Lesson 3 of 7 · Atomic structure
Protons, neutrons & electrons
To count particles in an atom or monatomic ion, use protons = Z, neutrons = A − Z, and electrons = Z − q. Here Z is the atomic number, A is the isotope’s mass number, and q is the signed charge in units of the elementary charge: +1 for Na⁺, −1 for Cl⁻ and 0 for a neutral atom.
Particle counter
Specify an atomic number (Z), mass number (A), and signed charge (q). For Na⁺ use +1; for Cl⁻ use −1; for a neutral atom use 0.
- protons
- 11 p⁺ = Z
- neutrons
- 12 n⁰ = A − Z
- electrons
- 10 e⁻ = Z − q
The counter checks the numbers you enter. It does not tell you whether the isotope has been observed or the ion is stable.
Read isotope notation
Read ²³Na⁺ as a sodium-23 ion with a charge of +1. Sodium tells you Z = 11, so there are 11 protons. The upper-left 23 is the mass number A, which counts protons plus neutrons. The upper-right plus sign means there is one fewer electron than protons.
Isotopes of one element have the same proton count and different neutron counts. Ions differ in electron count. Carbon-12 and carbon-14 are isotopes; Na and Na⁺ are different charge states of sodium.
| Given value | Meaning | Example: ²³Na⁺ |
|---|---|---|
| Z | Proton count | 11 |
| A | Protons + neutrons | 23 |
| Signed charge | Protons − electrons | +1 |
Mass number is an integer
An element’s atomic weight reflects the masses and proportions of its isotopes. Chlorine’s familiar value of 35.45 is therefore not the mass number of a single atom. Choose an isotope first: chlorine-35 has 18 neutrons, while chlorine-37 has 20.
Rounding an atomic weight does not reliably identify an isotope. Enter a specific mass number in the calculator. It checks your particle-count arithmetic; it cannot tell you whether that isotope has been observed, whether it is stable, or whether the ion can exist.
Reference: CIAAW: Abridged standard atomic weights
Work through the examples
Worked example 1
Count particles in ³⁵Cl⁻.
- Chlorine has Z = 17, so there are 17 protons.
- Neutrons = 35 − 17 = 18.
- Electrons = 17 − (−1) = 18.
Answer: 17 protons, 18 neutrons and 18 electrons.
Worked example 2
Compare carbon-12 and carbon-14 atoms.
- Both are carbon, so each has 6 protons.
- Subtract 6 from each mass number to find 6 and 8 neutrons.
- Both atoms are neutral, so each has 6 electrons.
Answer: The neutron count changes; the element and neutral electron count do not.
Avoid these mistakes
- Subtract a negative ionic charge correctly: Z − (−1) = Z + 1.
- Do not change the proton count when forming an ordinary ion.
- Mass number counts nucleons; it is not an exact mass in atomic mass units.
Check your understanding
Open printable worksheetTry each question before opening the answer key. When you check your answers, read the explanations too. Your choices stay in this browser.
Read the complete answer key
1. A — 12. A − Z = 24 − 12 = 12 neutrons.
2. A — 10. The 3+ ion has three fewer electrons than protons: 13 − 3 = 10.
3. C — Proton count. Atomic number is the proton count. Changing it changes the element.
4. B — No, specify an isotope. 35.45 is an atomic-weight value, not the mass number of a particular chlorine nucleus.
Common questions
Can a hydrogen atom have no neutrons?
Yes. Hydrogen-1 has one proton and no neutrons. A neutral hydrogen-1 atom also has one electron.
Are isotopes always radioactive?
No. Many isotopes are stable. Radioactivity depends on the particular nucleus, not simply on whether the word isotope is used.
References & model notes
Use these sources to check the definitions and data, or to read more about the topic. The lesson notes where a model or rounded value simplifies the chemistry.