Describe the theory associated with the radius of an atom as it (a) gains an electron (b) loses an electron
When an atom gains an electron, it forms an anion. The size of an anion is larger than that of the parent atom because the addition of one or more electrons results in increased repulsion among the electrons and a decrease in effective nuclear charge.
When an atom loses an electron, it results in the formation of a cation. A cation is smaller than its parent atom because it has fewer electrons while its nuclear charge remains the same, causing the nucleus to hold the remaining electrons more tightly.
Explanation
The answer explains the change in atomic radius upon ion formation based on the provided text. Gaining an electron increases electron-electron repulsion and decreases effective nuclear charge per electron, expanding the radius. Losing an electron leaves the same nuclear charge acting on fewer electrons, contracting the radius.