central atom

…more atoms attached to a central atom than can be accommodated by an octet of electrons. An example is sulfur hexafluoride, SF₆, for which writing a Lewis structure with six S―F bonds requires that at least 12 electrons be present around the sulfur atom:

Examples of the manner in which VSEPR theory is applied to species in which there is no central atom are provided by ethane (C₂H₆), ethylene (C₂H₄), and acetylene (C₂H₂), the Lewis structures

…the electron pairs of the central atom, disregarding the distinction between bonding pairs and lone pairs. These pairs are then allowed to move around the central atom (at a constant distance) and to take up positions that maximize their mutual separations. As in the methane molecule, four pairs adopt a…