Compounds and Chemical Formulae Revision Notes | GCSE Edexcel Higher Chemistry

1. What Are Compounds?

When you drink water, you're consuming a compound made from hydrogen and oxygen. But here's the thing - water behaves completely differently from the explosive hydrogen gas and the oxygen that keeps fires burning.

Think of compounds like a recipe. Just as you can't separate the flour from a baked cake by picking it apart, you can't separate the elements in a compound by physical methods. You need a chemical reaction - like using electricity to split water back into hydrogen and oxygen.

The key word here is 'fixed proportions'. Water is always made from exactly 2 hydrogen atoms and 1 oxygen atom. Never 3 and 1, never 1 and 2. This ratio is locked in.

2. Chemical Formulae - The Language of Compounds

Chemical formulae are like a shorthand code that tells us exactly what's in a compound. Instead of writing 'a compound containing 2 atoms of hydrogen and 1 atom of oxygen', we write $H_2O$.

The subscript numbers (the small numbers after the element symbols) tell us how many atoms of each element are present. If there's no number, it means there's just 1 atom of that element.

Worked example: Reading Chemical Formulae

What does the formula $CaCl_2$ tell us about calcium chloride?

1. Ca = calcium, Cl = chlorine — Use the periodic table to identify element symbols
2. Ca has no subscript = 1 atom Cl has subscript 2 = 2 atoms — No subscript means 1 atom
3. 1 calcium atom + 2 chlorine atoms — This gives the exact ratio in the compound

Answer: Calcium chloride contains 1 calcium atom and 2 chlorine atoms in fixed proportions

Let's look at some common examples. Methane ($CH_4$) has 1 carbon atom and 4 hydrogen atoms. Carbon dioxide ($CO_2$) has 1 carbon atom and 2 oxygen atoms. Notice how the subscript applies only to the element symbol immediately before it.

3. Naming Compounds from Formulae

There's a system to naming compounds that makes it logical once you know the rules. For simple compounds containing just two elements, we use the element names with a specific pattern.

The first element keeps its normal name. The second element gets an '-ide' ending. So $NaCl$ becomes sodium chloride, $MgO$ becomes magnesium oxide, and $CaF_2$ becomes calcium fluoride.

Worked example: Naming Binary Compounds

Name the compound $K_2S$

1. K = potassium — The first element keeps its normal name
2. S = sulfur — We'll modify this name in the next step
3. sulfur → sulfide — Second element gets '-ide' ending
4. potassium sulfide — First element name + second element with '-ide'

Answer: Potassium sulfide

Some compounds contain groups of atoms that act as a unit. These are called polyatomic ions. Common ones include sulfate ($SO_4^{2-}$), nitrate ($NO_3^-$), and carbonate ($CO_3^{2-}$). When naming these, we keep the group name intact.

4. How Compounds Form

Compounds don't just appear - they form through chemical reactions. When elements react, they rearrange their electrons to become more stable, creating chemical bonds that hold the atoms together.

During compound formation, there's always an energy change. Sometimes energy is released (like when hydrogen burns in oxygen to make water), sometimes energy must be supplied (like when we use electricity to split water back into hydrogen and oxygen).

The crucial point is that chemical reactions always involve forming new substances with different properties. When sodium (a metal that explodes in water) reacts with chlorine (a poisonous gas), they form sodium chloride - table salt that's safe to eat.

5. Exam Technique and Common Mistakes

Exam questions on compounds often test whether you can distinguish between elements, compounds, and mixtures. Remember the key differences: elements contain only one type of atom, compounds contain different elements chemically bonded in fixed ratios, and mixtures contain substances that aren't chemically bonded.

Watch out for these common traps: confusing the number of atoms with the number of molecules, forgetting that no subscript means 1 atom, and mixing up compound names with their constituent elements.

Exam precision note: In Compounds and Chemical Formulae - Atoms, elements and compounds - A simple model of the atom, symbols, relative atomic mass, electronic charge and isotopes, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.

Naming from a symbol equation uses the same logic as naming from a standalone formula. For example, in 2Na + Cl2 -> 2NaCl, the product formula is NaCl, so the compound is sodium chloride. In Mg + O2 -> MgO, the product is magnesium oxide. Students should practise spotting the product, reading the formula correctly, and then applying the naming rule: the first element keeps its name and the second usually ends in -ide in simple binary compounds.