The size of atoms generally tends to increase as you move down a group (vertical column) in the periodic table and decrease as you move across a period (horizontal row) from left to right. This trend is known as the periodic trend of atomic size or atomic radius.
Moving down a group, the atomic size increases because each successive element has an additional energy level (shell) of electrons compared to the previous element. These additional energy levels result in an increased distance between the nucleus and the outermost electrons, leading to larger atomic size.
Moving across a period from left to right, the atomic size decreases. This is because, within a period, the number of protons and electrons increases, but the additional electrons are added to the same energy level. The increasing positive charge of the nucleus exerts a stronger pull on the electrons, causing the atomic size to decrease.
However, it's important to note that there can be exceptions to this trend in certain cases due to variations in electron configuration and other factors. Additionally, when considering the size of ions, the presence of positive or negative charges can also influence the atomic size.
Overall, while the atomic size generally follows these trends, it's important to analyze specific elements and their electron configurations to make accurate comparisons.