Final answer:
Non-metals typically form anions that are larger than their parent atom due to increased electron-electron repulsion. Cations are smaller because removing electrons increases the effective nuclear charge on the remaining electrons. The trend within a group for cations of the same charge is an increase in the ionic radius with increased principal quantum number.
Step-by-step explanation:
The student's question pertains to the comparative sizes of ions and their parent atoms. In chemistry, particularly within the study of the periodic table and atomic structure, we observe that certain types of elements form ions that differ in size compared to their neutral atoms. Specifically, the types of elements that have ions larger than their atoms are the non-metals which tend to gain electrons and form anions. These anions are always larger than the parent atom.
This size difference is due to the addition of electrons resulting in increased electron-electron repulsion within the ion, spreading out the electron cloud. As a result, an anion experiences decreased effective nuclear charge per electron due to the greater number of electrons compared to protons in the nucleus. Furthermore, cations (positively charged ions) are always smaller than the neutral atom due to electron loss leading to decreased electron repulsion and increased effective nuclear charge felt by the remaining electrons. For instance, when looking at successive elements within the same group in the periodic table, if they form cations of the same charge, the ionic radii generally increase as the principal quantum number n increases.
Additionally, some transition metals can form cations with variable charges with no simple pattern like the main group elements. For example, iron (Fe) can form both 2+ and 3+ cations, which are each smaller than the neutral atom due to loss of electrons. To sum up, non-metals form larger anions when they gain electrons, while cations formed by metals are always smaller than their respective neutral atoms.