Coaching Centre
Compounds and chemical formulas
What a compound is, how compounds differ from the elements they contain, molecular and lattice compounds, reading chemical formulas, decomposition, and common compounds such as water, carbon dioxide, sodium chloride and iron oxide.
Sodium is a soft metal that bursts into flame when it touches water. Chlorine is a poisonous green gas that was used as a weapon in World War I. Join them together and you get sodium chloride: the salt you sprinkle on your chips.
What is a compound?
There are only 118 elements, but millions of different substances. Most substances are compounds: pure substances made of atoms of two or more different elements chemically bonded together in a fixed ratio.
A compound usually has completely different properties from the elements it is made of, as table salt shows. Water is another example: hydrogen is an explosive gas, oxygen is a gas that makes things burn, yet water is a liquid that puts out fires.
Because the atoms are bonded, a compound can't be separated by physical methods such as filtering or evaporating. It can only be broken down by a chemical reaction. Breaking a compound into simpler substances is called decomposition. When green copper carbonate is heated, for example, it decomposes into black copper oxide and carbon dioxide gas.
Reading chemical formulas
A chemical formula uses element symbols to show which elements are in a compound and how many atoms of each. A small number written below the line (a subscript) tells you how many atoms of the element just before it. No number means one.
| Formula | Name | Elements | Atoms in one molecule (or ratio) |
|---|---|---|---|
| H2O | Water | hydrogen, oxygen | 2 H + 1 O = 3 atoms |
| CO2 | Carbon dioxide | carbon, oxygen | 1 C + 2 O = 3 atoms |
| CH4 | Methane (natural gas) | carbon, hydrogen | 1 C + 4 H = 5 atoms |
| NH3 | Ammonia | nitrogen, hydrogen | 1 N + 3 H = 4 atoms |
| NaCl | Sodium chloride (table salt) | sodium, chlorine | 1 Na for every 1 Cl |
| NaHCO3 | Sodium bicarbonate (baking soda) | sodium, hydrogen, carbon, oxygen | 1 Na, 1 H, 1 C, 3 O |
| C6H12O6 | Glucose | carbon, hydrogen, oxygen | 6 C + 12 H + 6 O = 24 atoms |
A quick check: count the capital letters to find how many different elements there are. NaHCO3 has four capitals, so four elements.
Ethanol, the alcohol in drinks and hand sanitiser, has the formula C2H6O. Is it an element or a compound? How many atoms are in one molecule?
Three capital letters: C, H and O. More than one element bonded together, so it is a compound.
2 carbon + 6 hydrogen + 1 oxygen = 9 atoms in each molecule.
Molecules and lattices
Some compounds are made of separate molecules, each with exactly the same small group of atoms. Every water molecule is H2O; every carbon dioxide molecule is CO2. Very long molecules made of a pattern repeated over and over, like beads on a necklace, are called polymers: plastics, starch, proteins and DNA are all polymers.
Other compounds, especially those made from a metal and a non-metal, form a huge 3D lattice instead. There is no single "molecule" of salt, just billions of sodium and chloride particles in a repeating pattern. For a lattice, the formula shows the ratio of the elements.
- Start with oxygen and add two hydrogens. What have you made?
- Build methane (CH4) and ammonia (NH3). Why do carbon and nitrogen need different numbers of hydrogens?
- Start with carbon and add two oxygens. Some bonds are left unfilled. Click each C–O bond to make double bonds. What is the molecule?
- Build O2. Why is it called a molecule but not a compound?
- Challenge: build ethanol, C2H6O.
Some common compounds
- Water (H2O) is the most common compound on Earth's surface. So many substances dissolve in it that it is called the universal solvent.
- Carbon dioxide (CO2) is breathed out by living things and used by plants in photosynthesis. It traps heat in the atmosphere. Fire can't burn in it, so it's used in some fire extinguishers, and it puts the fizz in soft drinks.
- Iron oxide is rust, formed when iron reacts with oxygen and water. Iron oxides also give Central Australia's sand and rocks their red colour.
- Sodium chloride (NaCl) is table salt, essential in small amounts for our nerves and muscles.
Test yourself
How many atoms are in one molecule of methane, CH4?
1 carbon + 4 hydrogen = 5.
Which of these is a compound?
H₂O contains two different elements bonded together.
How many different elements are in baking soda, NaHCO3?
Count the capitals: Na, H, C, O.
A scientist can't break a new substance down into anything simpler, even with chemical reactions. It is most likely
Elements are the simplest substances.
Breaking a compound down into simpler substances is called
It needs a chemical reaction, often using heat or electricity.
Why is there no "molecule" of sodium chloride?
Lattice compounds have no separate molecules.
Element or compound?
A compound contains two or more elements chemically together. The small number in a formula is called a . In H2O there are hydrogen atoms. Very long molecules with repeating units are called . Salt forms a 3D .
Methane has the formula CH4. (a) Is methane an element or a compound? Justify your answer. (b) What does the formula tell you about one molecule of methane?
(a) Methane is a compound. Its formula has two different symbols, C and H, so it contains two different elements, carbon and hydrogen, that are chemically bonded together.
(b) The formula shows that each methane molecule contains one carbon atom (no subscript means 1) and four hydrogen atoms (subscript 4), making 5 atoms altogether. Every methane molecule has exactly this ratio of 1 carbon to 4 hydrogen.