Symbols and formulas

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Why

Chemists do not write “water consists of hydrogen and oxygen”, they write H₂O. Symbols and formulas are a shorthand that is read the same way all over the world.

Rule

Compound: A compound consists of atoms of different elements joined in a fixed ratio, for example water or carbon dioxide.

Molecule: A molecule is a small particle made of several joined atoms, for example a water molecule.

Formula: A formula uses element symbols to show which atoms a substance is made of. A small subscript number, the index, tells how many atoms there are.

The formula of water is $H_{2} O$ : one molecule has 2 hydrogen atoms (H) and 1 oxygen atom (O). If there is no index, it is 1 atom.

Some formulas and their atoms
Substance	Formula	Atoms in one particle
water	$H_{2} O$	2 H, 1 O
carbon dioxide	$CO_{2}$	1 C, 2 O
methane	$CH_{4}$	1 C, 4 H
ammonia	$NH_{3}$	1 N, 3 H
oxygen	$O_{2}$	2 O

Example

Reading the formula of methane

Methane has the formula $CH_{4}$ . C stands for carbon, H for hydrogen. The 4 says: for 1 carbon atom there are 4 hydrogen atoms. A methane molecule has 5 atoms in total.

Practice

Exercise

Three worked tasks, each with its solution:

Task no. 1
Match each element with its symbol.
Hint
The symbol has one or two letters and is in the table of the periodic table.
Solution
1. Every element has exactly one symbol.
Right answer: Bismuth – Bi; Astatine – At; Rubidium – Rb; Radium – Ra
Task no. 2
Match each element with its symbol.
Hint
The symbol has one or two letters and is in the table of the periodic table.
Solution
1. Every element has exactly one symbol.
Right answer: Lead – Pb; Strontium – Sr; Calcium – Ca; Polonium – Po
Task no. 3
Match each element with its symbol.
Hint
The symbol has one or two letters and is in the table of the periodic table.
Solution
1. Every element has exactly one symbol.
Right answer: Phosphorus – P; Fluorine – F; Selenium – Se; Tellurium – Te

Exercise

Solution

Right answer: 3

The index 3 is after the H. It applies only to the H, so there are 3 hydrogen atoms and 1 nitrogen atom.