6.1.6Radioactivity
Positive Ions — Edexcel GCSE Combined Science 1SC0 - Physics Revision Notes & Topical Questions
5 min read 8 parts Lesson 7 of 20
Part 1 of 7From atom to ion
1Neutral atom
- Neutral atom Equal numbers of protons (positive) and electrons (negative), so no overall charge.
2Energy absorbed
- Energy absorbed An outer electron gains enough energy, for example from radiation, to escape the attraction of the nucleus.
3Electron lost
- Electron lost The atom now has more protons than electrons.
4Positive ion
- Positive ion The overall charge is positive: losing one electron gives a +1 ion, losing two gives +2.
Part 2 of 7Counting particles
| Particle | Protons | Electrons | Charge |
|---|---|---|---|
| Sodium atom, Na | 11 | 11 | 0 |
| Sodium ion, Na+ | 11 | 10 | +1 |
| Magnesium ion, Mg2+ | 12 | 10 | +2 |
| Aluminium ion, Al3+ | 13 | 10 | +3 |
Part 3 of 7Key ideas
Nucleus unchanged
- Nucleus unchanged Forming an ion changes only the electrons;
- the number of protons and neutrons stays the same, so it is the same element.
Which atoms lose electrons
- Which atoms lose electrons Metals tend to lose their outer electrons to form positive ions.
Ionising radiation
- Ionising radiation Alpha, beta, gamma, X-rays and ultraviolet can knock electrons from atoms, creating ions;
- this can damage living cells.
Part 4 of 7Working out the charge
1
Count the protons from the atomic number; this never changes during ionisation.
2
Count the electrons the particle actually has, e.g. 10 for a sodium ion.
3
Subtract: charge = number of protons − number of electrons.
4
For sodium, 11 − 10 = +1, so the ion is written Na+. For magnesium with 12 protons and 10 electrons, the charge is +2, written Mg2+.
Part 5 of 7Ionisation and radiation
Ionising radiation
Alpha particles
- Alpha particles Strongly ionising because they have a large charge and mass, but they are stopped by paper.
Beta particles
- Beta particles Moderately ionising; they travel further, penetrating a few millimetres of aluminium.
Gamma rays
- Gamma rays Weakly ionising but very penetrating, requiring thick lead or concrete.
Part 6 of 7Metal atoms and positive ions in chemistry
Full outer shell
- Full outer shell A metal atom loses its outer electrons so that the remaining shell is full, which is a stable arrangement like a noble gas.
Group number
- Group number Group 1 metals form +1 ions, Group 2 metals form 2+ ions and Group 3 metals form 3+ ions, matching the number of outer electrons lost.
Ionic compounds
- Ionic compounds Positive ions are attracted to negative ions by strong electrostatic forces, forming a giant ionic lattice such as sodium chloride.
Part 7 of 7Negative ions for comparison
Gaining electrons
- Gaining electrons Non-metal atoms such as chlorine and oxygen tend to gain electrons,
- giving more electrons than protons and an overall negative charge, e.g. Cl− and O2−.
Why it matters
- Why it matters Positive and negative ions together make ionic compounds, and the total positive charge always equals the total negative charge.
Key terms
| Term | Meaning |
|---|---|
| Ion | An atom or group of atoms with an overall electric charge. |
| Ionisation | The process in which an atom loses or gains electrons to become an ion. |
| Positive ion | An atom that has lost one or more electrons. |
| Ionising radiation | Radiation with enough energy to remove electrons from atoms. |
| Electron shell | An energy level in which electrons surround the nucleus. |
| Electrostatic force | The force of attraction between oppositely charged particles. |
Avoid these mistakes
1
Saying an atom becomes positive by gaining protons; the number of protons never changes in ionisation.
2
Describing the loss of an electron as making the atom a different element.
3
Mixing up chargeslosing electrons gives a positive ion, gaining electrons gives a negative ion.
4
Stating that gamma is the most ionising radiation because it is the most penetrating; it is the least ionising of the three.
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