Formation of Ionic Bonds

Specification Point 1.21:
  • Explain how ionic bonds are formed by the transfer of electrons between atoms to produce cations and anions, including the use of dot and cross diagrams.
  • Metals: all metals lose electrons to another atom and become positively charged ions.
  • Non-metals: all non-metals gain electrons from another atom to become negatively charged ions.
  • Ionic bonds form when a metal atom transfers its outermost electron(s) to a nonmetal atom, forming oppositely charged ions.
  • The positive and negative charges are held together by the strong electrostatic attraction between the opposite charges.
  • This is what holds the ions in ionic compounds together.

Ionic bond in NaCl, Edexcel GCSE Chemistry

Diagram showing electron transfer from Na to Cl, creating electrostatic attraction between the ions


Specification Point 1.22:
  • Recall that an ion is an atom or group of atoms with a positive or negative charge.
  • An ion is an electrically charged atom or group of atoms formed by the loss or gain of electrons.
  • The loss or gain of electrons takes place to gain a full outer shell of electrons, a state which all atoms try to achieve due to its high stability.
  • The electronic structure of an ion 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.

Sodium loses an electron to form the sodium cation (Na+)

Oxide ion, Edexcel GCSE Chemistry

Oxygen gains 2 electrons to form the oxide anion (O2-)

Subatomic Particles in Ions

Specification Point 1.23:
  • Calculate the numbers of protons, neutrons and electrons in simple ions given the atomic number and mass number.
  • Recall that the atomic number is equal to the number of protons and the mass number is equal to the number of protons plus neutrons.
  • Atoms are neutral entities so the number of electrons and protons is the same, but this is not the case for ions.
  • For the subatomic particles in an ion:
    • Use the atomic number to work out how many electrons the atom started off with.
    • Use the charge on the ion to calculate the number of electrons in the atom and then add the electrons gained or subtract the electrons lost.
  • Brfor example has one single negative charge, so it has 1 electron more than protons.
  • The atomic number of Br is 35, so it must therefore have 35 + 1 = 36 electrons, 35 protons and 80 – 35 = 45 neutrons.

Bromine tile, Edexcel GCSE Chemistry

Bromine has an atomic number of 35 and a mass number of 80

  • Mg2+ has a 2+ positive charge, so it has 2 electrons less than protons.
  • The atomic number of Mg is 12, so it must therefore have 12 – 2 = 10 electrons, 12 protons and 24 – 12 = 12 neutrons.

Magnesium tile, Edexcel GCSE Chemistry

Magnesium has an atomic number of 12 and a mass number of 24

Edexcel GCSE Chemistry Notes

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.