The Formation of Ions

All chemical reactions involve atoms trying to get a full outer shell of electrons.

When an atom reacts with another atom, it will either
1. Lose electrons to form a stable positive ion.
2. Gain electrons to form a stable negative ion.
or  3. Share electrons to form a stable molecule.

For example: A sodium atom becomes a sodium ion.

Sodium has 1 electron in its outer shell.
It is in group 1 of the periodic table.

When sodium reacts, it will lose its outer electron.
Its outer shell will then have no electrons,
it is as though the outer shell has vanished!
The next shell in is full.
This full shell becomes the new outer shell:- Sodium is now stable.

The sodium ion still has 11 protons (11 positive charges), but now only 10 electrons (10 negative charges).
The sodium ion has an extra positive charge,
shown by the + sign.

The Formation of an Ionic Bond

The reaction between sodium and chlorine.

Chlorine has 7 electrons in its outer shell.
It is in group 7 of the periodic table.

When an atom of chlorine reacts,
it will gain one electron (from sodium).
The outer shell of chlorine will then have 8 electrons and be stable.
The stable chloride ion will have an extra electron,
and so will have an extra negative charge (shown as a - sign).

The force of attraction between the oppositely charged ions
is called an ionic bond.

Charges, Valance, and Oxidation Numbers.

When a metal reacts with a non-metal,
the metal will lose electrons to form a positively charge particle called positive ion aka cation.

The non-metal will gain electrons to form a negatively charged particle called a  negative ion aka anion.

Oxidation Number: the number of electrons, which a metal will lose, is the same as its group number in the periodic table. A metal in group 1 will form a 1⁺ ion. (Valence 1, oxidation number1⁺). A metal in group 2 will form a 2⁺ ion. (Valence 2, oxidation number 2⁺).

The number of electrons, which a non-metal will gain,
will be 8 minus its group number.

A non-metal in group 6 will form a 2^- ion.(Valence 2, oxidation state 2^-).

A non-metal in group 7 will form a 1^- ion.(Valence 1, oxidation state 1^-).

A non-metal ion is negative because it has gained electrons.