Bonding and molecular shape

Types of bonds

Atoms bond to reach lower energy. In an ionic bond, electrons transfer from a metal to a nonmetal, and oppositely charged ions attract in a lattice. In a covalent bond, atoms share electron pairs. Electronegativity is an atom's pull on shared electrons; a large difference makes a polar bond, with partial charges δ+ and δ−. Ionic lattice energy grows with higher ion charges and smaller ions, so MgO is held together far more strongly than NaCl.

Lewis structures

1. Add up the valence electrons of all atoms, adjusting for charge (add one per negative charge, subtract one per positive charge).
2. Place the least electronegative atom (not hydrogen) in the centre and join atoms with single bonds.
3. Complete octets on outer atoms, then put leftover electrons on the central atom.
4. If the central atom lacks an octet, form double or triple bonds.

Formal charge = valence electrons − nonbonding electrons − bonds. Structures with formal charges closest to zero are preferred. More bonds between two atoms means a shorter, stronger bond: triple < double < single in length.

VSEPR and shape

Electron groups (bonds and lone pairs) around a central atom repel and spread out. Count groups for the electron geometry, then ignore lone pairs to name the molecular shape. Two groups: linear. Three: trigonal planar (or bent with one lone pair). Four: tetrahedral (trigonal pyramidal with one lone pair, bent with two). Six: octahedral (square planar with two lone pairs on opposite sides). Hybridization follows the group count: 2 is sp, 3 is sp2, 4 is sp3.

Polarity and intermolecular forces

A molecule is polar if it has polar bonds that do not cancel by symmetry. Intermolecular forces, from weakest to strongest in general, are dispersion, dipole-dipole, and hydrogen bonding (an H bonded to N, O or F attracted to a lone pair on another N, O or F). They set boiling points.

Worked example

Find the shape and polarity of SO2.

1. Valence electrons: 6 + 2(6) = 18.
2. Sulfur is central with two oxygens. After completing octets, sulfur has one lone pair, giving a double bond to one oxygen and a single to the other, with resonance between them.
3. Sulfur has three electron groups (two bonds and one lone pair), so the electron geometry is trigonal planar and the molecular shape is bent, with an angle a little under 120°.
4. The two S–O bond dipoles do not cancel because the molecule is bent, so SO2 is polar.

Compare CO2: also 3 atoms, but carbon has no lone pairs, so it is linear and the dipoles cancel, making it nonpolar. Shape decides polarity, so always draw the shape before judging.