Quick Views Class X CH- 5 Periodic Classification of Elements

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Class X

 CH- 5 Periodic Classification of Elements

Syllabus of Periodic classification of elements: Need for classification, Modern periodic table, gradation in properties, valency, atomic number, metallic and non-metallic properties.

1. Classification of elements - Elements are classified in a fixed pattern in which the elements change their properties periodically. Periodic table helps us to study of physical and chemical properties of different elements.

Elements are mainly classified into metals and non-metals.

2. To find group and period  of elements - We can find the group of elements in the periodic table by the valence electrons and period by the number of shells in the electronic configuration. 

Ex – 

Ca  - At no. 20, 

E.C -  2,8,8,2  

Valence electron (the no. of electrons in last shells ) is 2 therefore group is 2 and 

the number of shells is 4, therefore period is 4.

Cl – At no. 17, 

E.C - 2,8,7  

Valence electron is 7 and the number of shells is 3. Therefore, Group is 17 and the period is 3.

3. Types of Elements in the periodic table:-

(i) Representative elements –

a. Group 1 –   Alkali metals

b. Group 2 –   Alkali Earth metals
c. Group 13 – Boron Family ( B, Al, Ga,….)
d. Group 14 – Carbon family ( C, Si, Ge, ….)
e. Group 15 – Nitrogen Family ( N, P, As, …..)
f. Group 16  – Oxygen family ( O, S,….)
g.Group 17 –  Halogen family ( F, Cl, Br, I,..)

(ii) Transition elements – Present in 3 to 12 groups. All elements are metal of
a. high melting and boiling points.
b. good conductor of heat and electricity.
c. magnetic materials.
d. having variable valency and
e. most of the elements are used as a catalyst.

(iii) Inert gases or Nobel gases: Group 18 ( He, Ne, Ar, Kr, Xe, Rn..)

4. Dobereiner’s Triads - Dobereiner showed that when the three elements in a triad were written in the order of increasing atomic masses; the atomic mass of the middle element was roughly the average of the atomic masses of the other two elements.

For example - Three elements are Lithium (Li), sodium (Na) and potassium (K) with the respective atomic masses 6.9, 23.0 and 39.0.

The average of the atomic masses of Li and K is (6.9 + 39.0)/2 = 22.95 nearly equal to 23 which is the mass of middle element Na.

5. Newlands’ Law of Octaves – According to Newland’s Law of Octaves, when elements are arranged in the order of increasing atomic mass, the properties of every eighth element are a repetition of the properties of that starting element.

6. Limitation of Newlands’ Law of Octaves- Newlands’ Law of Octaves worked well with lighter elements only. This law was applicable only up to calcium, after calcium every eighth element did not possess properties similar to that of the first.

7.Mendeleev’s Periodic Table - Mendeleev a Russian chemist arranged the elements in increasing order of their atomic masses and according to their chemical properties. He examined the relationship between the atomic masses of the elements and their physical and chemical properties. He selected hydrogen and oxygen as they are very reactive and formed compounds with most elements. The formulae of the hydrides and oxides formed by an element were treated as one of the basic properties of an element for its classification.

Mendeleev’s Periodic Table is arranged in 18 vertical columns called groups and 7 horizontal rows called periods.

8. Achievements of Mendeleev’s Periodic Table

(i) Mendeleev predicted the existence of some elements that had not been discovered at that time. Mendeleev named them by prefixing Eka (one) to the name of the preceding element in the same group. For instance, scandium, gallium, and germanium, discovered later, have properties. similar to Eka–boron, Eka–Aluminium and Eka–silicon, respectively. The properties of Eka–Aluminium predicted by Mendeleev and those of the element, gallium which was discovered later and replaced Eka-aluminium,

(ii) Noble gases like helium (He), neon (Ne) and argon (Ar) gases were discovered very late because they are very inert and present in extremely low concentrations in our atmosphere. One of the strengths of Mendeleev’s Periodic Table was that, when these gases were discovered, they could be placed in a new group without disturbing the existing order.

9. Limitations of Mendeleev’s Classification –

(i) The first limitation of Mendeleev’s Periodic Table was no fixed position can be given to hydrogen in the Periodic Table.

(ii) Isotopes were discovered long after Mendeleev had proposed his periodic classification of elements.

(iii) The atomic masses do not increase in a regular manner in going from one element to the next.

10. Isotopes- Isotopes of an element have similar chemical properties, but different atomic masses.

11.Making order out of Chaos by Moseley-- Henry Moseley showed that the atomic number of an element is a more fundamental property than its atomic mass. Accordingly, Mendeleev’s Periodic Law was modified and the atomic number was adopted as the basis of Modern Periodic Table and the Modern Periodic Law can be stated as follows: 

Properties of elements are a periodic function of their atomic number.

Elements thus arranged show periodicity of properties including atomic size, valency or combining capacity and metallic and non-metallic character.

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.

12. Position of Elements in the Modern Periodic Table:

(i) The elements present in any one group have the same number of valence electrons.

(ii) The number of shells increases as we go down the group.

(iii) Different elements with the same number of occupied shells are placed in the same period.

(iv) The number of shells is the same and the atomic number increases as we go from left to right the period.

(vi) The position of an element in the Periodic Table tells us about its chemical reactivity. The valence electrons determine the kind and number of bonds formed by an element.

13. Valency : The valency of an element is determined by the number of valence electrons present in the outermost shell of its atom. Ex – Atomic number of Na = 11, Electronic configuration = 2,8,1 and valency is 1.

14. Atomic size: The radius of an atom is known as atomic size. The atomic size is the distance between the center of the nucleus and the outermost shell of an isolated atom. The atomic radius of a hydrogen atom is 37 pm (picometre, 1 pm = 10^–12m).

15. Change in atomic radius - The atomic radius decreases in moving from left to right along a period. This is due to an increase in nuclear charge which tends to pull the electrons closer to the nucleus and reduces the size of the atom.

16. Change in atomic size - The atomic size increases down the group. This is because new shells are being added as we go down the group. This increases the distance between the outermost electrons and the nucleus so that the decrease in nuclear charge and the atomic size increases.

17. Position of Elements  - The metals are found towards the left-hand side of the Periodic Table while the non-metals are found on the right-hand side. In the middle elements which are classified as a semi-metal or metalloid because it exhibits some properties of both metals and non-metals.

18. Metalloids - Metalloids are also known as borderline elements –Ex - boron, silicon, germanium, arsenic, antimony, tellurium, and polonium – are intermediate in properties and are called metalloids or semi-metals.

19.Changes in Nuclear charge - 

I. The effective nuclear charge acting on the valence shell electrons increases across a period, the tendency to lose electrons will decrease, therefore electrons cannot be lost easily. Hence metallic character decrease as we go from left to right the period.

ii. Down the group, the effective nuclear charge experienced by valence electrons is decreasing because the outermost electrons are farther away from the nucleus. Therefore, these can be lost easily and electro positivity increases. Hence metallic character increases down a group.

In Periodic Table 

                                 

Left to Right  → At. Size decreases, At.no. Increases, No. of shells the same. Electro positivity decreases and Electronegativity Increases. Valency changes.

     

Downwards

          ↓          

  At. Size increases

  At.no increases                              

  No. of shells increases

  Valency same

  Metallic property increases

  Electro positivity increases

Questions for practice

Questions with Solutions