NCERT Class 10 The most Important Basic concepts of chapter 5 (periodic classification of elements)

 Very Important points of periodic classification of elements

1. There are one hundred and fourteen elements forming millions of Compounds.

2. There was a need to arrange the elements in certain order so that their properties can be easily summarised. This is called classification of elements.

3. Döbereiner grouped the elements with similar properties in triads so that the atomic mass of middle element is average of the other two.

4. Newlands gave the law of octaves similar to notes of music. He found that if the elements were arranged in the order of increasing masses, every eighth element starting from a given one gave properties similar to the first. This law was found applicable to only lower atomic masses.

5. Mendeléev arranged the elements in increasing order of their atomic masses. He found that properties of elements gradually changed from element to element and at certain interval they almost started repeating. It enabled him to predict the existence of some yet to be discovered elements. But at the same time certain anomalies were also noted.

6. Moseley discovered that atomic number is a fundamental property.

7. Modern periodic table is based on the fact that properties of the elements are periodic function of their atomic numbers. This table related periodicity in the properties of the element to the periodicity in their electronic configurations.

8. Modern periodic table is divided into 18 vertical columns called groups and 7 horizontal rows called periods.

9. Elements as arranged show periodicity in properties such as atomic radii, valency, ionisation energy, electron affinity and metallic character.

 

(i) Atomic radius increases from top to bottom along a group but decreases along a period from left to right.

(ii) Valency remains the same in the group while in it first increases up to 4 and then decreases along a period.

(iii) Ionisation energy decreases on moving down a group while it increases from left to right in a period.

(iv) Electropositive or metallic character increases from top to bottom for a group but decreases from left to right in a period.