Covalent Bonding This occurs when non metal atoms bond together. They share pairs of electrons to give oneanother complete outer shells. Here covalent molecules are formed. The simplest example is hydrogen. It has only one electron and the first shell can only hold two. H H Two separate hydrogen atoms Combine to give H H A hydrogen molecule, H 2

Combine hydrogen and chlorine covalently Hydrogen and chlorine are both non metals. They are both composed of diatomic molecules. They combine as follows: H 2(g) + Cl 2(g) > 2HCl (g) A hydrogen chloride molecule H Cl x H + x This is called a “dot and cross” diagram. It is used to show clearly which electrons come from which element. Do both elements have a Noble gas configuration in the product molecule?

Combine hydrogen and oxygen covalently If hydrogen and oxygen combine in this way, how many atoms of each element do we need? Work it out using a diagram. H OH A water molecule H O produces H We often represent covalent structures as “stick” diagrams or more correctly as structural formulae. Each shared pair of electrons is known as a single covalent bond. It is represented as a single straight line in a structural formula. O H H

Combine hydrogen and carbon How many hydrogen atoms do we need to give carbon a Noble gas structure in a simple hydrocarbon? H C In this space draw the complete dot and cross diagram, the structural formula, the chemical formula and the name of the compound H CH C H H H H H H CH 4 Methane

Double covalent bonds Some covalent molecules seem to have too few atoms to work with single covalent bonds. They have four shared electrons in some of their bonds. There are two well known examples, the alkenes and carbon dioxide. In the space below, draw the dot and cross diagrams and structural formulae of Ethene (C 2 H 4 ) and Carbon dioxide (CO 2 ) C C H H H H O C O C C H H H H O C O Are these molecules saturated or unsaturated?

The properties of simple covalent materials By simple molecules, we mean small molecules which are not linked up into giant structures. Examples are CO 2, H 2 O, CH 4, H 2. Compared to ionic compounds (which are giant structures), do they have high or low melting and boiling points? NaCl m.pt. = 801 o C CH 4 m.pt. = o C LiF m.pt = 876 o CH 2 m.pt. = -259 o C Why is this? Is it because covalent bonds are weak and easier to break than ionic bonds? No!!!. The covalent bonds inside the simple molecules are strong and are not broken when they are melted or even boiled. What does break are weak intermolecular bonds called Van der Waals bonds C H H H H C H H H H C H H H H Because of their low m.pts and b.pts., many simple covalent materials are gases and liquids at room temperature.

Are simple covalent materials electrical conductors? They are insulators. They never conduct, not as solids or as liquids. Why is this? They contain no charged particles which are free to move. All of their electrons are either in bonds or part of the inner shells of atoms.