Atoms, molecules and compounds

Atoms are the basic building blocks of elements. They are too small to be observed by the naked eye. Let's explore atoms, and molecules and compounds.

Elements are the building blocks of all matter and composed of one type of atom, such as \(\ce{He}\), \(\ce{H}\), \(\ce{C}\), \(\ce{Mg}\), and \(\ce{Fe}\). Some elements exist as molecules, such as \(\ce{S}_8\), \(\ce{O}_2\), and \(\ce{Cl}_2\).

Molecules and compounds

A molecule is a group of two or more atoms that have combined chemically and function as a unit. The simplest molecule that can exist is a diatomic molecule that contains two atoms. Two examples are \(\ce{O}_2\) and \(\ce{H}_2\). A compound contains different types of atoms in fixed proportions.

Both compounds and molecules are uncharged (neutral) species. Some examples are shown in the table.

Molecules	Compounds
\(\ce{H2}\)	\(\ce{H2O}\)
\(\ce{O3}\)	\(\ce{CO2}\)
\(\ce{S8}\)	\(\ce{C2H5OH}\)
\(\ce{CO2}\)	\(\ce{CH3COOH}\)
\(\ce{H2O}\)	\(\ce{FeSO4}\)

Chemical symbols

Each element has a unique name and chemical symbol. All of the discovered elements are organised in the modern periodic table. Some of these are shown in the table.

Element	Chemical symbol
Hydrogen	\(\ce{H}\)
Carbon	\(\ce{C}\)
Calcium	\(\ce{Ca}\)
Silver	\(\ce{Ag}\)
Copper	\(\ce{Cu}\)

Chemical formulas

Chemical formulas represent the chemical composition of molecules and compounds. They consist of the chemical symbols for the elements present in the compound and numerical subscript which shows the number of atoms of each element involved in the formation of the compound.

Consider the chemical formula for the compound sucrose, \(\ce{C12H22O11}\).

• This formula shows that sucrose contains three different elements: \(\ce{C}\), \(\ce{H}\) and \(\ce{O}\).
• The total number of atoms is \(12+22+11=45\).
• It contains \(12\textrm{ C}\) atoms, \(22\textrm{ H}\) atoms, and \(11\textrm{ O}\) atoms.

Now, consider the chemical formula for the compound calcium phosphate: \(\ce{Ca3(PO4)2}\).

• This formula shows that one unit of calcium phosphate is composed of three different elements: \(\ce{C}\), \(\ce{P}\) and \(\ce{O}\).
• The total number of atoms is \(3+2+8=13\).
• It contains \(3\textrm{ Ca}\) atoms, \(2\textrm{ P}\) atoms, and \(8\textrm{ O}\) atoms. For \(\ce{O}\) and \(\ce{P}\), the number of atoms is calculated by multiplying the subscript number by the number written after the brackets.

Your turn – atoms, molecules and compounds

Test yourself on your understanding of atoms, molecules and compounds.