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Once there are two or more atoms joined together, the group then becomes a molecule and Figure 3.2a shows a complete molecule of hydrogen, sharing the electrons.

You will notice that hydrogen only needs one more atom like itself to become stable. Oxygen, on the other hand, needs two hydrogen atoms to become a stable molecule of water (H₂O). See Figure 3.2b. If we give the carbon atom four hydrogen atoms it will become a stable molecule of methane, (CH₄) – Figure 3.2c, which is a gas; if we give the carbon atom two oxygen atoms (remember that oxygen has 2 arms) it will become a molecule of carbon dioxide (CO₂), also a gas – the one we breathe out (Figure 3.2d).

FIGURE 3.2: a) hydrogen molecule; b) water molecule; c) methane molecule; d) carbon dioxide molecule

The bonds making up the water and the methane molecules are called ‘single bonds’, those making the carbon dioxide molecule are called ‘double bonds’, because there are two parallel bonds. Double bonds give a molecule a certain amount of rigidity, but they can separate fairly easily to provide an opportunity for other atoms to join in and share electrons as we shall see later on.

Now it begins to get interesting! Carbon atoms have a special ability to keep joining with other carbon atoms to form long straight or branched chains. Each time a carbon atom (with two hydrogen atoms) joins the chain, the molecule so formed is bigger and heavier than the one preceding it (see Figure 3.3).

We are nearly there!

FIGURE 3.3: The chain increases by the addition of CH2 each time