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C5.1 What types of chemicals make up the atmosphere?

C5.1 What types of chemicals make up the atmosphere?

The Earth is surrounded by an atmosphere that is a mixture of gases – some of these gasses are molecular elements and some are molecular compounds.

Most non-metal elements are molecular and most of these consist of molecules with just two atoms joined together. Most compounds, between non-metal elements are also molecular.

Intermolecular forces – all of these substances have very strong covalent bonds between the atoms, but much weaker forces holding the molecules together. When one of these substance melts or boils it is these weak ‘intermolecular forces’ that break, not the strong covalent bonds.

Because the weak intermolecular forces break down easily these substances have low melting and boiling points. Molecular elements and compounds with small molecules have low melting and boiling points. This means they are gases at room temperature.

Unlike ionic compounds, pure molecular compounds do not conduct electricity because they do not have any free electrons, and the molecules do not have electric charges.

A covalent bond forms when two non-metal atoms share a pair of electrons. In covalent bonds, electrons are shared between the nuclei of two atoms – this causes a strong electrostatic attraction between the nuclei and shared electrons.

 

Covalent bonds are strong, and a lot of energy is needed to break them.

 

 

The easiest way to represent covalent bonds is by using straight lines, with each line representing a shared pair of electrons. These 2D molecular diagrams show which atoms are joined by covalent bonds, but do not show the shape of each molecule. It is possible to draw more complex 3D diagrams to show the true shape of each molecule.

Some molecules have a double covalent bond meaning they have two shared pair of electrons.

For example a molecule of oxygen (O2) consists of two oxygen atoms held together by a double bond: O=O

Others have a triple covalent bond meaning they have three shared pairs of electrons.

For example a molecule of nitrogen (N2) has two nitrogen atoms held together by a triple bond: N=N

 

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