Atoms are composed of a dense nucleus surrounded by electrons, which orbit the nucleus at specific levels, or shells. Atoms are most stable when they have a full outer (valence) shell of electrons; most atoms follow the octet rule, which states that atoms are most stable when they have eight valence electrons. Atoms can lose, gain, or share electrons to achieve full shells. The sharing of electrons between atoms creates covalent bonds.

In a covalent bond, the pairs of electrons shared by the atoms are most likely to be found in a region of high electron concentration between the atoms. When more than one atom is bonded to a central atom, the electron pairs in the molecule repel each other. Sometimes there are additional pairs of electrons in the valence shell. These electrons pairs exist as non-bonding, or lone, pairs that also repel other electron pairs and affect the shape of the molecule. The most stable arrangement for the molecule is when all electron concentrations are as far apart from each other as possible.

The valence shell electron pair repulsion (VSEPR) theory predicts the three-dimensional shapes of molecules based on the arrangement of the electron concentrations. In other words, the shape of the molecule is determined by the locations of the pairs of electrons that inhabit the outer shell of the central atom and repel one another.

There are five common geometries for molecules with less than four electron concentrations: linear, trigonal planar, bent, tetrahedral, and trigonal pyramid. In the linear case, two outside atoms are bonded to a central atom. The two electron concentrations repel each other such that the outside atoms stay on opposite sides of the central atom. Similarly, in the trigonal planar case, there are three outside atoms bonded to a central atom. Again, the electron concentrations repel each other to be as far apart as possible. As a result, the three atoms are arranged at the points of an equilateral triangle surrounding the central atom.

Lewis structures—also called electron dot diagrams—are a way of representing the valence electrons of an atom (both covalent bonds and lone pairs). By counting the number of electron concentrations (the number of bonding and non-bonding electron pairs), it is easy to predict the shape of the molecule using the VSEPR model.

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