Kaplan MCAT General Chemistry Review

Elements in Groups I and II (Groups 1 and 2) have such relatively low ionization energies that they are called the active metals. The active metals never exist naturally in their neutral elemental (native) forms; they are always found in ionic compounds, minerals, or ores. The loss of one electron (from the alkali metals) or the loss of two electrons (from the alkaline earth metals) results in the formation of a stable, filled valence shell. As you might imagine from the trend, the Group VIIA (Group 17) elements, the halogens, are a miserly group of penny pinchers and aren’t willing to give up their electrons to anybody. In fact, in their monatomic ion form, they are found only as anions, having greedily taken one electron from another atom to complete their octets. As you might guess, the halogens have very large ionization energies and the smaller the halogen atom, the higher the ionization energy.

The only group less willing to give up their valence electrons is the inert elements (noble gases). They already have a very stable electron configuration and are unwilling to disrupt that stability by losing an electron. Inert gases are among the elements with the highest ionization energies.

ELECTRON AFFINITY

The greedy halogens are among the worst of the bunch of elements that tend to hoard their electrons toward themselves. These elements also tend to be very anxious to gain the number of electrons necessary to complete their octets. Like nervous little squirrels frantically running around in search of nuts to pack into their accommodating cheek pouches, these elements go in search of other atoms that are willing to give up their electrons. When a gaseous atom of a particular elemental identity gains one or more electrons to complete its octet, it relaxes and breathes a sigh of relief. This “sigh of relief ” is a release of a quantity of energy called the electron affinity. Because energy is released when an atom gains an electron, we can describe this process as exothermic. By convention, the electron affinity is reported as a positive energy value, even though by the conventions of thermodynamics, exothermic processes have negative energy changes. Regardless of the sign, just remember that electron affinity is released energy. The stronger the electrostatic pull (that is, the Z_eff) between the nucleus and the valence shell electrons, the greater the energy release will be when the atom gains the electron. Thus, electron affinity increases across a period from left to right. Because the valence shell is farther away from the nucleus as the principal quantum number increases, electron affinity decreases in a period from top to bottom. Groups IA and IIA (Group 1 and 2) have very low electron affinities, preferring rather to give up one or two electrons, respectively, to achieve the octet configuration of the prior noble gas. Group VIIA (Group 17) elements have very high electron affinities because they need to gain only one electron to achieve the octet configuration of the immediately following noble gases in Group VIIIA (Group 18). Although the noble gases are the group of elements farthest to the right and would be predicted to have the highest electron affinities according to the trend, they actually have electron affinities on the order of zero, since they already possess a stable octet and cannot readily accept an electron. Elements of other groups generally have low electron affinity values.

Mnemonic

To recall the various trends, remember this: Cesium, Cs, is the largest, most metallic, and least electronegative of all naturally occurring elements. It also has the smallest ionization energy and the least exothermic electron affinity.

Mnemonic

In contrast to cesium, fluorine (F ) is the smallest, most electronegative element. It also has the largest ionization energy and most exothermic electron affinity.

ELECTRONEGATIVITY