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London forces and dipole-dipole interactions

Note that the geometries are named according to the atomic positions only, not the electron arrangement.

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AXE method: annotation and examples: AXE annotation, geometry, and examples for each shape.

Main geometries (without lone pairs of electrons):

Linear

In a linear model, atoms are connected in a straight line, and a bond angle is simply the geometric angle between two adjacent bonds. A simple triatomic molecule of the type AX2 has its two bonding orbitals 180° apart. Examples of triatomic molecules for which VSEPR theory predicts a linear shape include BeCl2 (which does not possess enough electrons to conform to the octet rule) and CO2. When writing out the electron dot formula for carbon dioxide, notice that the C-O bonds are double bonds; this makes no difference to VSEPR theory. The central carbon atom is still joined to two other atoms. The electron clouds that connect the two oxygen atoms are 180° apart.

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Lewis dot structure of carbon dioxide: Although the central atom (carbon) has four bonds, only two are sigma bonds; it is therefore is represented as AX2Ein the table.

Trigonal planar

Molecules with the trigonal planar shape are triangular and in one plane, or flat surface. An AX3molecule such as BF3 has three regions of electron density extending out from the central atom. The repulsion between these will be at a minimum when the angle between any two is 120o.

Tetrahedral

Tetra- signifies four, and -hedral relates to a face of a solid; “tetrahedral” literally means “having four faces. ” This shape is found when there are four bonds all on one central atom, with no lone electron pairs. In accordance with the VSEPR theory, the bond angles between the electron bonds are 109.5o. An example of a tetrahedral molecule is methane (CH4). The four equivalent bonds point in four geometrically equivalent directions in three dimensions, corresponding to the four corners of a tetrahedron centered on the carbon atom.