Bonding

Bonds between Atoms

Ionic Bonds

Atoms become ions by gaining or loosing electrons.

They do so in order to achieve the stable noble gas configuration with the full component of electrons in its outer shell.

Sodium has 11 electrons, with 1 in its third outer shell. If it loses this outer electron it achieves a stable noble gas configuration. Chlorine on the other hand has 17 electrons with 7 in the outer shell. It needs to gain an electron.

The transfer of an electron from Na to Cl yields simultaneous stability to both atoms and they are both held together by the attraction of opposite charges the ionic bond.

Covalent Bond

Atoms can also attain the stable noble gas configuration by sharing electrons = the Covalent Bond.

They are then held together by the attractive force of the two nuclei to the negatively charged electrons between them.

Single	Double	Triple

	Multiple

Comparison of covalent & ionic bonds

The strengths of ionic + covalent bonds are very similar (same order of magnitude).
Main difference is the distribution of electrons.
In a purely ionic compound the electron density falls to zero between the atoms whereas it is at it's highest between the atoms in a covalent bond.

ionic	covalent

The ionic bond is formed by oppositely charged ions attracting one another.

In covalent bonds the positively charged nuclei attract the shared electrons between them.

It is unusual for a bond to be either purely ionic or purely covalent. Intermediate bond types tend to predominate.

Intermediate Bond Types

Polarisation of ions

This gives rise to ionic bonds with increasing degrees of covalent character:

Pure Ionic	Mild Polarisation	Considerable covalent character

Factors leading to polarisation and have covalent character are summarised by Fajans rules:

•  polarising effect of cation(+) increases as

· size reduced
· charge increases

•  polarisability of anion(-) increase as

· size increases
· charge increases

Polarisation of covalent bonds

Polarisation of covalent bonds can lead to some ionic character.
This polarisation occurs because atoms have different tendencies to attract electrons towards themselves (electronegativity)

The most electronegative atoms are at the top right hand side of the periodic table.

Paulings electronegativity values describe the polarising power of an atom, the higher the value - the more polarising is the atom.

Paulings Electronegativities

In F₂ where the two atoms joined by the covalent bond are the same, the electro negativity values are the same and the electrons are shared equally. However in H-Cl the bond is polarised as Cl has a greater hold on the shared electrons.

Hd +

Where d + is delta positive and means (a little bit positive) and d - is delta negative meaning (a little bit negative).

Metallic Bonds

Metallic bonds can be viewed as ions in fixed lattices with the valence electrons delocalised throughout the whole lattice.

Bonds between Molecules

These are considerably weaker than either covalent or ionic bonds but can make significant contribution to the properties of compounds.

Van der waals forces

These are very weak, and arise from induced fluctuating dipoles in atoms and molecules.
The strength increases with the size of the atom as the number of electrons able to fluctuate increases.

Dipole – Dipole interactions

No dipole	Dipole	Dipole

Dipoles occur between molecules, which are permanently polarised with the negative and of one molecule attracting the positive end of the next.

Hydrogen bonds

Hydrogen bonds occur between molecules, which contain a highly electronegative atom linked covalently to a hydrogen atom.

Hydrogen bonds are intermediate in strength between covalent bonds and Vander waals and have an appreciable effect on the physical properties of compounds.