JEE Chemistry Classification of Elements and Periodicity in Properties Notes Set 02

Development of the Periodic Table

1. Dobereiner grouped elements in triads where the middle atomic mass is about the mean of the other two, for example Li, Na, K (7, 23, 39).
2. Newlands arranged elements by increasing atomic mass and noticed that every eighth element repeated the properties of the first (law of octaves). It failed beyond calcium.
3. Mendeleev's law stated that properties are periodic functions of atomic mass. He left gaps for undiscovered elements (eka-aluminium, eka-silicon) and corrected doubtful atomic masses, but pairs such as Ar-K and Te-I broke the mass order.
4. Moseley showed that the atomic number is the true basis. The modern law says properties of elements are periodic functions of their atomic numbers.

Structure of the Modern Table

1. There are 7 periods and 18 groups. Period number equals the number of the outermost shell. The number of elements in a period equals twice the number of orbitals filled in it: 2, 8, 8, 18, 18, 32 and a 7th period that is incomplete.
2. Group number rules: for s-block it is the number of valence electrons; for p-block it is 10 + (ns + np electrons); for d-block it is (n-1)d electrons + ns electrons.
3. Blocks are named by the subshell receiving the last electron: s-block (groups 1 and 2), p-block (groups 13 to 18), d-block (groups 3 to 12) and f-block (lanthanides and actinides).
4. General outer configurations: s-block ns1-2, p-block ns2 np1-6, d-block (n-1)d1-10 ns1-2, f-block (n-2)f1-14 (n-1)d0-1 ns2.
5. Representative (normal) elements are the s- and p-block elements except the noble gases. Transition elements are d-block, and inner transition elements are the f-block.
6. Elements 58 to 71 are the lanthanides (4f filling, 6th period) and 90 to 103 are the actinides (5f filling, 7th period). Elements beyond uranium (Z > 92) are the transuranic elements and are man-made.

Naming Elements with Z Above 100

Until a permanent name is approved, the three-letter symbol and name come from the digits of the atomic number: 0 nil, 1 un, 2 bi, 3 tri, 4 quad, 5 pent, 6 hex, 7 sept, 8 oct, 9 enn, joined in order with the ending -ium. For example Z = 104 is unnilquadium and Z = 105 is unnilpentium.