Valence shell electron pair repulsion (VSEPR) rules are a model used to predict the shape of individual molecules based upon the extent of electron-pair electrostatic repulsion.

The N=C=O angle should therefore be 180°, or linear. The carbon atom forms two double bonds. We see from Figure \(\PageIndex{2}\) that the geometry that minimizes repulsions is 4.

This approach gives no information about the actual arrangement of atoms in space, however. There are six nuclei, so the molecular geometry of SF1. Each double bond is a group, so there are two electron groups around the central atom. With three nuclei and three lone pairs of electrons, the molecular geometry of I1. The central atom, iodine, contributes seven electrons.

The structure that minimizes LP–LP, LP–BP, and BP–BP repulsions is4. We see from Figure \(\PageIndex{3}\) that the molecular geometry of COIn our next example we encounter the effects of lone pairs and multiple bonds on molecular geometry for the first time.3. Structure (b), with fewer LP–BP repulsions at 90° than (a), is lower in energy. Similarly, the carbon atom on the right has two double bonds that are similar to those in COUse the VSEPR model to predict the molecular geometry of propyne (HCount the number of electron groups around each carbon, recognizing that in the VSEPR model, a multiple bond counts as a single group. With four electron groups, we must learn to show molecules and ions in three dimensions.1. 2. The gas phase structures of the triatomic halides of the heavier members of The VSEPR theory can be extended to molecules with an odd number of electrons by treating the unpaired electron as a "half electron pair" — for example, Gillespie and Nyholm The central atom, beryllium, contributes two valence electrons, and each hydrogen atom contributes one. This molecular structure is 1.
For example, the HThe "AXE method" of electron counting is commonly used when applying the VSEPR theory. There are two bonding pairs and one lone pair, so the structure is designated as AX4. If they do not, then the molecule has a net dipole moment.Lewis electron structures give no information about [ "article:topic", "showtoc:no", "license:ccbyncsa" ][ "article:topic", "showtoc:no", "license:ccbyncsa" ]

With five nuclei surrounding the central atom, the molecular structure is based on an octahedron with a vertex missing. Placing five F atoms around Br while minimizing BP–BP and LP–BP repulsions gives the following structure:4. There are six electron groups around the Br, five bonding pairs and one lone pair. The central atom, sulfur, contributes six valence electrons, and each fluorine atom has seven valence electrons, so the Lewis electron structure isWith an expanded valence, this species is an exception to the octet rule.2. As shown in Figure \(\PageIndex{2}\), repulsions are minimized by placing the groups in the corners of a tetrahedron with bond angles of 109.5°.3. The Lewis electron structure is2. The central atom, carbon, contributes four valence electrons, and each oxygen atom contributes six. With three bonding groups around the central atom, the structure is designated as AX4.

Thus according to the VSEPR model, the C–N=C fragment should be bent with an angle less than 120°.The carbon in the –N=C=O fragment is doubly bonded to both nitrogen and oxygen, which in the VSEPR model gives carbon a total of two electron pairs. Thus both F atoms are in the axial positions, like the two iodine atoms around the central iodine in IThe VSEPR model can be used to predict the structure of somewhat more complex molecules with no single central atom by treating them as linked AXThe nitrogen atom is connected to one carbon by a single bond and to the other carbon by a double bond, producing a total of three bonds, C–N=C.
The bromine atom has seven valence electrons, and each fluorine has seven valence electrons, so the Lewis electron structure isOnce again, we have a compound that is an exception to the octet rule.2. The central atom, carbon, contributes four valence electrons, and each hydrogen atom has one valence electron, so the full Lewis electron structure is2. This means that both of these carbons are linear, with C–C≡C and C≡C–H angles of 180°.The terminal carbon atoms are trigonal planar, the central carbon is linear, and the C–C–C angle is 180°.Other examples of molecules with polar bonds are shown in Figure \(\PageIndex{9}\). Once again, both groups around the central atom are bonding pairs (BP), so CO1. Each E represents a lone pair of electrons on the central atom. However, we predict a deviation in bond angles because of the presence of the two lone pairs of electrons.1. The number of electron pairs in the valence shell of a central atom is determined after drawing the The electron pairs (or groups if multiple bonds are present) are assumed to lie on the surface of a sphere centered on the central atom and tend to occupy positions that minimize their mutual repulsions by maximizing the distance between them.The overall geometry is further refined by distinguishing between The difference between lone pairs and bonding pairs may also be used to rationalize deviations from idealized geometries. Step2: Apply VSEPR notation, A X E A=Number of central atoms X=Number of surrounding atoms E= Number of lone pairs on central atom For the above molecule VSEPR notation will be AX 2 E 3.