I. Very short answer type questions:
1. Define atoms.
Ans. Atoms are the tiny particles and the word atom first used by Democritus. Atoms are the building blocks of matter.
2. Write the name of particles present in atoms or Write the name of subatomic particles.
Ans. The name of particles present in atoms are electrons, protons, and neutrons.
3. Define atomic number.
Ans. The number of protons present in the nucleus of the atom of an element is called the atomic number.
4. Define mass numbers.
Ans. The sum of the number of protons and the number of neutrons present in the nucleus of the atom of an element is called the mass number of that element.
5. Define electronic configuration.
Ans. Arrangement of electrons around the nucleus of an atom is called electronic configuration.
6. Write the formula for calculating the maximum no.of electrons in an orbit?
Ans. The maximum number of electrons in each shell or orbit is determined by a formula 2n2. Where n is the number of shells.
7. What happens to an element Z if its atom gains three electrons?
Ans. When the atom of an element Z gains three electrons, it develops three negative charges and called negative ions.
8. J Chadwick discovered a subatomic particle that has no charge and has mass nearly equal to that of a proton. Name the particle and give its location in the atom?
Ans. The Particle is neutron and is present in the nucleus.
9. Which one of the Na+ and He has completely filled L and K shells? Give the reason to support your answer.
Ans. Na+ has completely filled L shell and He has completely filled K shells.
Reason: Atomic no. of Na = 11 E. C = 2,8,1
after loosing one electron E.C of Na = 2,8 , L shell is completely filled.
Atomic no. of He = 2 and E.C = 2 so its K shell is completely filled.
10. The atomic numbers of atoms of two elements are 18 and 20 respectively and their mass number is 40.
What is the relation between such pairs of atoms? Will they have the same property?
Ans. Atoms of different elements with different atomic numbers, which have the same mass number, are known as isobar. They have different chemical properties due to the different number of electrons and the same in physical properties.
11. Which elements have zero combining capacity or valency?
Ans. The atoms of inert elements, having a completely filled outermost shell show little chemical activity and their combining capacity or valency is zero.
12. How many electrons can be accommodated in the outermost shell of an atom?
Ans. Eight electrons can be accommodated in the outermost shell of an atom.
13. If the electronic configuration of an atom is 2, 8, 1, calculate the total number of electrons and write the name of the element.
Ans. The total number of electrons is 11 and this element is sodium.
14. What is the valency of oxygen if the atomic number is 8?
Ans. The atomic number of oxygen is 8 and electronic configuration is - 2, 6
So valency is = 8 - 6 = 2
15. Who concluded that the size of the nucleus is very small compared to the size of an atom?
Ans. Rutherford concluded that the size of the nucleus is very small compared to the size of an atom.
16. Define polyatomic ions.
Ans. A group of atoms carrying a charge is known as a polyatomic ion.
Ex - Ammonium NH4+ , Nitrate NO3-, carbonate HCO3- etc
II. Short answer type questions:
1. Write the properties of electron, proton, and neutron.
1. Write the properties of electron, proton, and neutron.
Ans. Electrons protons and neutrons are the subatomic particles of the atom.
Electrons -
. Electrons are negative in charge.
. The mass of an electron is 1.672 x 10-24
. They revolve in a fixed orbit around the nucleus.
Protons
. Protons are positive in charge.
. The mass of a proton is 9.108 x 10-19
. They are located inside the nucleus at the center of the atom
Neutrons
. Neutrons are electrically neutral in charge.
. The mass of a neutron is 1.676 x 10-24
. They are located inside the nucleus at the center of the atom.
2. An atom of an element has three electrons in its 3rd orbit which is the outermost shell and Write
(i) the electronic configuration
(ii) atomic number
(iii) number of protons, electrons, neutrons
(iv) valency.
Ans. Three electrons are present in the 3rd or last orbit,
(i) The electronic configuration is E.C = 2,8,3
(ii) The atomic number is =13
(iii) The no. of protons = 13, no. of electrons = 13 ,and no.of neutrons = 14
(i) Valency = 3
3. There are 16 protons and 15 Neutrons are present in an element. Calculate its atomic number and its atomic mass no.
State its valency.
Ans. Atomic number = No. of protons = 16
Atomic mass no. = No. of protons + No. of neutrons = 16 + 15 = 31
E.C = 2,8,6, Valency = 8 - 6 = 2
4. If an element X is available in the form of say, two isotopes 79Br35 (49.7 %) and 81Br35 (50.5 %), calculate the average atomic mass of the atom of X.
Ans. Atomic mass = 49.7/100 x 79 =39.26 g
Atomic mass = 50.5/100 x 81 = 40.91
Av. atomic mass = 39.26 + 40.91 = 80.17 or 80g
5. An atom of an element is represented as 19Y9 .
Ans. Atomic mass = 49.7/100 x 79 =39.26 g
Atomic mass = 50.5/100 x 81 = 40.91
Av. atomic mass = 39.26 + 40.91 = 80.17 or 80g
5. An atom of an element is represented as 19Y9 .
Write the
(a) Atomic number
(b) The electronic configuration
(c) No. of protons and
(d) Valency.
Ans. In an atom of element 19Y9
(i) Atomic number = 9
(ii) The atomic number of Y is = 9
E.C = 2,7
(iii) No. of protons = 9
(iv) Valency = 8 - 1 = 1
6. The electronic configuration of an element X is 2 , 8 , 2 :
(a) Find the number of electrons present in the atom of element X.
(b) Write its atomic number and
(c) Its atomic mass.
Ans. (a) No. of electrons present in X = 12 ( 2+ 8 + 2 = 12)
(b) Atomic number = No. of electrons = 12, It is magnesium Mg.
(c) Atomic mass = 24
7. List Bhor-bury rules for the distribution of electrons in different shells.
Ans. According to Neils Bohr rules: the distribution of electrons in different orbits or shells are represented by the letters K, L, M, N,… or the numbers, n=1,2,3,4,….formula 2n2, where ‘n’ is the orbit number or energy level index, 1,2,3,….
The maximum number of electrons in different shells are as follows:
K - 2, L - 8, M - 18, N - 32 .....and so on
The maximum number of electrons that can be accommodated in the outermost orbit is 8.
Electrons are not accommodated in a given shell unless the inner shells are filled.
8. Draw the atomic structure of an atom with atomic number 11.
Ans. Atomic number - 11
Element is Sodium Na.
Electronic Configuration - 2,8,1
Atomic structure
9. Composition of the nuclei of two atomic species X and Y are given below.
Element X Y
Proton 8 8
Neutron 8 10
i. Give the mass number.
ii. What is the relationship between the two elements?
Ans. Mass number of X = Proton number + Neutron number
Mass number = 8 + 8 = 16 g
Mass number of Y = Proton number + Neutron number = 8 + 10 = 18 g
The atomic number of both the atoms is 8 and mass number is different so they are isotopes.
10. Give one word of the following;
(i) Positively charged atom.
(ii) A group of atoms carrying a charge?
Ans. (i) Positively charged atom - Cation.
(ii) A group of atoms carrying a charge - Polyatomic ion.
11. (a) J. Chadwick discovered a subatomic particle that has no charge and has mass nearly equal to that of the proton. Name the particle and give its location in the atom.
(b) K and L shells of an atom completely filled electrons, then what would be
(i) The total number of electrons in the atom and
(ii) It's valency?
Ans. (a) The particle is neutron and it is present in the nucleus of the atom.
(b) If k and L shells of an atom are completely filled electrons, then what would be
(i) The total number of electrons in the atoms in which K and L shells are completely filled electrons is 10.
(ii) Valency is = 0
Reason : E .C = 2,8 , It is an inert element.
12. State three features of the nuclear model of an atom put forward by Rutherford.
Ans. Rutherford put forward the nuclear model of an atom, which had the following features:
(i) There is a positively charged center in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus.
(ii) The electrons revolve around the nucleus in well-defined orbits.
(iii) The size of the nucleus is very small as compared to the size of the atom.
13. Mention the postulates Niels Bohr put forward to overcome the objection raised against Rutherford's atomic model.
Ans. In order to overcome the objections raised against Rutherford’s model of the atom, Neils Bohr put forward the following postulates about the model of an atom:
(i) Only certain special orbits are known as discrete orbits of electrons are allowed inside the atom.
(ii) While revolving in discrete orbits the electrons do not radiate energy.
14. An element X has mass number 27 and it contains 13 protons.
(a) Write the symbolic representation of the element.
(b) Find the number of Neutrons in the element.
(c) Write the name of the element.
Ans. (a) The atomic number of an element is 13 so the symbolic representation of the element X will be Al.
(b) The number of Neutrons in the element = mass number - atomic number
= 27 - 13 = 14
(c) The name of the element is aluminium.
15. Sulphur dioxide is a colorless pungent-smelling gas and is a major air pollutant.
(a) Write the electronic configuration of its constituent element Sulphur and Oxygen. (Given - 32S16, 16O8 )
(b) Write the valency of Sulphur and Oxygen.
(c) Are sulphur and Oxygen isotope? Explain your answer.
Ans. (a) The electronic configuration of S whose atomic number is 16 = 2, 8, 6
The electronic configuration of O whose atomic number is 8 = 2,6(b) The valency of both sulpher and oxygen = 8 - 6 = 2
(c) No, sulpher and oxygen are not isotopes because their atomic number is not same.
16. Calculate the average atomic mass of chlorine if it exists commonly in two isotopes. 35Cl17 (75%) and 37Cl17 (25%)
Ans. The average atomic mass of chlorine if it exists commonly in two isotopes 35Cl17 (75%) and 37Cl17 (25%)
(75 x 35)/100 + (25 x 37)/100 = 26.25 + 9.25 =35.50
17. (a) What are the canal rays?
(b) State three characteristics of canal rays.
Ans. (a) Canal rays - Canal rays are α-particles which are doubly-charged helium ions. Since they have a mass of 4 u, the fast-moving canal ray or α-particles have a considerable amount of energy.
(b) Three characteristics of canal rays -
i. Canal rays are positively charged particles ii. fast-moving ray and deflected by the electric and magnetic field. iii.α-particles are much heavier than the protons.
18. If 14X6 is an isotope of element Y and has 2 neutrons less than X.
(a) Write the symbol of element Y showing its mass number and atomic number.
(b) Write the valency for these elements. What relation is given for the chemical properties of X and Y.
Ans. (a) If 14X6 is an isotope of element Y and has 2 neutrons less than X then the atomic number of Y = 6 and atomic mass = 14 - 2 = 12.
(b) Electronic configuration of X and Y = 2, 4 ( At. no. = 6)
The chemical properties of X and Y are the same.
19. An element 14X12 loses two electrons to form a cation which combines with the anion of element 35X17 formed by gaining an electron.
(a) Write the electronic configuration of element X
(b) Write the electronic configuration of the anion of Y
(c) Write the formula of the compound formed by the combination of X and Y.
Ans. (a) The atomic number of X is 12.
E.C = 2,8,2 Valency = +2 , Cation
(b) The atomic number of Y is 17
E.C = 2,8,7 Valency = -3 , Anion
(c) The formula of the compound will be = X3Y2.
21. (i) 24Mg12 and 26Mg12 are symbols of two isotopes of Mg. Compare atom of these isotopes with respect to
(a) Composition of their nuclei.
(b) Electronic configuration and valency.
(ii) Give the reason why two isotopes of magnesium have different mass numbers?
Ans. (i) (a) The atomic number of two isotopes of Mg are 24 and 26
In 24Mg12 - Number of Protons = 12, Number of Electrons = 12 and the
Number of Neutrons = 24 - 12 = 12
In 26Mg12 - Number of Protons = 12, Number of Electrons = 12 and the
Number of Neutrons = 26 - 12 = 14.
(b) E.C of 24Mg12 - 2,8,2 ( At. No. = 12) Valency = 2
E.C of 26Mg12 - 2,8,2 ( At. No. = 12) Valency = 2
(ii) Two isotopes of magnesium have different mass numbers due to different in neutron numbers.
22. Two elements X and Y combine in a ratio of 3 : 8 by mass and the compound Z is formed. Z is one of the essential components of photosynthesis to take place. If Z is also one of the greenhouse gases:
(a) Identify X,Y, and Z.
(b) Write the electronic configuration of X and Y.
(c) Write the atomicity of the molecule of Z.
Ans. Carbon dioxide is an essential component of photosynthesis to take place and it is also one of the greenhouse gases.
So two elements X and Y which are C and O combine in a ratio of 3 : 8 by mass and the compound Z that is CO2 is formed.
(b) E.C of C = 2,4 ( At.No. = 6) and E.C of O = 2,6 (At. No.= 8)
(c) The atomicity of CO2 = 3 ( One atom of carbon and two atoms of oxygen)
23. If the mass no. of an element is 23 and its atomic no.is 11 then
(a) Write its electronic configuration and its valency.
(b) Find the no. of neutrons in its nucleus.
(c) Mention the types of the ion formed by it.
Ans. Mass no. 23 and At. No. = 11 then
(a) E.C = 2,8,1 and Valency = 1
(b) No. of neutrons = Mass No. - Atomic number = 23 - 11 = 12
(c) It formes negative ion meas anion.
24. State the difference between isotopes and isobar. Give an example of each.
Ans. Atoms of the same elements with the same atomic numbers, which have different mass numbers, are known as isotopes. They have different physical properties due to the different number of neutrons and the same in chemical properties. Ex - 12C6 , 13C6 ,14C6 .
Atoms of different elements with different atomic numbers, which have the same mass number, are known as isobar. They have different chemical properties due to the different number of electrons and the same in physical properties. Ex- 23Ne10 23Na11
25. Name the scientists who discovered Proton, Electron, and Neutron. What is the charge and mass of a neutron?
Ans. In 1920 Rutherford discovered Proton, J.J. Thomson (1856- 1940), a British Physicists discovered electrons and in 1932, J. Chadwick discovered neutron which is another subatomic particle which had no charge and a mass nearly equal to that of a proton.
The charge of the neutron is zero and the mass is
(a) Atomic number
(b) The electronic configuration
(c) No. of protons and
(d) Valency.
Ans. In an atom of element 19Y9
(i) Atomic number = 9
(ii) The atomic number of Y is = 9
E.C = 2,7
(iii) No. of protons = 9
(iv) Valency = 8 - 1 = 1
6. The electronic configuration of an element X is 2 , 8 , 2 :
(a) Find the number of electrons present in the atom of element X.
(b) Write its atomic number and
(c) Its atomic mass.
Ans. (a) No. of electrons present in X = 12 ( 2+ 8 + 2 = 12)
(b) Atomic number = No. of electrons = 12, It is magnesium Mg.
(c) Atomic mass = 24
7. List Bhor-bury rules for the distribution of electrons in different shells.
Ans. According to Neils Bohr rules: the distribution of electrons in different orbits or shells are represented by the letters K, L, M, N,… or the numbers, n=1,2,3,4,….formula 2n2, where ‘n’ is the orbit number or energy level index, 1,2,3,….
The maximum number of electrons in different shells are as follows:
K - 2, L - 8, M - 18, N - 32 .....and so on
The maximum number of electrons that can be accommodated in the outermost orbit is 8.
Electrons are not accommodated in a given shell unless the inner shells are filled.
8. Draw the atomic structure of an atom with atomic number 11.
Ans. Atomic number - 11
Element is Sodium Na.
Electronic Configuration - 2,8,1
Atomic structure
9. Composition of the nuclei of two atomic species X and Y are given below.
Element X Y
Proton 8 8
Neutron 8 10
i. Give the mass number.
ii. What is the relationship between the two elements?
Ans. Mass number of X = Proton number + Neutron number
Mass number = 8 + 8 = 16 g
Mass number of Y = Proton number + Neutron number = 8 + 10 = 18 g
The atomic number of both the atoms is 8 and mass number is different so they are isotopes.
10. Give one word of the following;
(i) Positively charged atom.
(ii) A group of atoms carrying a charge?
Ans. (i) Positively charged atom - Cation.
(ii) A group of atoms carrying a charge - Polyatomic ion.
11. (a) J. Chadwick discovered a subatomic particle that has no charge and has mass nearly equal to that of the proton. Name the particle and give its location in the atom.
(b) K and L shells of an atom completely filled electrons, then what would be
(i) The total number of electrons in the atom and
(ii) It's valency?
Ans. (a) The particle is neutron and it is present in the nucleus of the atom.
(b) If k and L shells of an atom are completely filled electrons, then what would be
(i) The total number of electrons in the atoms in which K and L shells are completely filled electrons is 10.
(ii) Valency is = 0
Reason : E .C = 2,8 , It is an inert element.
12. State three features of the nuclear model of an atom put forward by Rutherford.
Ans. Rutherford put forward the nuclear model of an atom, which had the following features:
(i) There is a positively charged center in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus.
(ii) The electrons revolve around the nucleus in well-defined orbits.
(iii) The size of the nucleus is very small as compared to the size of the atom.
13. Mention the postulates Niels Bohr put forward to overcome the objection raised against Rutherford's atomic model.
Ans. In order to overcome the objections raised against Rutherford’s model of the atom, Neils Bohr put forward the following postulates about the model of an atom:
(i) Only certain special orbits are known as discrete orbits of electrons are allowed inside the atom.
(ii) While revolving in discrete orbits the electrons do not radiate energy.
14. An element X has mass number 27 and it contains 13 protons.
(a) Write the symbolic representation of the element.
(b) Find the number of Neutrons in the element.
(c) Write the name of the element.
Ans. (a) The atomic number of an element is 13 so the symbolic representation of the element X will be Al.
(b) The number of Neutrons in the element = mass number - atomic number
= 27 - 13 = 14
(c) The name of the element is aluminium.
15. Sulphur dioxide is a colorless pungent-smelling gas and is a major air pollutant.
(a) Write the electronic configuration of its constituent element Sulphur and Oxygen. (Given - 32S16, 16O8 )
(b) Write the valency of Sulphur and Oxygen.
(c) Are sulphur and Oxygen isotope? Explain your answer.
Ans. (a) The electronic configuration of S whose atomic number is 16 = 2, 8, 6
The electronic configuration of O whose atomic number is 8 = 2,6(b) The valency of both sulpher and oxygen = 8 - 6 = 2
(c) No, sulpher and oxygen are not isotopes because their atomic number is not same.
16. Calculate the average atomic mass of chlorine if it exists commonly in two isotopes. 35Cl17 (75%) and 37Cl17 (25%)
Ans. The average atomic mass of chlorine if it exists commonly in two isotopes 35Cl17 (75%) and 37Cl17 (25%)
(75 x 35)/100 + (25 x 37)/100 = 26.25 + 9.25 =35.50
17. (a) What are the canal rays?
(b) State three characteristics of canal rays.
Ans. (a) Canal rays - Canal rays are α-particles which are doubly-charged helium ions. Since they have a mass of 4 u, the fast-moving canal ray or α-particles have a considerable amount of energy.
(b) Three characteristics of canal rays -
i. Canal rays are positively charged particles ii. fast-moving ray and deflected by the electric and magnetic field. iii.α-particles are much heavier than the protons.
18. If 14X6 is an isotope of element Y and has 2 neutrons less than X.
(a) Write the symbol of element Y showing its mass number and atomic number.
(b) Write the valency for these elements. What relation is given for the chemical properties of X and Y.
Ans. (a) If 14X6 is an isotope of element Y and has 2 neutrons less than X then the atomic number of Y = 6 and atomic mass = 14 - 2 = 12.
(b) Electronic configuration of X and Y = 2, 4 ( At. no. = 6)
The chemical properties of X and Y are the same.
19. An element 14X12 loses two electrons to form a cation which combines with the anion of element 35X17 formed by gaining an electron.
(a) Write the electronic configuration of element X
(b) Write the electronic configuration of the anion of Y
(c) Write the formula of the compound formed by the combination of X and Y.
Ans. (a) The atomic number of X is 12.
E.C = 2,8,2 Valency = +2 , Cation
(b) The atomic number of Y is 17
E.C = 2,8,7 Valency = -3 , Anion
(c) The formula of the compound will be = X3Y2.
21. (i) 24Mg12 and 26Mg12 are symbols of two isotopes of Mg. Compare atom of these isotopes with respect to
(a) Composition of their nuclei.
(b) Electronic configuration and valency.
(ii) Give the reason why two isotopes of magnesium have different mass numbers?
Ans. (i) (a) The atomic number of two isotopes of Mg are 24 and 26
In 24Mg12 - Number of Protons = 12, Number of Electrons = 12 and the
Number of Neutrons = 24 - 12 = 12
In 26Mg12 - Number of Protons = 12, Number of Electrons = 12 and the
Number of Neutrons = 26 - 12 = 14.
(b) E.C of 24Mg12 - 2,8,2 ( At. No. = 12) Valency = 2
E.C of 26Mg12 - 2,8,2 ( At. No. = 12) Valency = 2
(ii) Two isotopes of magnesium have different mass numbers due to different in neutron numbers.
22. Two elements X and Y combine in a ratio of 3 : 8 by mass and the compound Z is formed. Z is one of the essential components of photosynthesis to take place. If Z is also one of the greenhouse gases:
(a) Identify X,Y, and Z.
(b) Write the electronic configuration of X and Y.
(c) Write the atomicity of the molecule of Z.
Ans. Carbon dioxide is an essential component of photosynthesis to take place and it is also one of the greenhouse gases.
So two elements X and Y which are C and O combine in a ratio of 3 : 8 by mass and the compound Z that is CO2 is formed.
(b) E.C of C = 2,4 ( At.No. = 6) and E.C of O = 2,6 (At. No.= 8)
(c) The atomicity of CO2 = 3 ( One atom of carbon and two atoms of oxygen)
23. If the mass no. of an element is 23 and its atomic no.is 11 then
(a) Write its electronic configuration and its valency.
(b) Find the no. of neutrons in its nucleus.
(c) Mention the types of the ion formed by it.
Ans. Mass no. 23 and At. No. = 11 then
(a) E.C = 2,8,1 and Valency = 1
(b) No. of neutrons = Mass No. - Atomic number = 23 - 11 = 12
(c) It formes negative ion meas anion.
24. State the difference between isotopes and isobar. Give an example of each.
Ans. Atoms of the same elements with the same atomic numbers, which have different mass numbers, are known as isotopes. They have different physical properties due to the different number of neutrons and the same in chemical properties. Ex - 12C6 , 13C6 ,14C6 .
Atoms of different elements with different atomic numbers, which have the same mass number, are known as isobar. They have different chemical properties due to the different number of electrons and the same in physical properties. Ex- 23Ne10 23Na11
25. Name the scientists who discovered Proton, Electron, and Neutron. What is the charge and mass of a neutron?
Ans. In 1920 Rutherford discovered Proton, J.J. Thomson (1856- 1940), a British Physicists discovered electrons and in 1932, J. Chadwick discovered neutron which is another subatomic particle which had no charge and a mass nearly equal to that of a proton.
The charge of the neutron is zero and the mass is
1.674929 x 10-27 kg, nearly equal to that of a proton.
III. Long answer type questions:
(a) An atom is electrically neutral.
(b) Noble gases show the least reactivity.
(c) The nucleus of an atom is heavy and positively charged.
(d) Ions are more stable than the atom.
Ans. (a) An atom is electrically neutral because atom has the same number of electrons (negatively charged ) and protons ( positively charged). After losing or gaining electrons atom becomes an ion.
(b) Nobel gases show the least reactivity because noble gases are stable elements as they have a filled valence shell of electrons and no need to lose or gain electrons to gain a stable electronic configuration.
(c) The nucleus of an atom is heavy and positively charged due to the presence of neutrons and protons.
(d) Ions are more stable than the atom because ions are formed by gaining or losing electrons due to which they gain stable electronic configuration.
2. (a) Helium atom has an atomic mass of 4u and two protons in its nucleus. How many neutrons does it have?
(b)The atomic mass of an element X is 16.2 u. What is the % age of isotope X having atomic number 8 and mass numbers 16 and 18?
Ans. (a) The atomic mass of Helium is 4u.
Atomic mass no. = no. of protons + no. of neutrons
The number of neutrons = Atomic mass no. - no. of protons
The number of neutrons = 4 - 2 = 2
(b) The atomic mass of X = 16.2 u
Let the % of the isotope of X with mass number 16 be = a and
the % of the isotope of X with mass number 18 be =100 - a
A/Q 16.2 = a/100 x 16 + (100 - a)/100 x 18
16.2 = 16a/100 + 1800/100 - 18a/100
18a/100 - 16a/100 = 18 -16.2 = 1.8
2a/100 = 1.80
a = 1.80 x 100/2 = 180/2 = 90%
100 - a = 10 %
The % age of isotope X is,having atomic number 8 and mass numbers 16 and 18 10% and 90%.
3. How many electrons, protons and neutrons are there in an element 19 X 9 ? What will be the valency?
(a) Define isotope and isobar with examples.
(b) What do the species 4B2 and 4C3 represent?
Ans. (a) Isotope - Atoms of the same elements with the same atomic numbers, which have different mass numbers, are known as isotopes. They have different physical properties due to the different number of neutrons and the same in chemical properties. Ex - 12C6 , 13C6 ,14C6 .
Isobar - Atoms of different elements with different atomic numbers, which have the same mass number, are known as isobar. They have different chemical properties due to the different number of electrons and the same in physical properties. Ex- 23Ne10 23Na11
(b) 4B2 represents Boron with atomic mass number 4 and atomic number 2 means the number of electrons 2 and the number of protons 2.
4C3 represents Carbon with atomic mass number 4 and atomic number 3 means the number of electrons 3 and the number of protons 3.
Both have the same atomic mass and different atomic number, so they are isobar.
They are different in chemical properties but the same in physical properties.
4. (a) List three subatomic particles of an element with its charge.
(b) A helium atom has an atomic mass 4 u and it's atomic no. is 2. How many neutrons does it have?
Ans. (a) Three subatomic particles are:
(i) Electron- Negative charge
(ii) Proton - Positive charge and
(iii) Neutron - Neutral.
(b) At. mass of He = 4 u and At. No = No. of protons = 2
No. of neutrons - At. mass no. - No. of electrons = 4 - 2 = 2
No. of neutrons = 2
5. Study the following table and answer the questions:
Elements
|
Protons
|
Neutrons
|
Electrons
|
A
|
2,
|
3,
|
2
|
B
|
10
|
9,
|
10
|
C
|
17
|
20
|
16
|
D
|
17
|
17
|
17
|
E
|
18
|
19
|
18
|
F
|
17
|
20
|
17
|
(b) What is the atomic no. of element B?
(c) Which two elements represent a pair of isotope and why?
(d) What is the valency of B and D?
(e) Identify the cation.
(f) Identify a pair of isobar.
Ans. (a) The mass no. of elements A = No. protons + No. of neutrons
= 2 + 3 = 5
The mass no. of elements B = No. protons + No. of neutrons
= 10 + 9 = 19
(b)The atomic no. of element B is = No of protons = No. of electrons = 10
(c) D and F represent a pair of isotope because they have the same no. of atomic no. that is 17 ( At. no = no. of protons = no. of electrons )
(d) At. no of B = 10 and E.C = 2,8. It is a stable element, so valency is zero.
At. no. of D = 17 and E.C = 2,8,7. Valency = 8 - 7 = 1.
(e) Cation - Cation is a positively charged ion, that would be attracted to the cathode in electrolysis.
(f) A pair of isobar = 40K, and 40Ca.
Isobars are atoms of different chemical elements that have the same number of nucleons.
6. List three differences between electrons, protons, and neutrons.
What observation in alpha-particle scattering experiment is the Rutherford to make the following:
a) Most of the space in an atom is empty.
b) The positive charge of the atom occupies very little space.
c) The mass of the atom is concentrated in a very small volume within the atom.
Ans. Three differences between electrons, protons, and neutrons:
Electron -
Following observations were made:
a) Most of the space in an atom is empty - Most of the fast-moving α-particles passed straight through the gold foil.
b) The positive charge of the atom occupies very little space - Very few particles were deflected from their path.
c) The mass of the atom is concentrated in a very small volume within the atom - A very small fraction of α-particles were deflected by 180 degrees.
7. (a) Which isotope is used in the treatment of cancer?
(b) the particle contains 11 protons and 10 electrons. Write the symbol and name of the particles.
(c) What do you think would be the observation if the alpha-particle
scattering experiment is carried out using a foil of a metal other than gold.
Ans. (a) An isotope of cobalt is used in the treatment of cancer.
(b) Number of protons = 11 and number of electrons = 10
The atomic number is 11 so the element is sodium and the symbol is Na.
(c) If the alpha particle scattering experiment is carried out using a foil of a metal other than gold, there would be no change in the observation. Rutherford selected a gold foil because he wanted as thin a layer as possible. Gold is malleable and a thin foil of gold can be easily made.
8. (a) State the limitations of J.J. Thomson’s model of an atom.
(b) Define valency by taking the example of Mg and O2
(c) S-2 has completely filled K, L and M shells. Find its atomic number.
Ans. (a) According to Rutherford the orbital revolution of the electron is not expected to be stable. But any particle in a circular orbit would undergo acceleration. During acceleration, charged particles would radiate energy and lose energy and finally fall into the nucleus. If this were so, the atom should be highly unstable and hence matter would not exist in the form that we know.
(b) Valency - Valency of an atom is the number of electrons lost, gained or shared by an atom during the course of the chemical reaction.
Atomic number of Mg = 12 and electronic configuration is - 2,8,2
Valence electron is 2.
Mg - 2e → Mg+2 Magnesium atom loses 2 electrons so valency is 2.
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