In the following model, the sp2 carbon atom is dark gray, oxygen is red and the hydrogens are cyan. Four representative molecular orbitals, including the HOMO and LUMO, may be examined by clicking the appropriate button. The different phases of the molecular orbitals are colored red and blue and are separated by nodal surfaces at which electron density is zero. A Lewis formula for acetone consists of six C–H sigma bonds, two C–C sigma bonds, a C–O sigmabond, a C=O pi bond and two non-bonding electron pairs. The lowest energy M.O. is essentially the C-O sigma bond. The LUMO is the π* orbital, and the remaining occupied orbitals correspond to similar orbitals of formaldehyde Because of overlap mixing, the molecular orbitals are not as localized as the Lewis structure suggests. The molecules and their orbitals may be rotated by clicking on the structure and dragging the mouse. Holding the shift key during this operation allows zooming.