atomic size

across a period, both no. of protons and no. of electrons increase. However the outermost electrons do not screen out the additional electron effectively (poor secondary screening effect) and the primary screening effect of the inner shell electrons remains unchanged. Therefore the effect of increasing nuclear charge outweights the effect of increasing no. of electrons, which in turn means that the effective nuclear charge experienced by the outermost electron increases across a period. As a result the outermost electron is attrated more to the nucleus. This results in contraction of electron cloud and hence reduction in atomic size.

2007-02-26 18:39:05 補充：
to be simple, just say the across a period there is a increasing net attraction on the outermost electron, and the atom is smaller