Final answer:
A negative ion, or an anion, is formed by the addition of one or more electrons to the valence shell of an atom. This causes the repulsion among the electrons and a decrease in the effective nuclear charge per electron, leading to an increase in the radius of the anion compared to the parent atom.
Step-by-step explanation:
An anion (negative ion) is formed by the addition of one or more electrons to the valence shell of an atom. This results in a greater repulsion among the electrons and a decrease in Zeff per electron. Both effects (the increased number of electrons and the decreased Zeff) cause the radius of an anion to be larger than that of the parent atom. For example, a sulfur atom ([Ne]3s²3p4) has a covalent radius of 104 pm, whereas the ionic radius of the sulfide anion ([Ne]3s²3pº) is 170 pm. For consecutive elements proceeding down any group, anions have larger principal quantum numbers and, thus, larger radii.