Section 1 of 5Ion Formation
Ion
- A charged particle formed when an atom gains or loses one or more electrons, so its numbers of protons and electrons differ.
Positive ion
- Formed when an atom loses electrons, leaving more protons than electrons, so it carries a positive charge.
Negative ion
- Formed when an atom gains electrons, leaving more electrons than protons, so it carries a negative charge.
Protons unchanged
- Forming an ion changes only the number of electrons, never the number of protons, so the element stays the same.
Section 2 of 5Calculating Ion Charge
Calculating Ion Charge
Charge rule
- An ion's charge equals its number of protons minus its number of electrons, so each extra electron gives a charge of −1.
Metals
- Metal atoms usually lose electrons to form positive ions, with a charge matching the number of electrons lost.
Non-metals
- Non-metal atoms usually gain electrons to form negative ions, which are called anions.
Electron count
- The number of electrons in an ion equals the proton number minus the ion's charge.
| Ion | Protons | Electrons | Charge |
|---|---|---|---|
| Na⁺ | 11 | 10 | +1 |
| Mg²⁺ | 12 | 10 | +2 |
| O²⁻ | 8 | 10 | −2 |
| Cl⁻ | 17 | 18 | −1 |
Section 3 of 5Forming Ions
1
A sodium atom loses one outer electron, forming the positive sodium ion Na⁺.
2
A chlorine atom gains one electron, forming the negative chloride ion Cl⁻.
3
Oppositely charged ions are attracted to each other, which holds ionic compounds together.
Section 4 of 5Worked Example
Question
- Calculate the number of electrons in a magnesium ion, Mg²⁺.
- Magnesium has a proton number of 12.
- Protons in Mg = 12.
- Charge = +2, which means two electrons were lost.
- Electrons = 12 − 2 = 10.
Answer The Mg²⁺ ion has 10 electrons.
Section 5 of 5Exam Checklist & Tips
7.1.3 – Key Points for the Exam
- Ion A charged atom or group of atoms.
- Positive ion (cation) An ion with more protons than electrons.
- Negative ion (anion) An ion with more electrons than protons.
- Electron loss The process by which an atom loses electrons to form a positive ion.
- Electron gain The process by which an atom gains electrons to form a negative ion.
Common Mistakes
- Changing the proton number when forming an ion; only the electron number changes.
- Giving a negative ion a positive charge, or stating that positive ions gain electrons.
- Writing the wrong number or sign for an ion's charge, such as Mg⁺ instead of Mg²⁺.
- Assuming an ion's mass changes significantly from losing electrons; the electron mass is negligible.