What are the smallest constituents of matter?

Atoms are extremely small constituents of matter.
There are 118 different atoms.

Example: the iron atom 🧲
Mass: 9.3×10^-23 g
There are 10²² (10 thousand billion billion) iron atoms in 1 g of iron.

Diameter:
0.248 nm = 0.000000000248 m

A chain of iron atoms measuring 1 mm contains 4 million iron atoms.

An atom is represented by a coloured sphere.
An atom is designated by a symbol containing one uppercase letter (14 atoms) or one uppercase letter followed by a lowercase letter (104 atoms).

Name	Symbol	Model
Hydrogen atom	H
Oxygen atom	O
Carbon atom	C
Nitrogen atom	N

A molecule is made of several atoms bonded together.
A molecule is designated by a formula.

Example:
A water molecule contains two hydrogen atoms and one oxygen atom.
Its formula is: H₂O
Its model is:

The formula of a molecule contains at least one atomic symbol and often one or more numbers.

1. A symbol indicates the name of the atom.
2. A subscript number is associated with the symbol before it.
3. When there is no number, the quantity is equal to 1.

Examples:

• The ascorbic acid molecule (vitamin C), with formula C₆H₈O₆, contains:
• — 6 carbon atoms C,
• — 8 hydrogen atoms H, and
• — 6 oxygen atoms O.

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• The ethanol molecule, with formula C₂H₆O, contains:
• — 2 carbon atoms C,
• — 6 hydrogen atoms H, and
• — 1 oxygen atom O.

Remark:
In both rectangles, four oxygen atoms are represented.