Atomic size: Definition, examples, Trend in periodic table

Question: What is meant by term atomic size? Give its trend in periodic table.

Answer.

Atomic Size The size or radius of an atom is the average distance between the nucleus of an atom and its valence shell.

Trend in Periodic table

In period:

Generally, atomic size decreases from left to right across the period. This is because valence shell remains same in the period and nuclear charge is increased by one unit in each atom as we move from left to right across the period. At the same time, valence electron is added to the same shell which gradually increases the attractive forces between nucleus and valence electrons. As a result, atomic size decreases.

For example, atomic size of beryllium is smaller than lithium. This is because Be (at right side of lithium) has one more proton and electron as compared to lithium and hence more force of attractions. So, it has smaller atomic size as compared to Li.

In group: Generally, atomic size increases from top to bottom within the group. This is because size of valence shell (which determines size of atom) increases due to additional shell of electrons as we go from top to bottom along the group. As a result, atomic size increases.

For example, atomic size increases from Li to Na. This is due to fact that Na (down the group than Li)

has one more shell of electrons as compared to Li and hence more atomic size than Li.

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