Here, we will discuss how the variation of atomic radii in group and period. We will study why do atomic radii increases in the group and decreases along the period.

Variation of atomic radii in the group and period

1. In general, atomic radii increases in the group from top to bottom. It decreases along the period from left to right.
2. The alkali metal is having largest radius in the periodic table. Halogens are having smallest radius in the periodic table.
3. The size of inert gases are however larger than preceding halogens due to Van der Waal’s radius.

Why the at. radii decreases from left to right in a period

1. As we move from left to right in the periodic table, the nuclear charge increases by 1 unit.
2. But the incoming electron adds up in a new shell each time.
3. Therefore, the effective nuclear charge or Z_eff increases as we move from left to right in the period.
4. Hence, the electrons of all the shells are pulled closer to the nucleus.
5. This results in the decrease of the at. radii along the period.

But sometimes, we observe the exceptional behaviour. For example, there is a increase in size when we move from F to Ne. The reason is in case of inert gases, the inter electronic repulsions are maximum. Also, inert gases have Van der Waal’s radii which is larger than covalent radii.

Variation of at. radii with atomic number in second period

Li ( 152 pm ) < Be ( 111 pm ) < B ( 88 pm ) < C ( 77 pm ) < N ( 70 pm ) < O ( 74 pm ) < F ( 72 pm) < Ne (160 pm )

Why atomic radii increases from top to bottom in the group

1. As we move down the group, the incoming valence electron adds up in the new energy level for successive elements.
2. Nuclear charge also increases with the increase in atomic number.
3. But the effect of increased nuclear charge decreases due to the shielding effect of intervening electrons.
4. Hence, the net effective nuclear charge or Zeff decreases down the group.
5. This results in increase in atomic radii as we move from top to bottom in the group.

Check out the post on types of radii here :

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