Periodic Classification of Elements

CONCEPTS

1. Early chemists classified elements as metals and non-metals on the basis of a set of physical and chemical properties.

2. Dobereiner classified elements on the basis of “Law of Triads” which states that “atomic mass of the middle element of a triad is almost the arithmetic mean of the other two elements”.

3. A triad of elements is a group of three elements arranged in the order of increasing atomic masses, such that a group of such elements have similar physical and chemical properties.

4. Newland classified elements on the basis of “Law of Octaves” stated by him. According to this law : ‘When the elements are arranged in the order of increasing atomic mass, the properties of the eighth element (starting from a given element) are a repetition of the properties of the first element.

5. Mendeleev stated the law for the classification of elements based on the increasing atomic masses of the elements and the similarity in their physical and chemical properties.

6. Mendeleev’s Periodic Law states “the physical and chemical properties of elements are the periodic function of their atomic masses”.

7. Mendeleev’s Periodic Table is a tabular chart, representing systematic arrangement of elements in groups and periods in the order of their increasing atomic masses.

8. In the original Mendeleev’s Periodic Table : 
(i) There are eight vertical columns called groups. The groups from I to VII are subdivided into two groups, i.e., subgroup ‘a’ and subgroup ‘b’. Thus, on the whole, there are 15 vertical columns.
(ii) The properties of elements in the same subgroup or main group are similar.
(iii) The horizontal rows in the Periodic Table are called periods.
(iv) In a period, the properties of elements gradually change from metallic to non-metallic character.
(v) There are a few gaps in the Periodic Table. These gaps were left knowingly as these elements were not known at that time.

9. H.G.J. Moseley modified Mendeleev’s Periodic Law by changing atomic mass to atomic number, which is a more fundamental property of the element. It states : “the physical and chemical properties of elements are the periodic function of their atomic numbers”.

10. In the modern version of Mendeleev’s Periodic Table :
(i) There are nine vertical columns called groups.
(ii) These groups are numbered from 0 to 8 and not from 1 to 9.
(iii) Zero group consisting of rare gases was not known when Mendeleev prepared his original periodic table.
(iv) Groups from I to VII are divided into subgroups A and B.
(v) Horizontally, periodic table is divided into seven rows, running from left to right. These rows are called periods. There are 7 (seven) periods in all.

11. Modern Periodic Law : It states : ‘‘Properties of elements are the periodic function of their atomic numbers’’. 

12. Neils Bohr reconstructed the periodic table commonly called the Long form of the Periodic Table.

13. Long form of the Periodic Table : In the long form of the periodic table, the elements are arranged in groups and periods on the basis of the electronic configuration of elements.

14. Characteristic of the Long form of the Periodic Table.
(a) Characteristics of the Groups : 
1. There are 18 groups in the long form of the periodic table.
2. Group 1 is on the extreme left hand side and group 18 on the extreme right hand side of the periodic table.
3. Groups 1, 2 and 13 to 17 contain normal elements. The normal elements are sometimes called representative elements. In these elements, all the inner shells are completely filled with electrons, except the outermost shell which is incomplete.
4. The elements in group 18 are known as noble gases or inert gases. They have 8 electrons in their valence shell, except helium, which has 2 electrons in the valence shell.
5. The elements in group 3 to group 12 are called transition elements. In transition elements, the outermost shell as well as the shell next to the outermost shell (penultimate shell) are incomplete.
(b) Characteristics of the Periods : 
1. There are seven periods in all, such that each period has consecutive (or continuous) atomic numbers.
2. The number of elements in a period correspond to the maximum number of electrons which can be accommodated in one shell of the element.
3. The number of the period to which an element belongs is given by the number of the outermost shell (quantum number).

15. Rare Earth or Lanthanides : They are inner transition elements from atomic number 57 (Lanthanum) to atomic number 71 (Lutetium). They are kept outside the periodic table to mark their peculiar properties.

16. Actinides : They are inner transition elements from atomic number 89 (Actinium) to atomic number 103 (Lawrencium). They are kept outside the Periodic Table to mark their peculiar properties.

17. On moving from left to right across a period, the number of electrons in the valence shell increases by one in groups 1, 2, 13, 14, 15, 16, 17 and 18.

18. On moving from left to right across a period, the valency of elements with respect to hydrogen increases in group 1, 2, 13, and 14 and decreases in the groups 15, 16 and 17 till it is zero in group 18.

19. On moving from left to right across a period, the valency of elements with respect to oxygen increases in groups 1, 2, 13, 14, 15, 16, 17, and then it is zero in group 18.

20. On moving from left to right across a period, the atomic size of the elements decreases in groups 1, 2, 13, 14, 15, 16, 17 and then suddenly increases.

21. On moving from left to right across a period, the metallic character of the elements gradually decreases and non-metallic character gradually increases, till in the 18th group it becomes a noble gas.

22. On moving from left to right across a period, the chemical activity of the elements gradually decreases, then it starts increasing, such that the element in the last group is chemically inert.

23. On moving from left to right across a period, the basic nature of the oxides gradually decreases and the acidic nature of the oxides gradually increases.

24. As one moves down a group, the number of electrons in the valence shell in groups 1 and 2 are the same as the number of the group. The number of electrons in groups 13 to 18 is equal to : the number of the group –10.

25. All the elements in a group have the same valency which is equal to the number of electrons in the valence shell.

26. The atomic size of the elements increases as one moves down the group.

27. (a) While moving down in a group of metals (1, 2 and 13), the metallic character of elements increases.
(b) While moving down in a group of non-metals (14, 15, 16 and 17), the electronegative character of non-metals decreases.

28. (a) While moving down in a group of metals (1, 2 and 13), the chemical activity of metals increases.
(b) While going down in a group of non-metals (14, 15, 16, 17), the chemical activity decreases.

29. (a) While going down in a group of metals, the physical properties, such as melting point and boiling point, decrease gradually.
(b) While going down in a group of non-metals, the physical properties, such as melting point and boiling point, increase gradually.