Periodic Properties of Elements in the PT The properties of the elements, which exhibit trend, include – Atomic Radius, Electro negativity, Ionization Energy, Electron Affinity. These properties of elements change gradually as we move from left to right of the table or from top to bottom of a group. These trends can be predicted using the periodic table and can be explained and understood by analyzing the electron configuration of the elements. Elements tend to gain or lose valence electrons to achieve stable octet formation as these properties mostly depend on the Outer Shell E.C. Physical Properties: Atomic Volume, Atomic Size that includes Atomic Radius, Ionic Radius Chemical Properties: Ionization Energy, Electron Affinity, Electro negativity, Valency, Metallic Character, Redox state etc.

Atomic Size: The distance between the centre of the nucleus to the outermost shell of an atom is said to be the Atomic Radius. However, it is impossible to measure the Atomic Radius like that as its impossible to isolate a single atom for measurement. Also its impossible to determine the exact position of an e - in the Valence Shell. Hence it is expressed in terms of Covalent Atomic Radius, which is the distance between two successive nucleii of 2 covalently bonded like atoms in a molecule of di – atomic elements like H 2, O 2, N 2, Cl 2 etc. r cov = d/2

Periodic Variation of Atomic Radius: On moving down a group, valency increases (directly proportional to Atomic Number). As a result, the Valence Outermost Shell get far away from the nucleus. This results in an increase in Atomic Radius. On moving from left to right of a PT, the atomic radius, however, decreases. This happens because of Shielding Effect of the Nucleus.

Ionization Enthalpy/ Ionization Energy/ Ionization Potential: The amount of energy required to remove the loosely bound electron from an atom. A(g) + I.E. = A + (g) + e - Expressed in eV/atom. The amount of energies required to remove the subsequent electrons in other shells are referred to as Successive Ionization Enthalpies. I. E. 1 > I. E. 2 > I. E. 3 Periodic Variation of Ionization Enthalpy: The Ionization Enthalpy increases along the period from left to right of the table. The Ionization Enthalpy decreases from top to bottom of the table. However, I.E. decreases from group 2 to 3 and 15 to 16 going from left to right of the table. (EXCEPTION)

Electron Gain Enthalpy/ Electron Affinity: It is the amount of energy released when an atom accepts an electron to form an anion. X (g) + e - = X - (g) + E.A. Expressed in eV/atom. Similarly, an atom can also accept the second and third e - and the energies released for these process is called Successive Electron Gain Enthalpies. (Δ eg H 1, Δ eg H 2, Δ eg H 3 …..) Periodic Variation of Electron Gain Enthalpy: Δ eg H increases in moving across the period from left to right of the table. Δ eg H decreases in moving from top to bottom of the group of the table.

Electro negativity It is the tendency of an atom in a molecule to attract the shared pair of electrons more towards itself. Electro negativity depends more on – Charge on the Nucleus, Size of the atom, and Shielding Effect. Here atom A and atom B shares a lone pair of e -, where B has a fair share of the lone pair as it has the capacity to pull this pair more towards itself, hence B is more electronegative than A. Here it results in the formation of a Polar Covalent bond as B becomes a more negative pole and A becomes a more positive pole. Ideal examples of molecules having this type of bond and more electronegative atom are HCl, O – H bond in H 2 O

Arunava Das, PGT Chemistry, GDGPS Siliguri, Darjeeling, West Bengal & Ex – Research Scholar UGC CSIR DBT JRF Molecular Biology Lab, Indian Institute of Science, Bangalore and Soil Biology Lab, GKVK, Bangalore