The periodic trend for ionic size depends on whether the atom forms a positive ion or a negative ion.
Metals tend to lose electrons and form positive ions. The ion size of a metal is smaller than its neutral atom. This is because when an electron is lost, the positive charge from the nucleus now attracts the remaining electrons more strongly. These ions are shown in red in the diagram.
Nonmetals tend to gain electrons to form negative ions. The ion size of a nonmetal is larger than its neutral atom. This is because when an electron is gained, the positive charge from the nucleus is distributed over more negative electrons. These ions are shown in blue in the diagram.
Within a family, whether positive or negative ion, notice that the ion size increases from top to bottom in the column of elements.
