structure and bonding: valence bond theory

valence bond theory

• valence bonding - electron pairs localized between 2 atoms

• qualitative valence bonding used to describe structure and bonding of molecules

valence bond theory of H2

• bring 2 H atoms from a long way apart to closer together - wavefunctions overlap

• probability of finding an electron in the region of overlap is increased - energetically favourable

• when bond length reaches a balance of repulsion and attraction, electrons simultaneously attracted to both nuclei - 2 electrons of opposite spind can be accommodated in overlapping orbitals

• bonds formed by overlap of atomic orbitals

• resulting bond has cylindrical symmetry - no nodes around internuclear axis, sigma bond

• as you bring H atoms closer together, they start bumping into eachother and repel

valence bond theory of HF and F2

• sigma bond formed by overlap of H 1s orbital with F 2p orbital

• remaining 2s and 2p electrons on F not involved in bonding - lone pairs

• F2 forms a sigma bond by overlap of the 2 singly occupied 2p orbitals - one on each F

promotion

• put energy in to excite an electron from a lower energy orbital to a higher energy one (eg 2s to 2p)

• the energy required is more than the energy given out by the bond being formed

hybridisation

• hybrid orbitals created by mixing s, p and d orbitals

• hybridisation considered if component atomic orbitals are close in energy

• no of hybrid orbitals = no of atomic orbitals

• built by combining an s orbital with as many p/d orbitals as are necessary to accommodate the bonding electrons and lone pairs on the atom

• character of hybrid orbitals depends on the atomic orbitals involved and the percentage contribution of each atomic orbital

• hybrid orbitals more directional than atomic orbitals so give better orbital overlap - stronger bonds

resonance

• when bonding in molecules/ions cannot be represented by a single lewis structure so a combination of resonance structures make up a resonance hybrid which depicts a more accurate model of the molecule’s bonding

• guidelines:

1. charge separation should reflect relative electronegativities of atoms

2. adjacent charges should be of opposite sign

3. avoid building up large positive/negative charge on any one atom