Elements, Ions and Compounds

All substances are made from atoms of elements.

Molecules are made from two or more atoms.

• If the atoms in the molecule are the same, then the molecule is that of an element.

• If the atoms are different, then the molecule is a compound.

Ions can be formed from metals, non-metals, or combinations of elements.

Ions can be:

• positively charged – they have lost one or more electrons and have more protons than electrons, which is why they have a positive charge

• negatively charged – they have gained one or more electrons and have more electrons than protons, which is why they have a negative charge.

Compounds are made from atoms of two or more elements and can include metals and non-metals (ionic compounds) or just non-metals (molecular compounds).

While ions are all charged, molecules can have:

• no charge – non-polar

• or a slight charge – polar.

The diagram below summarises some of the differences.

Particles with a charge (ions or polar molecules) have different properties from molecules with no charge (non-polar).

• Ions and polar compounds attract oppositely charged particles and play important roles in the structure of molecules.

• Non-polar compounds do not dissolve in water but will dissolve in lipids (fats/oils) – they are said to be lipid-soluble.

Elements in living organisms

The six most common elements, accounting for 99% of the mass of the human body are shown in the table below.

The remaining 1% of the mass of the human body is made of about 10 other elements, mainly:

Ions

Most of these key elements are found as inorganic ions.