Classification of Elements and Periodicity in Properties

Modern Periodic Table

Electronic Configuration in Periods

• Period indicates the value of ‘n’ (principal quantum number) for the outermost or valence shell.

• Successive periods in the periodic table are associated with the filling of the next higher principal energy level (n = 2, n = 3, etc).

• First period (n = 1) →­­ hydrogen (1s¹) and helium (1s²) [2 elements]

• Second period (n = 2) → Li (1s² 2s¹), Be (1s² 2s²), B (1s² 2s² 2p¹) to Ne (2s² 2p⁶) [8 elements]

• Third period (n = 3) → filling to 3s and 3p orbitals gives rise to 8 elements (Na to Ar)

• Fourth period (n = 4) → 18 elements (K to Kr) − filling of the 4s and 4p orbitals

3d orbital is filled up before 4p orbitals (3d orbitals → energetically favourable)

• 3d-transition series → Sc (3d¹ 4s²) to Zn (3d¹⁰ 4s²)

• Fifth period (n = 5) → 18 elements (Rb to Xe)

• 4d-transition series starts at Ytterbium and ends at Cadmium.

• Sixth period (n = 6) → 32 elements; electrons enter 6s, 4f, 5d, and 6p orbitals successively. Elements from Z = 58 to Z = 71 are called 4f-inner transition series or lanthanoid series (filling up of the 4f orbitals).

• Seventh period (n = 7) → electrons enter at 7s, 5f, 6d, and 7p orbitals successively. Filling up of 5f orbitals after Ac (Z = 89) gives 5f-inner transition series or the actinoid series.

Electronic Configuration in Groups

• Same number of electrons is present in the outer orbitals (that is, similar valence shell electronic configuration).

• Electronic configuration of group 1 elements is given in the following table.

Atomic number	Symbol	Electronic configuration
3	Li	1s2 2s1(or) [He]2s1
11	Na	1s2 2s2 2p6 3s1(or) [Ne]3s1
19	K	1s2 2s2 2p6 3s2 3p6 4s1(or) [Ar]4s1
37	Rb	1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 5s1(or) [Kr]5s1
55	Cs	1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 6s1(or)[…