Electron counting in Balsa is based on the well-known electron-pair bonding model developed by Lewis and others ("Lewis model") . This model assumes that a bond spans exactly two atoms and is associated with a nonzero, positive, and even electron count. These electrons are drawn in equal quantities from each atom's set of valence electrons. A single bond draws one electron from each atom (two total), a double bond two (four total), and a triple bond three (six total). The electron count of an atom whose element attribute equals None is always zero. Bonding does not change it.