Electron affinity indicates the energy change when an electron is added to a gaseous atom or ion. It can be thought of as the ability of an atom to attract an additional electron. Elements at the left of a row prefer to lose their few valence electrons to form an octet of electrons, whereas elements at the right of a row prefer to gain extra electrons to form an octet. Electron affinity increases moving from the left of a row to the right. An exception to this rule are the noble gases. These elements already have a stable octet of electrons and have electron affinities near zero. As a general rule, electrons located in shells farther from the nucleus feel a weaker force of attraction to the nucleus. Electron affinity therefore decreases moving down a column as atoms add more shells and become larger. Electron affinities can be positive indicating a desire to shed an electron (Be, Mg), or negative, indicating a desire to gain an electron (Cl, F).
