As the atomic number increases, the size of an atom generally decreases.
The size of an atom is determined by the distribution of its electrons and the attractive force between the electrons and the nucleus. As the atomic number increases within a period (horizontal row) of the periodic table, the number of protons in the nucleus increases. This means there is a stronger positive charge in the nucleus, exerting a greater pull on the electrons.
The increased positive charge in the nucleus leads to stronger electrostatic attraction between the electrons and the nucleus. As a result, the electrons are pulled closer to the nucleus, reducing the size of the atom. Additionally, as electrons are added to the same energy level (shell), they are held closer to the nucleus due to increased electron-electron repulsion.
However, it's important to note that there are exceptions to this trend. For example, the atomic size may slightly increase when moving from a noble gas to the alkali metal in the same period due to the addition of a new energy level. Additionally, when considering atoms within a group (vertical column), the atomic size generally increases as the atomic number increases, since each successive element adds a new energy level or shell of electrons.