element was roughly the average of the atomic masses of the other two elements of tried but all the elements could not be arranged in such triads. Ex. N (14) , P (31.0) ,  AS (74.9) - 

Mean of mass of N and As
= 14 + 74.9 = 44.45 

        2 

which is not equal to atomic mass of P.

43. What is meant by the modern periodic table? How do the following change on moving from top to bottom in a group of modern periodic table?
(i) Number of valence electrons.
(ii) Number of occupied shells.
(iii) Size of atoms
(iv) Metallic character of elements
(v) Effective nuclear charge experienced by valence electrons.
Ans. A tabular arrangement of elements in groups and periods highlighting the regular trends in properties of elements is called a periodic table.
(i) Number of valence electrons - remains same.
(ii) Number of occupied shells - The number of shells increases successively.
(iii) Size of atoms - Increases
(iv) Metallic character of elements - Increases
(v) Effective nuclear charge experienced by valence electrons - Decreases.

44. The element Be  Mg  and Ca each having two electrons in their outermost shells are in the period 2, 3 and 4 respectively of the modern periodic table. Answer the following questions giving justification in each case:

(i) Write the group to which these elements belong.
(ii) Name the least reactive metal.
(iii) Name the element having largest atomic radius.
Ans. (i) The group in which these elements belong is 2.
(ii) Reactive metal is Be.
(iii) Name of element having largest atomic radius - Ca

45. Based on the group valency of elements state the formula for the following giving justification for each:(i) Oxides of first group elements.

(ii) Halides of the element of group 13 and
(iii) Compound formed when an element of group 2 combines with an element of group 16.
Ans. Formula for the
(i) Oxides of first group elements - X₂O
(ii) Halides of the element of group 13 - XY₃
(iii) Compound formed when an element of group 2 combines with an element of group 16 - XY

46. (i) Who proposed modern periodic law?

(ii) What was the basis of arranging the element in it.
Ans.  (i) Henry Moseley proposed the modern periodic table.
(ii) The property of elements are the periodic functions of their atomic number.
The electrons in the atoms of four elements A B C D are distributed in three shells having 1,3,5 and 7 electrons in the outermost shell respectively.

47. State the period in which these elements can be placed in the modern periodic table. Write the electronic configuration of the atoms of A and D and the molecular formula of the compound formed when A and D combine.

Ans. The elements belong to 3rd period as they have three shells.
Electronic configuration of  A= 2, 8,1 -  It is Sodium 
Electronic configuration of D=2, 8, 7 – It is chlorine 
When they combine it forms Sodium chloride (AD).

48. Three elements X, Y and Z have atomic number 7, 10 and 14 respectively.

(i) Identify the group number of these elements in the periodic table.
(ii) Mention one unique feature each of elements Y and X.
(iii) Which of the above two elements belongs to the same period?
Ans. (a) Element X has atomic number 7.  E.C of X is 2, 5. Thus it belongs to Group 5th. Element is N. 
Element Y has atomic number 10. E.C of Y is 2, 8. Thus it belongs to Group 18th.Element is Ne.
Element Z has atomic number 14. E.C of Z is 2, 5. Thus it belongs to Group 14th. Element is Si.
(b) Y is noble gas (Ne) and X is nonmetal element (N).

49. How does metallic character of a metal depend on the size of atom?

Ans. The metallic character of the elements vary:
(i) In a group – Metallic character increases as one move down a group in the periodic table.
Reason: On moving down a group the atomic number, atomic size and nuclear charge increases but nuclear pull decreases due to large atomic size . Hence elements can lose electrons easily.
(ii) In a period of the modern periodic table – Metallic character decreases across a period, moving from left to right in the periodic table.
Reason: On moving across a period nuclear pull increases due to increase in atomic number, and thus atomic size decreases. Hence element cannot lose electrons easily.

50. (a) How does the electronic configuration of an atom related to its position in the modern periodic table?

(b) Would you place the two isotopes of chlorine Cl-35 and Cl-37 in different slots of the periodic table because of their different atomic mass or in the same slot because their chemical properties are the same? Justify your answer.
Ans. (a) The electronic configuration of an element tells us :
1) The number of valence electrons in outermost shells and 2) The number of shells.
The position of an element in a certain group will depend upon the number of valence electrons. For Ex. Electronic configuration of Ca- 2, 8, 8, 2. Thus, it belongs to the 2nd group.
The position of an element in a certain period will depend upon the number of shells.
For Ex. The electronic configuration of Magnesium is 2, 8 , 2. The number of shells is 3 and hence, it belongs to the third period.
Hence, the electronic configuration decides the position of an element in the periodic table.
(b) We place the two isotopes of chlorine Cl-35 and Cl-37 in the same slot because their chemical properties are the same. It is because periodic table is based on the atomic numbers of the element and both the isotopes of chlorine have the same atomic number (Z = 17).

51. Using the given part of the periodic table answer the following questions with reason:

(i) Name the element having smallest atomic size.
(ii) Write electronic configuration of element E.
(iii) Identify the elements which have similar physical and chemical properties as the element Y.

Group→ Period↓	1	2	13	14	15	16	17	18
3	X		B	C	E	D
4	Y
5	Z

Ans. (a) Element E

(b) E. C of E – 2, 8, 6
(c) X and Z.
(a) Why do we classify elements?
(b) What are the two criterias used in the development of Modern periodic table?

52. State the position of (i) metal (ii) nonmetal or (iii) metalloid in the periodic table.
 Ans. (a) Elements are pure substances made up of one type of atoms. They are the basic units of all type of matter. In order to study elements in an organized matter, they need to be classified.
(b) 1 ) Modern periodic table is based on atomic number and 2) Chemical properties of elements. 
(c) Metals are kept in the left side of the periodic table, Metalloids are in middle and Nonmetals are in the right side of the periodic table.

53.Which element has:

(i) Two shells both of which are completely filled with elements?
(ii) The electronic configuration 2, 8, 2?
(iii) A total of three shells, with four electrons in its valence shell?
(iv) A total of two shells, with three electrons in its valence shell?
(v) Twice as many electrons in its second shell as in its first shell?
 Ans.(i) Element in which two shells are filled is Ne (At. No. 10; E.C – 2, 8)
(ii) The element of electronic configuration 2, 8, 2 is Mg ( At. No 12)
(iii) The element is Si (E.C – 2, 8, 4; At. No. 14)
(iv) The element is B (E.C – 2, 3; At. No. 5)
(v) The element is C (E.C – 2, 4; At. No. 6)
An element X ( 2, 8, 2 ) combine separately with SO₄^2-, PO₄^3-

54. (i) Write the formula of the two compounds so formed

(ii) To which group of the modern periodic table does the element X belongs?
(iii) Will it form covalent or ionic compound? Give reasons.
 Ans. (a) Element X is Mg ( At. No. -12),
When Mg react with SO₄^2-  it forms - MgSO₄ and
When react with PO₄^3-  it forms- Mg₃ PO₄^3- 
(b) Group 2 as its valency is 2.
(c) Mg form ionic compound because Mg is a metal and lose electron to form cation.

55. Given below are some elements of the modern periodic table.

₄Be ₉Fe ₄Si ₁₉k ₂₀Ca
(i)Select the element that has one electron in the outermost shell and write its electronic configuration.
(ii)Select two elements that belong to the same group. Give reasons for your answer.
(iii) Select two elements that belong to the same period. Which one of the two has bigger atomic size?
Ans. (i) ₁₉k has one electron in the outermost shell.
E.C- 2, 8, 8, 1
(ii) Two elements that belong to the same group are Na and K.
(iii) Two elements that belong to the same period are Mg and Ca. Mg has bigger atomic size, because as we go from left to right in the periodic table, atomic size decreases.

56. An element X belongs to 3rd period and group II of the periodic table. State .

(a) The number of valence electrons,
(b) The Valency,
(c) Name of the element,
(d) Whether it is a metal or nonmetal.
Ans.(a)The number of valence electrons,
(b) Valency 2
(c) Mg
(d) It is a metal.

57. From the following part of the periodic table, answer the following questions:

(i) Which is the most reactive metal?
(ii) Name the family of fluorine Q, R and T.
(iii) Name one element which of group 2 and 15.
(iv) Give the name of the element P placed below carbon.
(v) Compare X and P with respect to the size of atoms.

1 Lithium	2	13	14 Carbon	15	16 Oxygen	17 Fluorine
X			P			Q
Y						R
Z						T

Ans. (i) Z
(ii) Halogens
(iii) Magnesium and nitrogen
(iv) Silicon
(v) X has bigger size than P because X has less effective nuclear charge.

58. (i) Which two criteria did Mendeleev used to classify the elements in his periodic table?

(ii) State Mendeleev periodic table.
(iii) Why is it not possible to give a fixed position to hydrogen in Mendeleev's periodic table?
Ans. (i) Mendeleev arranged the elements on the basis of their 2 fundamental property, the atomic mass, and also on the similarity of chemical properties.
(ii) On this basis, Mendeléev formulated a Periodic Law,the elements were arranged on the basis of their fundamental property, the atomic mass, and also on the similarity of chemical properties.,which states that ‘the properties of elements are the periodic function of their atomic masses’.
(iii) Like alkali metals, hydrogen combines with halogens, oxygen and sulphur to form compounds having similar formulae, On the other hand, just like halogens, hydrogen also exists as diatomic molecules and it combines with metals and non-metals to form covalent compounds.

59. Two elements X and Y belong to group 1 and 2 respectively in the same period of the periodic table. Compare these elements with respect to

(i) Number of electrons in the outermost orbit
(ii) Their valencies
(iii) Metallic character
(iv) Their atomic size
(v) Formula of the chloride
(vi) Formula of their Sulphate.
Ans. (i) 1 and 2
(ii) 1 and 2
(iii) X is more metallic than Y
(iv) Atomic size of X is greater than
(v)  YXCl and YCl₂
(vi)YX₂SO₄  And YSO₄

60. Given below are atomic radii of same elements of second period .

Element -                                    B   O    N   C
At.rad.in pm (picometre)         88  66  74  77            1Pm = 10^-12m.
(i) Arrange them in decreasing order of their atomic radii.
(ii) Are the elements now arranged in the pattern of a period in the Periodic Table ?
Ans. (i) B (88), C (77), N (74), O (66)
(ii) Yes  the elements are arranged in the pattern of a period in the Periodic Table.

61. An element X reacts with an element Y to form a compound.(Atomic number 17 and 20).(i) Write the position of this element in the modern periodic table.

(ii) Write the formula of the compound formed.
Justify your answer in each case.
Ans. (i) At. number of X is 17.
Its valency is 1 (E.C - 2, 8, 7)
X is Cl.
X belongs to 3rd period and group 17.
At. number of Y is 20.
Its valency is 2 ( E.C - 2, 8, 8, 2) i
Y is Ca.
(ii) The molecular formula of the compound formed - YX₂  ( CaCl₂)
Y belongs to period 4 and group 2. 

62. (a) Name any two noble which are found in atmosphere?

(b) Why the noble gases are placed in a separate group?
Ans. (a) Noble gases are the elements of Group 18.The noble gases, in order of their density, are helium, neon, argon, krypton. They are called noble gases they don't react with anything. For this reason they are also known as inert gases.
(b) Noble gases are placed in a separate groups because they are chemically unreactive and has different properties than other elements, which make them different from all others.

63. Four elements P, Q, R and S have atomic numbers 12, 13, 14 and 15 respectively. Answer the following questions giving reasons:
(i) What is the valency of Q?

(ii) Classify these elements as metals and non-metals.
(iii) Which of these elements will form the most basic oxide?
Ans. (i) The valency of Q is 3. ( E.C - 2, 8, 3)
(ii) P and Q are metals as they loose electrons to complete their octet.and R and S are non metals as they gain electrons to complete their octet..
(iii) P and Q will form basic oxide as they are metals.

64. Consider two elements A and B atomic number 17 and 19.(i) Write the position of these elements in the modern periodic table giving justification.

(ii) Write the formula of the compound formed when A combines with B.
(iii) Draw the electrons dot structure of the compound and state the nature of the bond formed between the two elements.
Ans. (i) Electronic configuration of A - 2, 8, 7  Valency = 8 -7 = 1
(i) Electronic configuration of B - 2, 8, 8, 1  Valency =  1
 The position of these elements in the modern periodic table is group 1 because valency 1 and periods are 2 and 3 because number of shells are 2 and 3.
(iii).A is Cl and B is K, so compound is KCl and bond is electrovalent bond.
Dot structure of KCl - 
 

65. Which is greater in size ?

(a) An atom or a cation
(b) An atom or anion
(c) Fe2+  or  Fe3+
Ans. (a) Parent atom is greater in size than cation because cation have lesser electrons, while their nuclear charge remains the same. The remaining electrons are, therefore held more tightly by the protons in the nucleus and thus their radii.are smaller than the parent atoms. 
(b) Anion is greater than their parent atoms because  they have one or more additional electrons, but without an additional proton in the nucleus to help moderate the size.

66. Name two elements in each case :

(a) Alkali metal
(b) Alkaline earth metal
(c) Halogen
(d) Inert gas
(e) Transition elements.
Ans. Two elements in:
(a) Alkali metal - Na and K.
(b) Alkaline earth metal - Mg and Ca.
(c) Halogen - Cl and Br
(d) Inert gas - Ne and Ar
(e) Transition elements - Fe and Zn.

67. Helium is an unreactive gas and neon is a gas of extremely low reactivity. What, if anything, do their atoms have in common and different?

Ans. (a) Lithium, sodium, potassium are all belongs to same group I, have same valency means chemical property same and all are alkali metals.
(b) Helium and Neon both have same valency zero and both are in the same group 18th. Both are inert in nature that means chemical properties same.Difference is that Helium has duplet in its outer shell K and neon has octave in its outer shell L.

Do these yourself  👇

1. Define periodicity.
2. An element has 2 electrons in its N shell.
(a) What is its atomic number?
(b) State its position in periodic table.
(c) Is it metal or non metal?
3.Answer the following questions in respect of elements  31P15
(a) Give its electronic configuration
(b) To which group and period does it belong?
(c) What is its Valency?
(d) Is it metal or non metal?
(e) Is it reducing agent or oxidizing agent?
(f) Give its formula with chlorine.
4. Chlorine, fluorine and bromine are kept in same group on the basis of similar properties.
(a) What are those similar properties?
(b) What is the common name of these group?
5.How do the following change on moving from left to right in a periodic table?
(a) Atomic structure
(b) Chemical reactivity
6.How does the chemical reactivity of
(a) Alkali metals vary?
(b) Halogen vary?
7.How could the Modern Periodic Table remove various anomalies of Mendeléev’s Periodic Table?
8.Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice?
9.Name :

(a) three elements that have a single electron in their outermost shells. 

(b) two elements that have two electrons in their outermost shells. 

(c) three elements with filled outermost shells.
(a) Lithium, sodium, potassium are all metals that react with water to liberate hydrogen gas. Is there any similarity in the atoms of these elements?
10. By considering their position in the Periodic Table, which one of the following elements would you expect to have maximum metallic characteristic? Ga Ge As Se Be
11.Use Mendeléev’s Periodic Table to predict the formula for the oxides of the following elements: K, C, AI, Si, Ba.
12. What were the criteria used by Mendeléev in creating his Periodic Table?