Ions & Ionic Bonds (Cambridge (CIE) IGCSE Chemistry)

The formation of ions

How are ions formed?

• An ion is an electrically charged atom or group of atoms formed by the loss or gain of electrons

• This loss or gain of electrons takes place to obtain a full outer shell of electrons

• The electronic structure of ions of elements in Groups 1, 2, 6 and 7 will be the same as that of a noble gas - such as helium, neon, and argon

• Negative ions are called anions and form when atoms gain electrons, meaning they have more electrons than protons

• Positive ions are called cations and form when atoms lose electrons, meaning they have more protons than electrons

• All metals lose electrons to other atoms to become positively charged ions

• All non-metals gain electrons from other atoms to become negatively charged ions

Formation of a cation

Formation of positively charged sodium ion

Formation of an Anion

Formation of negatively charged chloride ion

What is ionic bonding?

The Formation of Ionic Bonds 

• Ionic compounds are formed when metal atoms react with non-metal atoms

• Metal atoms lose their outer electrons which the non-metal atoms gain to form positive and negative ions

• The positive and negative ions are held together by strong electrostatic forces of attraction between opposite charges

• This force of attraction is known as an ionic bond and they hold ionic compounds together

Dot-and-cross diagrams

• Dot and cross diagrams are diagrams that show the arrangement of the outer-shell electrons in an ionic or covalent compound or element

• The electrons are shown as dots and crosses

• In a dot and cross diagram:

  • Only the outer electrons are shown

  • The charge of the ion is spread evenly which is shown by using brackets

  • The charge on each ion is written at the top right-hand corner

 Electrostatic forces between the positive Na ion and negative Cl ion

Ionic bonds between Group I & Group VII elements

Group I & Group VII Ions

• Sodium is a Group 1 metal so loses one outer electron to another atom to gain a full outer shell of electrons

• A positive sodium ion with the charge 1+ is formed, Na⁺ 

• Chlorine is a Group 7 non-metal so gains one electron to have a full outer shell of electrons

• A negative chloride ion with a charge of 1– is formed, Cl^–

• The ions are then attracted to one another and held together by electrostatic forces

• The formula of the ionic compound is thus NaCl

• The large square brackets should encompass each atom and the charge should be in superscript and on the right-hand side, outside the brackets

Diagram representing the formation of the ionic bond in sodium chloride

Sodium loses one electron and chlorine gains an electron

Dot-and-cross diagram of sodium chloride