Ionic bonding

Ions are electrically charged particles formed when atoms lose or gainelectrons. They have the same electronic structures as noble gases.

Metal atoms form positive ions, while non-metal atoms form negative ions. The strong electrostatic forces of attraction between oppositely charged ions are called ionic bonds.

Ions

How ions form

Ions are electrically charged particles formed when atoms lose or gain electrons. This loss or gain leaves a complete highest energy level, so the electronic structure of an ion is the same as that of a noble gas - such as a helium, neon or argon.

Metal atoms and non-metal atoms go in opposite directions when they ionise:

• Metal atoms lose the electron, or electrons, in their highest energy level andbecome positively charged ions
• Non-metal atoms gain an electron, or electrons, from another atom tobecome negatively charged ions

Positively charged sodium and aluminium ions

Negatively charged oxide and chloride ions

How many charges?

There is a quick way to work out what the charge on an ion should be:

• The number of charges on an ion formed by a metal is equal to the group number of the metal
• The number of charges on an ion formed by a non-metal is equal to the group number minus eight
• Hydrogen forms H⁺ ions.

 

	Group 1	Group 2	Group 3	Group 4	Group 5	Group 6	Group 7	Group 0
Example element	Na	Mg	Al	C	N	O	Cl	He
Charge	1+	2+	3+	Note 1	3-	2-	1-	Note 2
Symbol of ion	Na+	Mg2+	Al3+	Note 1	N3-	O2-	Cl-	Note 2

Note 1: carbon and silicon in Group 4 usually form covalent bonds by sharing electrons.

Note 2: the elements in Group 0 do not react with other elements to form ions.

Representing positive ions

You need to be able to show the electronic structure of some common metal ions, using diagrams like these:

Lithium, Li

Lithium is in Group 1. It has one electron in its highest energy level. When this electron is lost, a lithium ion Li⁺ is formed.

Sodium, Na

Sodium is also in Group 1. It has one electron in its highest energy level. When this electron is lost, a sodium ion Na⁺ is formed.

Neon atom

Note that a sodium ion has the same electronic structure as a neon atom (Ne).

But be careful - a sodium ion is not a neon atom. This is because the nucleus of a sodium ion is the nucleus of a sodium atom and has 11 protons - but the nucleus of a neon atom has only 10.

Magnesium, Mg

Magnesium is in Group 2. It has two electrons in its highest energy level. When these electrons are lost, a magnesium ion Mg²⁺ is formed.

A magnesium ion has the same electronic structure as a neon atom (Ne).

Calcium, Ca

Calcium is also in Group 2. It has two electrons in its highest energy level. When these electrons are lost, a calcium ion Ca²⁺ is formed.

A calcium ion has the same electronic structure as an argon atom (Ar).

Representing negative ions

You need to be able to show the electronic structure of some common non-metal ions, using diagrams like these:

Fluorine, F

Fluorine is in Group 7. It has seven electrons in its highest energy level. It gains an electron from another atom in reactions, forming a fluoride ion, F^-.

Note that the atom is called fluorine, but the ion is called fluoride.

Neon atom

Note that a fluoride ion has the same electronic structure as a neon atom (Ne).

Once again, a fluoride ion is not a neon atom, because thenucleus of a fluoride ion is the nucleus of a fluorine atom, with 9 protons, and not of a neon atom, with 10.

Chlorine, Cl

Chlorine is in Group 7. It has seven electrons in its highest energy level. It gains an electron from another atom in reactions, forming a chloride ion, Cl^-.

Oxygen, O

Oxygen is in Group 6. It has six electrons in its highest energy level. It gains two electrons from one or two other atoms in reactions, forming an oxide ion, O^2-.

When metals react with non-metals, electrons are transferred from the metal atoms to the non-metal atoms, forming ions. The resulting compound is called anionic compound.

Consider reactions between metals and non-metals, for example:

• sodium + chlorine → sodium chloride
• magnesium + oxygen → magnesium oxide
• calcium + chlorine → calcium chloride

In each of these reactions, the metal atoms give electrons to the non-metal atoms. The metal atoms become positive ions and the non-metal atoms become negative ions.

There is a strong electrostatic force of attraction between these oppositely charged ions, called an ionic bond. The animation shows ionic bonds being formed in sodium chloride, magnesium oxide and calcium chloride.

Group 1 and Group 7

The elements in Group 1 of the Periodic Table are called the alkali metals. They form ionic compounds when they react with non-metals. Their ions have a single positive charge. For example, sodium forms sodium ions, Na+.

The elements in Group 7 of the Periodic Table are called the halogens. They form ionic compounds when they react with metals. Their ions have a single negative charge. For example, chlorine forms chloride ions, Cl–.

Sodium chloride

Sodium chloride, NaCl, forms when sodium and chlorine react together. It contains oppositely charged ions held together by strongelectrostatic forces of attraction – the ionic bonds. The ions form a regular lattice in which the ionic bonds act in all directions.

Dot-and-cross diagrams

You need to be able to draw dot-and-cross diagrams to show the ions in some common ionic compounds.

Sodium chloride, NaCl

Sodium ions have the formula Na⁺, while chloride ions have the formula Cl^-. You need to show one sodium ion and one chloride ion. In the exam, make sure the dots and crosses are clear, but do not worry about colouring them.

Magnesium oxide, MgO

Magnesium ions have the formula Mg²⁺, while oxide ions have the formula O^2-. You need to show one magnesium ion and one oxide ion.

Calcium chloride, CaCl₂

Calcium ions have the formula Ca²⁺. Chloride ions have the formula Cl^-.

You need to show two chloride ions, because two chloride ions are needed to balance the charge on a calcium ion.

Formulae of ionic compounds

Ionic compounds are represented by formulae. Symbols and numbers show the atoms in the compound.

For example: ZnCO₃ is the formula for zinc carbonate.

One zinc atom (Zn) and one carbon atom (C) are chemically bonded with three oxygen atoms (O₃). Notice that we don't need to write a 1 next to the Zn or C.

Brackets in formulae

Sometimes brackets are used.

For example: Fe(OH)₃ is the formula for iron(III) hydroxide.

Iron(III) hydroxide consists of one iron atom joined with three oxygen and three hydrogen atoms. The formula is written like this because the oxygen and hydrogen atom often act together.

Constructing formulae

The formula of a compound can be worked out if the ions in it are known. For example, the compound formed from Na⁺ and SO₄^2- will consist of two Na⁺ ions to every one SO₄^2- ion so that the compound is neutral overall. The formula is therefore Na₂SO₄.

Here are the formulae of some common ions.

Positive ions (cations)

Name	Formula	Name	Formula	Name	Formula
ammonium	NH4+	magnesium	Mg2+	zinc	Zn2+
hydrogen	H+	calcium	Ca2+	lead	Pb2+
lithium	Li+	barium	Ba2+	iron(II)	Fe2+
sodium	Na+	silver	Ag+	iron(III)	Fe3+
potassium	K+	copper(II)	Cu2+	aluminium	Al3+

Negative ions (anions)

Name	Formula	Name	Formula	Name	Formula
fluoride	F-	hydrogen carbonate	HCO3-	sulfide	S2-
chloride	Cl-	hydroxide	OH-	sulfate	SO42-
bromide	Br-	nitrate	NO3-	carbonate	CO3-
iodide	I-	oxide	O2-