Tue August 14, 2012 By: Rahul Joshi

why the size of an atom decreases as we move left to right in a period?

Expert Reply
Tue August 14, 2012
When we move from left to right across a period, the number of shells in the atom remain the same but the number of valence shell electrons increase. So, the effective nuclear charge on the electrons also increase resulting in smaller size of atoms. For eg, take the case of period 2 elements, Li, Be, B, C, N, O, F, Ne. The number of orbitals (shells) in all these elements is same (i.e., 2 shels) but the number of valence electrons increase as we move from left to right. This results in an increase in nuclear charge on outermost shell (L shell) of the atoms moving from left to right. Hence, the atomic size decreases.
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