KS3 · Chemistry
Chemical formulae and symbols
H₂O and H₂O₂ look almost the same. One is water; the other is hydrogen peroxide, which is corrosive and harmful. The difference? One small number.
Chemistry · Reading formulae
Element or compound?
Pick a formula, then put it where it belongs. Read the symbols: how many different types of atom can you see?
An element has one type of atom. A compound has two or more types of atom bonded together.
Still to sort
Element (0)
Only one type of atom.
Where the line is: A small number doesn't make a compound. N₂ has two atoms, but they are both nitrogen.
Compound (0)
Two or more types of atom, chemically joined.
Where the line is: Look for a second capital letter — that's a second element.
Every symbol starts with a capital letter, so counting the capitals tells you how many elements a formula contains. The small numbers only say how many atoms — they never add a new type.
Counting the atoms
View
Explore
tap a part of a formula ↓
How many atoms of each type does each formula show? Decide, then switch to the atoms to check.
Water, methane and carbon dioxide, each drawn beside its atoms.
Chemistry · Formulae as ratios
Sodium chloride forms a giant structure, so there isn't one small NaCl unit to count. Drag to take more or fewer sodium atoms: the chlorine keeps pace, and the ratio stays 1:1. A real piece of sodium chloride has far more atoms than this — but the same ratio. KBr works the same way: one bromine for every potassium.
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
Capital letters tell you which atoms. Small numbers tell you how many. Change one small number and it's no longer the same substance.
What you need to know
- A chemical symbol is one or two letters that stand for an element, and the first letter is always a capital: Na is sodium, K is potassium, Br is bromine.
- A chemical formula shows which types of atom a substance contains, and either the number or the ratio of atoms of each.
- An element contains only one type of atom. N₂ is two nitrogen atoms bonded together; O₂ is two oxygen atoms joined together.
- A compound is two or more elements chemically joined (bonded), so it contains two or more different types of atom — for example CO, HCl and H₂O.
- A subscript counts only the atom whose symbol it follows, and a symbol with no subscript means one atom: write NO, not N₁O₁.
- Subscripts are written small and just below the line (C₂H₆, not C²H⁶). Raised numbers are used in maths for squaring, not in formulae.
- In a giant structure the formula gives a ratio: NaCl means one chlorine atom for every sodium atom.
- Compounds can contain the same elements in different ratios and still be different substances with different properties — like water (H₂O) and hydrogen peroxide (H₂O₂).
The big picture
A chemical formula tells you what a substance is made of. The symbols say which types of atom are there; each small number (subscript) counts only the atom just before it, and no number means one atom. One type of atom makes an element, like N₂; two or more types bonded together make a compound, like H₂O. For a giant structure such as NaCl, the formula gives a ratio of atoms rather than a count. The same elements in a different ratio make a different compound with different properties.
Key points
Worked example
Problem
A molecule of ammonia is made of one nitrogen atom and three hydrogen atoms. Write the formula for ammonia, then say whether ammonia is an element or a compound.
⚠ Watch out
Letting a subscript count the wrong atoms. In CO₂ the 2 counts only the oxygen — it does not double the carbon, and it does not double the whole formula. Read each number with the one symbol just before it.
Memory hook
Capitals name the atoms; the little number underneath counts the one in front. No number? Just one.
Check yourself
Look at N₂O. Which elements does it contain, how many atoms of each, and is it an element or a compound? Say the ratio of nitrogen to oxygen out loud before you check.
Flashcards
(14)What is a chemical symbol?
Which elements are Na, K and Br?
What does a chemical formula tell you?
What makes a substance an element?
What makes a substance a compound?
Which atoms does a subscript count?
A symbol in a formula has no number after it. How many of that atom are there?
Where do the numbers in a formula go?
What does NaCl tell you about sodium chloride?
How do you turn atom counts into a ratio?
Copper sulfate contains one copper, one sulfur and four oxygen. How many elements, and what ratio?
H₂O and H₂O₂: same substance or not?
Name three different compounds made from just nitrogen and oxygen.
Ethanol is C₂H₆O and ethanoic acid is C₂H₄O₂. How do their properties compare?
Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.
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