What are the trends in atomic radius across the periodic table?

The atomic radius generally decreases across a period from left to right and increases down a group.

In the periodic table, the atomic radius refers to the size of an atom. This size is determined by the distance from the nucleus to the outermost shell where electrons are located. As you move across a period from left to right, the atomic radius tends to decrease. This is because as you move across a period, each atom has one more proton and electron than the last. The additional proton increases the positive charge of the nucleus, pulling the electrons closer and thus decreasing the atomic radius.

On the other hand, as you move down a group, the atomic radius increases. This is due to the addition of energy levels or shells as you move down a group. Each new energy level is further from the nucleus, increasing the atomic radius. Despite the increase in the number of protons, the effect of the additional energy levels is greater, leading to an overall increase in atomic radius.

It's also worth noting that the trend of decreasing atomic radius across a period is not always consistent. For instance, the transition metals (groups 3-12) have a generally constant atomic radius. This is because the additional electrons are added to an inner shell, not the outermost shell, which doesn't significantly affect the atomic radius.

In summary, the atomic radius is influenced by the number of protons in the nucleus and the number of energy levels or shells. As you move across a period from left to right, the atomic radius decreases due to the increased nuclear charge. Conversely, as you move down a group, the atomic radius increases due to the addition of energy levels.