Define atomic size. Give its unit of measurement. In the modem periodic table what trend is observed in the atomic radius in a group and a period and why is it so?

Atomic Size is the distance between the centre of an atom i.e from the nucleus to the outermost (valence) shell of that atom.

Atomic size is measured in Angstroms, where 1 Angstrom = 10^-10m.

In the modern periodic table, the atomic size of elements increases down the group, and the atomic size decrease along the period from left to right. Down the group, as the number of energy shells of the atoms keeps on increasing from top to bottom, hence, the atomic radius also increases.

Along the period, the atomic number of elements increases, which leads to an increase in the number of protons, thus, effective nuclear charge increases. So, the atomic radius decreases across the period.