Chemistry

2.11 - Ionic Bonding

8 min

Intro & ions recap

Forming an ionic bond

Dot and cross diagrams

Magnesium chloride example

note 1 of 2

Ionic bonding

There are three types of strong chemical bond - ionic, covalent, and metallic.
Ionic bonding happens between a metal and a non-metal.
The metal atom transfers electrons to the non-metal atom:
- The metal loses electrons to become a positive ion.
- The non-metal gains electrons to become a negative ion.
- Both ions end up with a full outer shell.
The ionic bond is the strong electrostatic attraction between these oppositely charged ions.

Dot-and-cross diagrams

These show the electron transfer that produces the ions.
Electrons from one atom are drawn as dots and those from the other as crosses. An arrow shows an electron moving from the metal to the non-metal.
Usually only the outer shell is drawn, as that's the only one that changes.
Each finished ion goes in square brackets, with its charge in the top right corner.

Examples of ion formation

Sodium chloride (NaCl) - The sodium atom loses its one outer electron to become Na⁺; the chlorine atom gains it to become Cl^-.
Magnesium chloride (MgCl₂) - The magnesium atom loses two outer electrons, one to each of two chlorine atoms, becoming Mg²⁺; each chlorine gains one electron to become Cl^-.

Two rows of dot-and-cross diagrams with arrows moving crosses from metal to chlorine atoms: sodium and chlorine form bracketed Na+ and Cl− ions, and magnesium and two chlorines form Mg2+ between two Cl− ions.

Limits of a dot-and-cross diagram

The size of the ions
The true arrangement of the ions in the structure
How many ions a real sample contains

note 2 of 2

Worked example - Dot-and-cross diagram for K₂O

Draw a dot-and-cross diagram to show the formation of potassium oxide, K₂O.

Step 1 - Identify the two ions and their charges

Potassium is in Group 1 - a potassium atom has one outer electron and loses it to form K⁺.
Oxygen is in Group 6 - an oxygen atom has six outer electrons and gains two to form O^2-.

Step 2 - Work out how many of each ion are needed

Each potassium atom gives away one electron, but each oxygen atom needs two.
So one oxygen atom needs an electron from two separate potassium atoms.
Two K⁺ ions and one O^2- ion balance the charges to zero (+2 and 2−).

Step 3 - Show the electron transfer

Draw the two potassium atoms and the oxygen atom.
Draw an arrow from each potassium atom's outer electron to the oxygen atom.
The oxygen atom gains two electrons to complete its outer shell of eight.

Step 4 - Draw the finished ions

Draw only the outer shell of each ion.
Put each ion in square brackets with its charge in the top right corner.
Draw the two potassium ions either side of the oxide ion.

Step 5 - Answer

Two K⁺ ions, each with an empty outer shell, drawn either side of one O^2- ion with a full outer shell of eight.
Check: two ions of +1 and one ion of 2− cancel to zero.

Two potassium atoms each pass their one outer electron, a cross, by arrow to an oxygen atom, forming an O2− ion with a full outer shell between two empty positive ions, labelled potassium oxide.

key terms

TermDefinition

Ionic bondThe strong electrostatic attraction between oppositely charged ions.

end of notes

Key points to remember

Ionic bonding

There are three types of strong chemical bond - ionic, covalent, and metallic.

Ionic bonding happens between a metal and a non-metal.

The metal atom transfers electrons to the non-metal atom:

The metal loses electrons to become a positive ion.
The non-metal gains electrons to become a negative ion.
Both ions end up with a full outer shell.

The ionic bond is the strong electrostatic attraction between these oppositely charged ions.

Dot-and-cross diagrams

These show the electron transfer that produces the ions.

Electrons from one atom are drawn as dots and those from the other as crosses. An arrow shows an electron moving from the metal to the non-metal.

Usually only the outer shell is drawn, as that's the only one that changes.

Each finished ion goes in square brackets, with its charge in the top right corner.

Examples of ion formation

Sodium chloride (NaCl) - The sodium atom loses its one outer electron to become Na⁺; the chlorine atom gains it to become Cl^-.

Magnesium chloride (MgCl₂) - The magnesium atom loses two outer electrons, one to each of two chlorine atoms, becoming Mg²⁺; each chlorine gains one electron to become Cl^-.

Worked example - Dot-and-cross diagram for K₂O

Draw a dot-and-cross diagram to show the formation of potassium oxide, K₂O.

Step 1 - Identify the two ions and their charges

Potassium is in Group 1 - a potassium atom has one outer electron and loses it to form K⁺.
Oxygen is in Group 6 - an oxygen atom has six outer electrons and gains two to form O^2-.

Step 2 - Work out how many of each ion are needed

Each potassium atom gives away one electron, but each oxygen atom needs two.
So one oxygen atom needs an electron from two separate potassium atoms.
Two K⁺ ions and one O^2- ion balance the charges to zero (+2 and 2−).

Step 3 - Show the electron transfer

Draw the two potassium atoms and the oxygen atom.
Draw an arrow from each potassium atom's outer electron to the oxygen atom.
The oxygen atom gains two electrons to complete its outer shell of eight.

Step 4 - Draw the finished ions

Draw only the outer shell of each ion.
Put each ion in square brackets with its charge in the top right corner.
Draw the two potassium ions either side of the oxide ion.

Step 5 - Answer

Two K⁺ ions, each with an empty outer shell, drawn either side of one O^2- ion with a full outer shell of eight.
Check: two ions of +1 and one ion of 2− cancel to zero.

2.11 - Ionic Bonding

Ionic bonding

Dot-and-cross diagrams

Examples of ion formation

Limits of a dot-and-cross diagram

Worked example - Dot-and-cross diagram for K2O

Step 1 - Identify the two ions and their charges

Step 2 - Work out how many of each ion are needed

Step 3 - Show the electron transfer

Step 4 - Draw the finished ions

Step 5 - Answer

Key points to remember

2.11 - Ionic Bonding

Ionic bonding

Dot-and-cross diagrams

Examples of ion formation

Limits of a dot-and-cross diagram

Worked example - Dot-and-cross diagram for K2O

Step 1 - Identify the two ions and their charges

Step 2 - Work out how many of each ion are needed

Step 3 - Show the electron transfer

Step 4 - Draw the finished ions

Step 5 - Answer

Key points to remember

Worked example - Dot-and-cross diagram for K₂O

Worked example - Dot-and-cross diagram for K₂O