Chemical Bonding and Molecular Structure
10.0 Valence Bond Theory
10.0 Valence Bond Theory
This theory was proposed by Linus Pauling, who was awarded the Noble Prize for Chemistry $1954$.
Atoms with unpaired electrons tend to combine with other atoms which also have unpaired electrons. In this way the unpaired electrons are paired up, and the atoms involved, all attain a stable electronic arrangement. This is usually a full shell of electrons (i.e. a noble gas configuration).
Two electrons shared between two atoms constitute a bond. The number of bonds formed by an atom is usually the same as the number of unpaired electrons in the ground state, i.e. the lowest energy state. However, in some cases the atom may form more bonds than this.
This occurs by excitation of the atom (i.e. providing it with energy) when electrons which were paired in the ground state are unpaired and promoted into suitable empty orbitals. This increases the number of unpaired electrons, and hence it increases number of bond which can be formed.
A covalent bond results from the pairing of electrons (one from each atom). The spins of the two electrons must be opposite (anti-parallel) because of the Pauli exclusion principle that no two electrons in one atom can have all four quantum numbers the same.
1. In $HF$, $H$ has a singly occupied $s-$orbital that overlaps with a singly filled $2p$ orbital on $F$.
2. In $H_2O$, the $O$ atom has two singly filled $2p$ orbitals, each of which overlaps with a single occupied s-orbital from two $H$ atoms.
3. In $NH_3$, there are three singly occupied $p$ orbitals on $N$ which overlap with $s$ orbitals from three $H$ atoms.
4. In $CH_4$, the $C$ atom in its ground state has the electronic configuration $1s^2, 2s^2, 2p_x^1, 2p_y^1$ and only has two unpaired electrons, and so can form only two bonds. If the $C$ atom is excited, then the $2s$ electrons may be unpaired, giving $1s^2, 2s^1, 2p_x^1, 2p_y^1, 2p_x^1$. There are now four unpaired electrons which overlap with singly occupied $s$ orbitals on four $H$ atoms.
$CH_4$ molecule uses its three $p-$orbitals $p_x$, $p_y$ and $p_z$, which are mutually at right angles to each other, and the $s$ orbital is spherically symmetrical. Hence they form tetrahedral structure.
