Ions

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Ions

The electrons in the outermost shell, the one furthest from the nucleus, are called valence electrons. It is these electrons that are involved in chemical reactions. The elements in a column or 'Group' on the Periodic Table all have the same number of valence electrons and so they behave in a similar way in chemical reactions. For example, Group 1 elements all have one valence electron, and they are all highly reactive metals.

Atoms are more chemically stable if their outermost shell (also called their valence shell) is full. Atos can become stable in three ways:

By giving electrons to another atom/losing valence electrons
By taking electrons from another atom/gaining valence electrons
By sharing electrons with another atom

Atoms that have gained or lost electrons are called ions.

Ion Formation

In their element form, atoms are neutral - they have the same number of protons (positive charge) and electrons (negative charge). These equal-but-opposite charges cancel each other out.

But when an atom gains or loses electrons to form an ion, it unbalances the electrical charge of the atom.

The atom losing electrons has more positive protons than negative electrons. So atoms that lose electrons form positive ions (called cations). See the example of the Sodium ion below.
The atom gaining electrons has more negative electrons than it has positive protons. So atoms that gain electrons form negative ions (called anions). See the example of the Chloride ion below.

Charges on Ions

You can predict what type of ion an atom will form by looking at its electron arrangement. The periodic table of ions below show the trends of charges.

Those atoms with an outermost shell less than half-full, lose electrons. Atoms of metal elements and atoms of hydrogen lose electrons and form positive ions (cations).
- - Group 1 elements have one outer shell electron, which they lose to form positive ions with a net charge of 1+
  - Group 2 elements have two outer shell electrons, which they lose two electrons to form ions with a charge of 2+
Those atoms with an outermost shell more than half-full, gain electrons. Atoms of non-metals gain electrons and form negative ions (anions).
- - Group 16 elements with six outer shell electrons gain two more, to achieve a charge of 2 -
  - Group 17 elements have seven outer shell electrons that are quick to gain one more, to achieve a charge of 1 -
Atoms with half-full outermost shells tend to share electrons with other atoms. They do not form ions.
Those atoms with a full outermost shell are stable and are unreactive.
- - Group 18 elements don't gain or lose electrons; they don't form ions.
Notice that metals form positive ions called cations, while non-metals form negative ions called anions.
Transition metals are special cases that follow these trends.

Naming Ions

Ions are written as the chemical symbol of the atom with their charge shown on the top right.

Remember that electrons are negatively charged so an atom gaining electrons becomes a negative ion (an atom that loses electrons becomes positive).
The charge shows the imbalance between the number of protons and the number of electrons. A charge of + means the ion has more protons than electrons because the atom has lost electrons.
The charge of - means that the ion has more electrons than protons because the atom has gained electrons.
A number in front of the charge tells you how many electrons have been lost or gained, e.g. an oxide ion is written O²⁻ because an oxygen atom gains 2 electrons in a reaction.

Here are the common ions, their names and their symbols:

The neames of the metal ions are written as the name of the metal followed by the word ion. The same is true for hydrogen. E.g. sodium ion, magnesium ion, aluminium ion.
The names of the non-metal ions end in 'ide' replacing the normal ending of the element's name. E.g. chlorine becomes chloride, oxygen becomes oxide, fluorine becomes fluoride and sulfur becomes sulfide.
Some ions are made up fo a group of atoms with an overall charge. They are called polyatomic ions, and have different naming conventions. If the polyatomic ion contains oxygen, the names end in 'ate', with the exception of hydroxide ions.

Forming Neutral Ionic Compounds

Ionic compounds can be formed when a chemical reaction takes place. You know a chemical reaction is taking plae when you observe:

bubbling or fizzing, indicating that a gas is being produced
a change in physical appearance including colour and state
a change in smell
the production of light or sound
heat can be absorbed from the surroundings or produced and released into the surroundings causing a change in temperature

At an invisible level, electrons are on the move. Atoms that make up the reacting elements are gaining and losing electrons to form negative and positive ions. For example, the diagram shows a sodium atom losing an electron, reacting with a chlorine atom (which gains the electron) to form a sodium ion and a chloride ion.

Then, the oppositely charged Na⁺ and Cl⁻ ions are strongly attracted together to form the compound sodium chloride, held together by a strong chemical bond called an ionic bond.

Ions combine together to form compounds so that there is no excess charge. That is, the number of positive charges must equal the number of negative charges. So ionic compounds made during a chemical reaction are neutral.

Copper sulfate does not look like it has copper in it, and the salt on your table at home (sodium chloride) doesn't seem to be made of a sivery metal combined with a green gas. This is because the substances are no longer present in their element form - they have changed to form ionic compounds.

Ionic Formulae

A chemical formula is a shorthand way of writing the name of a chemical compound. It uses the symbols of the chemicals taking part in the reaction. There are two rules for working out the formulae for a simple compound:

Rule 1: If the ion charges are equal, then one of each will combine. See the example of magnesium oxide below.

Rule 2: If ion charges are unequal, then drop the number in front of the charge to the line (from superscript to subscript) and swap sides. See the example below for sodium oxide - the charges for sodium and oxide were dropped and swapped.

In some cases, you will need to:

Simplify subscripts if they have a common factor (see the example below for aluminium phosphide).
Use brackets for more than one polyatomic ion (see the example below for calcium phosphate).

In the formulae for ionic compounds, no charges are shown (because an ionic compound is neutral) and the positive ion is drawn and named first.

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