1.6.1Principles of Chemistry
Formation of Ions — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 11 parts Lesson 34 of 50
Part 1 of 4Atoms, electrons and stable shells
Neutral atoms
- Neutral atoms An atom has equal numbers of protons and electrons, so the positive and negative charges cancel and it has no overall charge.
Full outer shells
- Full outer shells Noble gases, in Group 0, have full outer shells and are very unreactive.
- Other atoms react by gaining, losing or sharing electrons to reach the same stable arrangement.
What an ion is
- What an ion is An ion is an atom, or group of atoms, that has lost or gained electrons and so carries an electric charge.
- Only the number of electrons changes; the number of protons in the nucleus stays the same.
Part 2 of 4How ions form
| Feature | Positive ion (cation) | Negative ion (anion) |
|---|---|---|
| Change in electrons | Atom loses electrons | Atom gains electrons |
| Typical elements | Metals | Non-metals |
| Overall charge | Protons outnumber electrons, so positive | Electrons outnumber protons, so negative |
| Example | Na → Na+ + e− | Cl + e− → Cl− |
| Name | Same as the element: sodium ion | Ends in -ide: chloride ion |
Part 3 of 4Predicting the charge from the group
1Find the group
- Find the group The group number is the number of electrons in the outer shell.
2Metals in Groups 1, 2, 3
- Metals in Groups 1, 2, 3 They lose 1, 2 or 3 outer electrons, giving ions with charges 1+, 2+ and 3+.
3Non-metals in Groups 5, 6, 7
- Non-metals in Groups 5, 6, 7 They gain 3, 2 or 1 electrons to fill the shell, giving 3−, 2− and 1− ions.
4Result
- Result The ion has the same electron arrangement as the nearest noble gas, for example Na+ has 2,8 like neon.
Part 4 of 4Worked examples
Sodium to Na+
- Sodium to Na+ Sodium atom: 11 protons, 11 electrons, arrangement 2,8,1.
- It loses its outer electron to become 2,8 with 11 protons and 10 electrons, a charge of 1+.
Oxygen to O2−
- Oxygen to O2− Oxygen atom: 8 protons, 8 electrons, arrangement 2,6.
- It gains two electrons to become 2,8 with 8 protons and 10 electrons, a charge of 2−.
Magnesium to Mg2+
- Magnesium to Mg2+ Magnesium atom is 2,8,2; losing the two outer electrons gives 2,8 and a charge of 2+.
Writing ion symbols
- Writing ion symbols The charge is written as a superscript with the number first, then the sign: Mg2+, not Mg+2.
Key terms
| Term | Meaning |
|---|---|
| Ion | An atom or group of atoms with an overall electric charge because it has lost or gained electrons. |
| Cation | A positively charged ion, formed when an atom loses electrons. |
| Anion | A negatively charged ion, formed when an atom gains electrons. |
| Noble gas configuration | A full outer shell of electrons, which is a very stable arrangement. |
| Outer shell | The highest energy level that holds electrons; its electrons are used in bonding. |
Avoid these mistakes
1
Saying that protons are lost or gained when ions form. Only electrons move.
2
Thinking a metal ion is bigger or more negative because it 'gained' electrons. Metals lose electrons and become positive.
3
Confusing the sign of the chargelosing negative electrons makes the particle positive.
4
Writing the charge as Cl−1 or Mg+2; the number comes before the sign.
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