Understanding Ions

Typically, an atom will have the same number of electrons as protons. Because the number of positively charged particles (protons) is the same as the number of negatively charged particles (electrons), the overall charge of the atom is zero. Atoms in this state are neutral.

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However, it is possible for atoms to gain or lose electrons, giving them a charge. These charged atoms are called ions.

Remember that an atom’s electrons are organised into electron shells, filling from the innermost shell to the outermost shell. This is shown in the table below:

The outermost shell is only completely full for Group 0 elements (the noble gases). For all other elements, the outermost shell is not full. For example, consider sodium and chlorine:

Atoms with incomplete electron shells are less stable. They become more stable when they have full electron shells, meaning they are prone to losing or gaining electrons.

Because electrons have a negative charge, this means that:

• When a neutral atom gains one or more electrons, it forms a negatively charged ion, known as an anion.
  
• When an atom loses one or more electrons, it forms a positively charged ion, known as a cation.

Typically, positively charged ions will have lost their outermost electron shell completely. This makes the next-outermost shell the new outermost shell, which is already full of electrons.

It is important to keep in mind that for the formation of ions, only the loss or gain of electrons is considered. Protons are never gained or lost to become more electrically stable, as this would result in the formation of a new element.