Question

Define atomic size. How does it vary in a period and in a group?

Answer 1

Hint: Atomic size is a very important characteristic of an atom to understand the reactivity. The position of the element on the periodic table along with the number of shells for electrons present in the atom.

Complete answer:
Atomic size is described as the atomic radius, which is the distance between the centre of nucleus and the outermost electron in the outer shell. Atomic size is affected by the following factors:
- Number of Shells: Atomic size increases with the increase in number of electron shells. So, atomic radius is directly proportional to the number of electron shells.
- Nuclear charge: As the nuclear charge increases the atomic size decreases, due to increase in attractive force. So, atomic size is inversely proportional to nuclear charge.
- Screening effect: In an atom having more electrons, particularly signifying more electrons shells, it is observed that the inner orbits decrease the attraction between the electrons in the outer orbit and nucleus. Thus, they act as a screen or shield between electrons of or orbit and nucleus. This is known as a screening effect. As the screening effect increases atomic size also increases. So atomic radius is directly proportional to screening effect. The shielding ability of inner electrons decreases in the order of $s > p > d > f$.

Across the period from left to right there is a decrease in atomic size with increase in nuclear charge of the element. Atomic size increases down the group because of the addition of extra shells. So, as we move from group 1 to group 18 the size of the atom will decrease due to increased effective nuclear charge. And as we move from period 1 to period 7 the size will increase due to increase in shells.

Note:
The smaller the atomic radius, the more influence the nucleus has on reactivity. Since the nucleus of an atom contains positively charged protons it also attracts the electrons. For example, Halogen atoms want to get electrons so the added force of nuclear pull makes them more reactive.