chem1602 A - lecture 2

• The potential energy of two atoms varies with the distance between them. A pair of atoms are considered to have a chemical bond between them when they lie in the 'energy well', i.e. at a lower energy.

                         THE LEWIS MODEL:

Covalent bonds occur when valence electrons are SHARED between two atoms. Maximum stability is reached when each atom has a 'filled valence shell'.

• OCTET RULE: each atom shares electrons with neighbouring atoms to achieve a total of 8 valence electrons (an 'octet').
• This model explains why some bonds are shorter - double bond is 2 shared electrons, triple bond is 3 shared electrons.
• CHARGED MOLECULES/ ION: molecules can accept/lose electrons to fill valence shell.
• RESONANCE HYBRIDS = if it is possible to draw more than one structure, the true structure is an average, 'resonance hybrid',
• DOESN'T EXPLAIN HYPERVALENCY: require more than 8 valence electrons.
• DOESN'T EXPLAIN WHY OXYGEN IS PARAMAGNETIC: unexplained unpaired electrons.

Valence Shell Electron Pair Repulsion Theory

• The electrons in bonds and lone-pairs around an atom can be considered as 'charge clouds' which REPEL each other.
• THE LOWEST ENERGY arrangement is when they are AS FAR APART AS POSSIBLE.
• Lone pairs REPEL MORE than bond pairs
• MULTIPLE BONDS are treated as a SINGLE ELECTRON PAIR - therefore we do not have to worry about any resonance structures.
• This theory also explains why molecules like nucleic acids (e.g. guanine) are PLANAR
• IF A LONE PAIR has a CHOICE between an equatorial position or an axial position, it will occcupy the EQUATORIAL SITE. This is because in the equatorial position it is repelled less by the two axial bonding pairs than it would be by the three equatorial bonding pairs if it was in the axial position.