The ions formed in molecules from Group VIIA atoms (the halogens, such as \(\mathrm{Cl}\) ) are almost exclusively \(\mathrm{X}^{-}\) ions rather than \(\mathrm{X}^{2-}\) ions. Explain this result.

Elements in Group VIIA of the periodic table are known as halogens. This exclusive group consists of fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). Halogens are highly reactive nonmetals, and this reactivity is primarily due to their electron configuration. Each halogen has seven electrons in its outermost shell and is one electron short of achieving a full octet, which is particularly stable.

Halogens are widely known for their tendency to participate in chemical reactions that allow them to gain that one elusive electron, completing their outer shell. Interestingly, the reactivity of halogens decreases from fluorine to astatine, with fluorine being the most reactive. This is due to the increasing atomic size and decreasing electronegativity as one moves down the group.

Valence electrons are the outermost electrons of an atom and play a key role in chemical bonding and reactivity. The number of valence electrons determines an element’s ability to form bonds with other atoms. In the case of the halogens (Group VIIA elements), the presence of seven valence electrons is characteristic.

These seven valence electrons create an interesting scenario. Rather than losing multiple electrons to achieve a stable configuration, halogens prefer to gain a single electron to fill their valence shell. This propensity significantly influences the types of ions they form, and it explains why the common ions of halogens carry a -1 charge.

A stable electron configuration often refers to an atom's electron arrangement resembling that of the nearest noble gas, which has a full valence shell. Atoms strive to reach this stable state through chemical reactions that allow them to either gain or lose electrons. For halogens, attaining stability involves gaining one electron to fill their valence shell.

The desire to achieve a complete set of eight valence electrons, known as the octet rule, is a driving force in the chemical behavior of many elements. In the specific cases of halogens, reaching a stable configuration means forming an ion with a -1 charge. The process is favored over forming a 2- charge because it requires less energy and is constrained by the natural repulsion between electrons.

Ionization energy is the energy required to remove an electron from an atom in its gaseous state. It's an important concept when discussing ion formation and stability. High ionization energy means it's more difficult to remove an electron, thereby making it less likely for an atom to lose electrons. In contrast, it's energetically more favorable for an atom with high ionization energy to gain electrons.

For halogens, which all have high ionization energies, it is energetically less favorable to lose electrons. However, they readily gain one electron due to lower energy requirements as compared to gaining two. The formation of a 1- ion is thus more likely both in terms of energy conservation and the electrons' inherent repulsive forces, as adding to a negative ion requires overcoming this repulsion, which requires even more energy.