In forming molecules, electron shells fill themselves out by sharing pairs of electrons. This is basic chemical affinity: forging a covalent bond, with electrons portioning their orbits among atoms.
There are several types of covalent bonds, which vary by the nature of electron sharing. Sigma, pi, and 3-center 2-electron bonds are illustrative.
A sigma bond (σ) is the strongest type of covalent bond, corresponding to valence shell sharing as previously described.
A pi bond (π) is formed by overlapping atomic orbital lobes. Pi bonds are more diffuse than sigma bonds, and so somewhat weaker.
In a >3-center 2-electron bond (3c-2e), 3 atoms share 2 electrons. This odd bonding comes up an electron short. Typically, the bonding orbital is not equally allocated, but skewed toward 2 of the 3 atoms in the molecule. The simplest example of 3c-2e bonding is H3+: the trihydrogen cation.
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Covalent bonding takes energy. The strength of a chemical bond – bond energy – is measured by the energy needed to separate the bonded atoms of a molecule.
Electronegativity is electron sex appeal: the ability of an atom or molecule to attract electrons. Electronegativity is an inelegant term, and confusing, because it applies to the positive appeal that an element has for an electron, albeit an electron is negatively charged.
On the other end of the electron exchange program, some elements are willing to donate electrons. The measure of electron donation willingness is termed electropositivity.