Atomic sizes across a period.

Atomic sizes ( in terms of radius) gradually decrease from Left to right across a period of elements. This is because within a period, all electron (s) are added to the same shell . However, at the same time the protons are being added to the nucleus making it more positively charged. The effect of increasing the protons number is greater than increasing the electron number . This means that the nucleus attracts electrons more firmly to the nucleus, as a result, the atomic radius decreases .