Final answer:
The size of an atom generally decreases as you move from left to right across a period due to the increasing positive charge of the nucleus, which pulls the electrons closer.
Step-by-step explanation:
As you move from left to right across a period, the atomic size, or atomic radius, tends to decrease. This occurs because the increasing charge of the nucleus pulls on the electrons more strongly. During this progression across a period, protons are added to the nucleus, which increases the nuclear charge, and electrons are being added to the same principal energy level (shell). The growing positive charge in the nucleus enhances the force of attraction on the valence electrons, causing them to be pulled closer to the nucleus, hence reducing the atomic size.
There are exceptions to this trend, such as the fact that the atomic radius of oxygen is slightly larger than that of nitrogen. However, generally, as we progress toward the right on the periodic table, the effective nuclear charge (Zeff) experienced by the electrons increases, leading to a decrease in the size of the atom. It is important to note that anions have larger radii than their parent atoms due to an increase in electron count without a corresponding increase in nuclear charge, while cationic radii are smaller because the number of valence electrons has changed while the nuclear charge has remained constant.