0.4 Chemical bonding  (Page 3/9)

Covalent bonding and valency

1. Explain the difference between the valence electrons and the valency of an element.
   
2. Complete the table below by filling in the number of valence electrons and the valency for each of the elements shown:

   Element	No. of valence electrons	No. of electrons needed to fill outer shell	Valency
   $\mathrm{F}$
   $\mathrm{Ar}$
   $\mathrm{C}$
   $\mathrm{N}$
   $\mathrm{O}$

3. Draw simple diagrams to show how electrons are arranged in the following covalent molecules:
   1. Water ( $\mathrm{H}{}_2\mathrm{O}$ )
   2. Chlorine ( $\mathrm{Cl}{}_2$ )

Properties of covalent compounds

Covalent compounds have several properties that distinguish them from ionic compounds and metals. These properties are:

1. Melting and boiling points The melting and boiling points of covalent compounds is generally lower than that for ionic compounds.
2. Flexibility Covalent compounds are generally more flexible than ionic compounds. The molecules in covalent compounds are able to move around to some extent and can sometimes slide over each other (as is the case with graphite, this is why the lead in your pencil feels slightly slippery). In ionic compounds all the ions are tightly held in place.
3. Solubility in water Covalent compounds generally are not very soluble in water.
4. Electrical conductivity Covalent compounds generally do not conduct electricity when dissolved in water. This is because they do not dissociate as ionic compounds do.

Lewis notation and molecular structure

Although we have used diagrams to show the structure of molecules, there are other forms of notation that can be used, such as Lewis notation and Couper notation . Lewis notation uses dots and crosses to represent the valence electrons on different atoms. The chemical symbol of the element is used to represent the nucleus and the core electrons of the atom.

So, for example, a hydrogen atom would be represented like this:

A chlorine atom would look like this:

A molecule of hydrogen chloride would be shown like this:

The dot and cross in between the two atoms, represent the pair of electrons that are shared in the covalent bond.