C1 E) Compounds
Compounds come about due to a reaction between the elements. For example, iron can react with sulfur to produce iron sulphide. The word and chemical equation for this is shown below:
The properties of the compounds produced by chemical reactions are usually very different to the properties of the elements/ reactants that were involved in the reaction. For our reaction, the iron is shiny and the sulfur is yellow. The iron sulphide produced by the reaction is a dull grey substance, which is different from the characteristics of iron and sulfur seperately.
There is often a detectable energy change when a reaction takes place – either energy can be released or taken in during a reaction. Reactions that release energy are known as exothermic reactions, and reactions that take in energy are known as endothermic reactions (we will be looking at exothermic and endothermic reactions in more detail later on in this GCSE science course).
The only way that we can separate the elements in a compound is by undertaking a chemical reaction, which can be rather tricky. For example, if we wanted to separate iron sulphide (FeS) back into iron and sulfur, we would need to undertake a chemical reaction.
Earlier I mentioned that the elements/ atoms in compounds are joined together by chemical bonds. These chemical bonds are known as ionic or covalent bonds. We are going to be looking at ionic and covalent bonds in more detail in later sections, but here is a quick summary of them.
Ionic Bonding
Ionic bonding involves a metal and a non-metal. The metal gives electrons to the non-metal. This results in the metal becoming a positive ion (it has lost electrons) and the non-metal becoming a negative ion (it has gained electrons). There is a very strong force of attraction between the oppositely charged positive metal ion and the negative non-metal ion, which results in the metal and non-metal being bonded together – they are bonded together because the oppositely charged metal and non-metal are attracted to each other. Some examples of ionically bonded substances are sodium chloride (NaCl) and magnesium oxide (MgO).
Covalent Bonding
Covalent bonding involves non-metals sharing pairs of electrons. Examples of covalently bonded compounds are carbon dioxide (CO2) and methane (CH4).
Molecules
We represent compounds by using formulas. The subscript numbers in the formulas tell us the number of that element in the compound (subscript numbers are numbers below the letters).
For example, water has the formula H2O. Every water molecule will contain 2 hydrogen atoms and 1 oxygen atom.
Another example is methane, which has the formula CH4. All methane molecules will contain 1 carbon atom and 4 hydrogen atoms.
Sometimes molecules will have brackets involved. The subscript outside the bracket tells us the number of times that we have the bracket. For example, copper nitrate has the formula Cu(NO3)2. The 2 outside the bracket tells us that there is 2 lots of everything inside the bracket. In copper nitrate, there is 1 copper atom, 2 nitrogen atoms and 6 oxygen atoms.