chem問題.........

The atomic size will decrease across a period because Moving from left to right across a period, the atomic radius decreases. The nucleus of the atom gains protons moving from left to right, increasing the positive charge of the nucleus and increasing the attractive force of the nucleus upon the electrons. True, electrons are also added as the elements move from left to right across a period, but these electrons reside in the same energy shell and do not offer increased shielding.

Also,the nuclear charge and the shielding electrons combine to make the effective nuclear charge. That is a very important factor when you are comparing elements in a period. As you go across a period, the nuclear charge increases and the number of energy levels stays the same. Consequently, the number of shielding electrons stays the same and the effective nuclear charge increases. As the effective nuclear charge increases, it pulls the electrons in closer and closer to the nucleus. So as you go across a period, the increase in the nuclear charge causes a decrease in the atomic size because the electrons in the valence energy level are pulled in closer and closer.

Here are another anwser for your question:
http://www.dartmouth.edu/~genchem/0304/spring/6glueck/PeriodicProp.html

Hope these information can help you.

Across the periodic table, atomic size will decrease as the attractive force increase across the period.

If moving down the group, the atomic size will be increase since the increase of the electrons layer..