The proton number gives the protons, the nucleon number minus the proton number gives the neutrons, and the charge tells you how the electron count differs from the protons. Three lines of working are enough for any particle.
This lesson is part of matter and atomic structure. It follows distinguishing atoms, ions and molecules.
What does the symbol tell you?
Write the particle as nucleon number on top, proton number below, and the charge on the right. For example, ²³₁₁Na⁺ has nucleon number 23 and proton number 11.
- Protons = proton number.
- Neutrons = nucleon number − proton number.
- Electrons = protons − charge. A 1+ ion has 1 fewer electron than protons. A 2− ion has 2 more.
Worked example: Na⁺ and S²⁻
Sodium ion, ²³₁₁Na⁺.
- Protons = 11.
- Neutrons = 23 − 11 = 12.
- Electrons = 11 − 1 = 10.
Sulfide ion, ³²₁₆S²⁻.
- Protons = 16.
- Neutrons = 32 − 16 = 16.
- Electrons = 16 + 2 = 18.
| Particle | Protons | Neutrons | Electrons | Net charge |
|---|---|---|---|---|
| Na atom | 11 | 12 | 11 | 0 |
| Na⁺ ion | 11 | 12 | 10 | +1 |
| S atom | 16 | 16 | 16 | 0 |
| S²⁻ ion | 16 | 16 | 18 | −2 |
The slip that costs marks
The common slip is to subtract the charge from every ion. For S²⁻ that gives 16 − 2 = 14 electrons, which is wrong. A negative ion has gained electrons, so it has more electrons than protons.
The quick check: compare the two totals. Cations have more protons than electrons, and anions have more electrons than protons. If your answer breaks that rule, recount.
Extending to a larger ion
Try ²⁷₁₃Al³⁺. Protons = 13, neutrons = 27 − 13 = 14, electrons = 13 − 3 = 10.
Al³⁺ and Na⁺ both have 10 electrons, even though they are different elements. Particles with the same number of electrons are called isoelectronic.
Use the atomic structure and periodic position practice tool to drill fresh symbols once the routine is steady.
Check yourself
Find the numbers of protons, neutrons and electrons in ³⁵₁₇Cl⁻ and in ²⁴₁₂Mg²⁺.
Answer
Cl⁻: protons 17, neutrons 35 − 17 = 18, electrons 17 + 1 = 18.
Mg²⁺: protons 12, neutrons 24 − 12 = 12, electrons 12 − 2 = 10.
Check: the anion has more electrons than protons, and the cation has fewer. Both results agree.
What to study next
Go to interpreting isotopes and relative atomic mass, where atoms with different neutron counts appear. Then test yourself with the chapter practice set.
If you want a teacher to watch your counting step by step, see online one-to-one Chemistry tuition.