Atoms, elements and compounds · Model
Formulae
One of these bottles is drinking water. The other bleaches hair and has been used as rocket fuel. The difference is one atom.
Start here
H2O and H2O2. One extra oxygen atom.
Both are clear, colourless liquids made of nothing but hydrogen and oxygen. One falls out of the sky and you drink it. The other burns skin, bleaches hair, and in concentrated form has been used to launch rockets.
What is actually different about them?
Nothing is dissolved in anything. Each particle of one has two hydrogen atoms and one oxygen; each particle of the other has two hydrogens and two oxygens. That single extra atom, in every particle, makes a different substance — and the formula is how you say so.
A formula lists which elements are in a substance and counts the atoms of each. The small number written low, after a symbol, counts the atoms of that element in one particle.
The model · change one number
Build a formula and see what you have made
0 of 3 real substances found
Two elements, and a count for each. Most combinations are not substances at all — which is the first thing worth knowing about formulae.
Commit first. If you change the 2 in H2O to a 3, what have you got?
Two elements
Hydrogen atoms
Oxygen atoms
H2O — water
Two hydrogen atoms and one oxygen atom in every particle. You are about 60% of this.
Key fact
A formula says which elements are present and how many atoms of each. Change one of those numbers and you have not changed the amount — you have changed the substance.
Five words
Say your answer out loud before you turn each card over. If you cannot say it, you do not know it yet.
Where the model runs out
NaCl is not a particle with one of each in it
Everything so far has treated a formula as a recipe for one particle. For salt that is not what is happening, and the model has to bend.
A molecule · CO2
Separate particles float about, each one made of exactly one carbon and two oxygens. You could, in principle, pick one out. The formula counts the atoms in that particle.
A giant structure · NaCl
A single grain of salt is billions of sodium and chlorine atoms locked in one repeating stack, with no separate particles at all. NaCl is a ratio — one sodium for every chlorine — not a count of what is in one particle.
Decide before you tap a card: what does the formula for salt tell you?
Think again
“2H2O and H2O2 both have four atoms written down, so they must be the same thing.”
One of those is a big number at the front, and one is a small number at the back. Commit before you read on.
2H2O is two particles of water. Two of the same thing you drink. H2O2 is one particle of hydrogen peroxide, which is a different substance entirely.
A big number in front multiplies whole particles and changes how much you have. A small number after a symbol counts atoms inside one particle and changes what you have. It is the difference between two glasses of water and a bottle of bleach.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
How many atoms are there altogether in one particle of H2SO4?
Rung 2 · The one that catches people
What is the difference between 2CO2 and C2O4?
Rung 3 · Explain
Explain why H2O and H2O2 are different substances rather than different amounts of the same one, and say what would happen if you tried to make water by removing an oxygen atom.
Rung 4 · Take it somewhere new
A student writes the formula for salt as Na1Cl1 and says it means one particle containing one sodium and one chlorine atom. Two things are wrong. Say what they are, and give the version a chemist would write.
Key note
A formula gives the elements in a substance and the number of atoms of each. A small number after a symbol counts atoms inside one particle; a big number in front counts particles. For a giant structure like NaCl the formula gives the ratio, because there is no single particle to count.
Going further
Glucose is C6H12O6 and ethanoic acid — vinegar — is C2H4O2. Divide both down and they reach the same ratio: one carbon, two hydrogens, one oxygen. Same proportions, wildly different substances, because what differs is how many atoms are in each particle and how they are joined. It is the first hint that a formula is not the whole story, and the reason chemists eventually needed structures as well as formulae.
Before this lesson
Next in this unit
At GCSE this becomes
- Balancing equations, and working out a formula from the masses that react.
Where to next
Ask Mr Badmus AI
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