Ions (Edexcel GCSE Chemistry)

Revision Note

Test Yourself
Stewart

Author

Stewart

Expertise

Chemistry Lead

Ions

What are ions?

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

Deducing subatomic particles in ions

  • An atom is neutral and has no overall charge
  • Ions on the other hand have either gained or lost electrons causing them to become charged
  • The number of subatomic particles in atoms and ions can be determined given their atomic (proton) number, mass (nucleon) number and charge
  • Ions have a different number of electrons to the number of protons, depending on their charge
    • A positively charged ion has lost electrons and therefore has fewer electrons than protons
    • A negatively charged ion has gained electrons and therefore has more electrons than protons

Worked example

Determine the number of protons, neutrons & electrons of the following ions:

  1. Mg2+ ion
  2. F ion

Answer 1: 

  • The atomic number of a magnesium atom is 12 and the mass number is 24
  • Therefore, the number of protons in a Mg2+ ion is 12
    • Remember: The number of protons does not change when an ion is formed
  • An atom of magnesium is electronically neutral, which means that the number of protons equals the number of electrons
    • However, the 2+ charge in an Mg2+ ion means it has lost two electrons
    • So, it has 12 - 2 = 10 electrons 
  • Number of neutrons = mass number – atomic (proton) number 
    • Number of neutrons = 24 – 12
    • Number of neutrons = 12

Answer 2:

  • The atomic number of a fluorine atom is 9 and the mass number is 19
  • Therefore, the number of protons in an F– ion is 9
    • Remember: The number of protons does not change when an ion is formed
  • An atom of fluorine is electronically neutral, which means that the number of protons equals the number of electrons
    • However, the 1- charge in an F ion means it has gained one electron
    • So, it has 9 + 1 = 10 electrons 
  • Number of neutrons = mass number – atomic (proton) number 
    • Number of neutrons = 19 – 9
    • Number of neutrons = 10

Exam Tip

  • The number of electrons that an atom gains or loses is the same as the charge
  • For example:
    • If a magnesium atom loses 2 electrons, then the charge will be 2+
    • If a bromine atom gains 1 electron then the charge will be 1-

Formation of Ions

  • 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, 3, 5, 6 and 7 will be the same as that of a noble gas - such as helium, neon, and argon

Deducing Dot & Cross Diagrams for Ionic Compounds

  • Sodium is a group 1 metal so will lose one outer electron to another atom to gain a full outer shell of electrons
  • A positive sodium ion with the charge 1+ is formed

Formation of positively charged Sodium ion1

Diagram showing the formation of the sodium ion

  • Chlorine is a group 7 non-metal so will need to gain an electron to have a full outer shell of electrons
  • One electron will be transferred from the outer shell of the sodium atom to the outer shell of the chlorine atom
  • A chlorine atom will gain an electron to form a negatively charged chloride ion with a charge of -1

Formation-of-negatively-charged-Chloride-ion1, IGCSE & GCSE Chemistry revision notes

Diagram showing the formation of the chloride ion

Exam Tip

For exam purposes you need only show the outer electrons in dot & cross diagrams.When writing about ions, we use the notation 1-, 2+ etc. to describe the charge of the ion, with the number first followed by the sign (+/-). It is incorrect to write them the other way around as this refers to the oxidation state, not the charge.

You've read 0 of your 0 free revision notes

Get unlimited access

to absolutely everything:

  • Downloadable PDFs
  • Unlimited Revision Notes
  • Topic Questions
  • Past Papers
  • Model Answers
  • Videos (Maths and Science)

Join the 80,663 Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Stewart

Author: Stewart

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.