American Chemical Society (ACS) Chemistry Practice Exam

When an electropositive atom, which typically has a low ionization energy, becomes an ion by losing one or more electrons, it generally becomes smaller. This occurs because the removal of electrons reduces electron-electron repulsion within the atom and often leads to a stronger effective nuclear charge acting on the remaining electrons. As a result, the nucleus can draw the remaining electrons closer, resulting in a decrease in atomic size.

In contrast, an atom gaining electrons (forming an anion) would increase in size due to additional electron-electron repulsion, not applicable in this case since we are discussing cations formed from electropositive atoms. The size change can vary depending on the specific elements involved, but in the context of electropositive atoms losing electrons, a general trend towards decreased size is observed.

Thus, the correct understanding here is that electropositive atoms become smaller when they form positive ions due to the loss of electrons and the resulting changes in the effective nuclear charge on the remaining electrons.