Bonding

Covalent structures

Covalent structures

In a covalent bond, two atoms share a pair of electrons. A molecular orbital is formed by the overlap of the two atomic orbitals. Only two electrons can be contained within a molecular orbital, as with an atomic orbital. This means that overlap is only possible if both atomic orbitals contain only one electron each (a normal covalent bond) or if one orbital contains two and the other none (a dative covalent bond).

Normal and dative covalent bonds

The number of unpaired electrons within an atom dictates how many normal covalent bonds are possible.

An atom which has full valence shell orbitals is able to give the pair of electrons needed for a dative covalent bond. This includes elements from groups V, VI and VII and 0. An atom with empty valence shell orbitals can accept the electrons. This includes elements from groups I, II and III.

Sigma and pi bonds

It is possible for atomic orbitals to overlap in two ways.

The most common way is along the internuclear axis. This create a s-bond. Any single bond between two atoms is a s-bond.

Only one s-bond is possible between two atoms. This is because otherwise there would be too many electrons present in one s,mall space, resulting in repulsion.

If two orbitals overlap, behind and in front or above and below, the internuclear axis, a ?-bond is formed. They are only formed by p-orbitals overlapping, not s-orbitals.

In a double bond, you will always find a s-bond and a ?-bond. A triple bond consists of one s-bond and two ?-bonds.

Strength of covalent bonds

In general, covalent bonds are strong. The smaller the atoms are the stronger the bond as the electrons are closer to the nuclei

Molecular, giant covalent and layered substances

Covalent bonding can create three different substances:

  • molecular
  • giant covalent
  • giant covalent layered

Molecular

A lot of the time no more bonds are possible beyond a few atoms, forming a molecule. A molecular substance, like iodine, is formed of a large mount of molecules or discreet units. These molecules are held together by intermolecular forces which, although not as strong as covalent bonds, can keep the substance in a liquid or solid state.

As a gas, the intermolecular forces are broken but the bonds within the molecule are not.

Characteristic properties include:

  • Melting and boiling points: the melting and boiling points tend to be low as intermolecular bonding is weak. As intermolecular forces quickly decrease with distance there is not much difference between the two points.
  • Electrical conductivity: there is hardly any electrical conductivity as there are no delocalised electrons or ions present either in a liquid or solid state.
  • Consistency: in general, molecular covalent substances are soft and crumbly due to their weak intermolecular and non-directional bonding.

Giant covalent

Bonding capacity does not always end at a molecule and can lead to the formation of a lattice. In this structure there are no discrete molecules and between adjacent atoms exists covalent bonding. These are known as giant covalent substances and a prime example is the allotrope of carbon, diamond.

Characteristic properties include:

  • Melting and boiling points: before such a substance can melt or boil the covalent bonds must be broken. This means that their melting and boiling points are usually very high. They depend on how many bonds are present and the strength of the bonds. The difference between the melting and boiling points is low, however, as once broken covalent bonds are completely broken due to the fact that they are very directional.
  • Electrical conductivity: there is hardly any electrical conductivity as there are no delocalised electrons or ions present either in a liquid or solid state.
  • Consistency: due to the strength of the covalent bonding these substances are hard and brittle. In fact, diamond is the hardest substance known to man and is used for applications like drills.

Giant covalent layered

Substances like graphite are composed of an infinite lattice of atoms bonded covalently in two dimensions, creating layers. The layers are attracted to one another by intermolecular forces and there tends to be delocalised electrons between them.

Characteristic properties include:

  • Electrical conductivity: because of the delocalised electrons, substances like graphite are very good conductors in the x and y directions, even when solid. However, there is limited electrical conductivity in the z direction (perpendicular to the planes) as delocalisation only occurs in two dimensions.
  • Density: the density is lower than in giant covalent structures because of the relatively big distance between planes.
  • Hardness: as the planes are able to slide over each other quite easily substances like graphite are a lot softer than diamond. Hence graphite is used for applications like industrial lubricant.