How do atomic radii of elements vary across a period and on moving down the group? Explain with reasons.

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Solution

Atomic radii: The atomic radius of an element is a measure of the size of its atom, usually the mean or typical distance from the center of the nucleus to the outermost electron.
Trends in Period:
As we move across a period, the atomic radius of atoms generally decreases. Within a period, protons are added to the nucleus as electrons are added to the same principal energy level.
These electrons are pulled closer to the nucleus because of its increased positive charge.
As the force of attraction between nuclei and electrons increases, the size of the atoms decreases.
Hence, the atomic radii decrease along the period due to an increase in effective nuclear charge.
Trends in group:
As we move down a group, atomic radii increase. The atomic number increases down a group and there is an increase in the positive nuclear charge. But there is also an increase in the number of occupied principle energy levels.
As electron cloud size increases, atomic radii increases.
Hence, atomic radii will increase as we go down group.

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