Question

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

Answer 1

Atomic size is the distance between the center of an atom i.e. from the nucleus to the outermost shell (valence shell) of that atom.
Atomic size is measured in Angstroms (A), where 1 Angstrom = 10^-10 meters.

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 so, the atomic radius also increases.

During the period, the atomic number of elements increases which leads to an increase in the number of protons inside the nucleus and thus, the effective nuclear charge increases. So, the atomic radius decreases over the period.