Final answer:
Atomic radii increase down a group in the periodic table due to the additon of electron shells and decrease across a period from left to right due to increasing nuclear charge.
Step-by-step explanation:
The atomic radius trends in the periodic table are fundamental concepts in Chemistry. The size of an atom generally increases down a group due to the addition of electron shells as the principal quantum number increases. As one moves from top to bottom in a column, the valence electron shell expands, causing the radius to increase because these additional electron shells are further away from the nucleus, surpassing the increasing nuclear charge's ability to pull electrons inwards.
On the other hand, as you go across a period from left to right, the atomic radii decrease. This is because, while electrons are added to the same principal energy level, the number of protons in the nucleus also increases, leading to a greater positive charge that pulls the valence electrons closer to the nucleus.
The added electrons feel a larger effective nuclear charge, which causes the size of the atom to decrease horizontally across a period. Despite the same energy level being filled across the row, the increasing number of protons in the nucleus wins out in this tug-of-war, leading to a reduction in atomic radius.
Thus, the general periodic trend is that atomic radii decrease from left to right across a row and increase from top to bottom down a column. Atomic radii plotted against atomic number typically demonstrates this pattern, with some minor deviations.