Question

what accounts for trend in atomic radii within a peroid

Answer 1

Across the period:

> Atomic Number Increases
> Therefore nuclear charge increases
> Electron shielding remains remains the same
> The number of valence electrons increases
> Therefore the attraction between the valence shell and the nucleus increases across the period.
> Therefore the valence shell gets pulled closer and closer across the period
> Therefore the atomic radii decreases gradually across the period.

Just to clarify by 'across the period' i mean left to right. or in order of increasing atomic number.

Hope that helps.

Answer 2

In case you aren't familiar with 'electron shielding' that is essentially when the inner electron shells reduce the attractive force of the nucleus felt by the outermost (valence) shell.