### Periodic Classification of Elements

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It is not possible to measure the atomic radius precisely since the electron cloud surrounding the atom does not have a sharp boundary. One practical approach to estimate the size of an atom of a non-metallic element is to measure the distance between two atoms when they are bound together by a single bond in a covalent molecule and then dividing by two. For metals, we define the term "metallic radius" which is taken as half the internuclear distance separating the metal cores in the metallic crystal. The van der Waal's radius represents the overall size of the atoms which includes its valence shell in a non bonded situation. It is the half of the distance between two similar atoms in separate molecules in a solid. The atomic radius decreases across a period and increases down the group. Generally, same trends are observed in case of ionic radius. The ionic radius of the species having the same number of electrons depends on the number of protons in their nuclei.
It is not possible to measure the atomic radius precisely since the electron cloud surrounding the atom does not have a sharp boundary. One practical approach to estimate the size of an atom of a non-metallic element is to measure the distance between two atoms when they are bound together by a single bond in a covalent molecule and then dividing by two. For metals, we define the term "metallic radius" which is taken as half the internuclear distance separating the metal cores in the metallic crystal. The van der Waal's radius represents the overall size of the atoms which includes its valence shell in a non bonded situation. It is the half of the distance between two similar atoms in separate molecules in a solid. The atomic radius decreases across a period and increases down the group. Generally, same trends are observed in case of ionic radius. The ionic radius of the species having the same number of electrons depends on the number of protons in their nuclei.
The first $(\Delta_1H_1)$ and second $(\Delta_1H_2)$ ionisation enthalpies (in kJ $mol^{-1})$ and the $(\Delta_{eq}H)$ electron gain enthalpy (in kJ $mol^{-1})$ of a few elements are given below:
 Elements $\Delta_IH_1$ $\Delta_I H_2$ $\Delta_{eg}H$ (A) P 520 7300 -60 (B) Q 419 3051 -48 (C) R 1681 3374 -328 (D) S 1008 1846 -295 (E) T 2372 5251 +48 (F) U 738 1451 -40
According to inert pair effect, the lower state becomes more stable on descending the group. The effect is commonly described to stability of an electron configuration with entirely filled subshells in the inert pair effect, a metal loses all the p-electrons in its outermost subshell, leaving a filled $s^{2}$ subshell, the  pair of $s-$electrons seems relatively inert and is less easily removed.
Name the following with reference to the elements of the modern periodic table.