Ions & Ionic Bonds (CIE IGCSE Chemistry)

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? 

• 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

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