1.8 - Formation of Ions

How an atom becomes an ion, and its charge

• An ion is a charged particle. It can be:
  • A single atom that has gained or lost electrons (e.g. a chloride ion), or
  • A group of atoms bonded together carrying an overall charge (e.g. a hydroxide ion).
• Atoms form ions to get a full outer shell of electrons - the stable arrangement a noble gas already has.
• An atom with an incomplete outer shell reaches this by losing or gaining electrons. It then has more protons than electrons (or more electrons than protons), so it carries a charge.

Metals lose, non-metals gain

• Metal atoms lose outer-shell electrons to form positive ions - called cations.
• Non-metal atoms gain electrons into their outer shell to form negative ions - called anions.
• The size of the charge is set by how many electrons move: lose two electrons and the charge is 2+; gain three and the charge is 3−.

Reading the charge off the periodic table

Every element in a group has the same number of outer electrons, so it loses or gains the same number to reach a full shell - making the charge predictable.

The electronic configuration of an ion

• To work out an ion's electronic configuration from its atom:
  1. Write out the atom's electron arrangement.
  2. Add or remove electrons from the outer shell to match the ion's charge.
• For elements in Groups 1, 2, 3, 5, 6 and 7, the ion ends up with the electron arrangement of the nearest noble gas. Transition metal ions such as Fe²⁺, below, do not follow this rule.

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Worked examples of ion configurations

• Na (2,8,1) loses 1 electron → Na⁺ (2,8) - same as neon.
• Cl (2,8,7) gains 1 electron → Cl^- (2,8,8) - same as argon.
• Mg (2,8,2) loses 2 electrons → Mg²⁺ (2,8).
• O (2,6) gains 2 electrons → O^2- (2,8).

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Using the numbers alone

• You can find the number of particles without drawing shells:
  • Protons are unchanged from the atom, so they equal the atomic number.
  • Electrons = atomic number adjusted by the charge - subtract the charge for a positive ion, add it for a negative ion.
• Fluorine has atomic number 9 (9 protons); F^- has one more electron = 10 electrons.
• Iron has atomic number 26 (26 protons); Fe²⁺ has two fewer electrons = 24 electrons.

Worked example - Configuration of an ion

Finding an ion's electronic configuration follows three steps.

Applying the same steps to fluorine (arrangement 2,7): as a Group 7 non-metal it gains one electron to complete its outer shell, forming F^- with the arrangement 2,8 - the same as neon.

Each ion reaches whichever noble gas is nearest to it in the periodic table.

Writing an ion-electron equation

• An ion-electron equation shows what happens to the electrons when an atom becomes an ion.
• The symbol e^- stands for one electron.

How to write one

1. Use the element's group number to find the charge on the ion it forms:
   • Group 1 metal - loses 1 electron → 1+ ion
   • Group 2 metal - loses 2 electrons → 2+ ion
   • Group 6 non-metal - gains 2 electrons → 2− ion
   • Group 7 non-metal - gains 1 electron → 1− ion
2. Write the atom on one side and the ion it becomes on the other.
3. Add the electrons to balance the charge:
   • Losing electrons - they go on the same side as the ion.
   • Gaining electrons - they go on the same side as the atom.

The number of electrons always matches the size of the ion's charge, so the charge balances on both sides.

Worked example - Ion-electron equation

Barium is in Group 2, so a barium atom loses two electrons to form a Ba²⁺ ion.

Applying the same to oxygen (Group 6), which gains two electrons - the electrons go on the same side as the atom (the left):

O + 2e^- → O^2-

The two electrons added on the left balance the 2− charge on the ion on the right.