Use the trends in ionization energy and electron affinity to explain why calcium fluoride has th

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Use the trends in ionization energy and electron affinity to...

Key Concepts

Periodic Trends

Periodic trends, including changes in ionization energy and electron affinity across the periodic table, provide insight into the likelihood of atoms forming particular ions. Understanding these trends helps predict which ions are energetically favorable for an element, and thereby, which stoichiometric combinations will yield the most stable ionic compounds. In this case, the trends support calcium’s tendency to form a doubly charged cation and fluorine's ability to form a single negative ion, resulting in the compound CaF?.

Charge Balance and Stoichiometry in Ionic Compounds

In ionic compounds, the overall charges of the constituent ions must balance to result in a neutral compound. Calcium, forming a Ca²? ion, requires two anions to balance its +2 charge, while each fluorine forms an F? ion with a -1 charge. The 2:1 ratio of fluoride ions to calcium ions leads to the empirical formula CaF?, ensuring that the total positive and negative charges cancel each other.

Ionization Energy

Ionization energy is the energy required to remove an electron from a gaseous atom. Elements with lower ionization energies, particularly metals, can lose electrons to form positive ions; however, the removal of additional electrons requires progressively more energy. In the case of calcium, although the first electron is relatively easy to remove, the second ionization energy is high, yet still attainable in the formation of a stable Ca²? ion when balanced against energy release from subsequent bond formation.

Electron Affinity

Electron affinity is the energy change that occurs when a neutral atom gains an electron, typically releasing energy. Halogens like fluorine have high electron affinities because they are one electron short of a complete octet, making the formation of F? ions energetically favorable. This significant energy release when fluorine gains an electron helps drive the formation of stable ionic compounds.

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