On this page
Every atom is built from just three particles. Given two or three numbers, you can work out exactly how many of each an atom has. This is one of the first calculations in chemistry, and it’s quick once you know where each number comes from.
The three particles
| Particle | Where | Charge | Relative mass |
|---|---|---|---|
| Proton | Nucleus | +1 | 1 |
| Neutron | Nucleus | 0 | 1 |
| Electron | Around the nucleus | −1 | about 1/1836 |
Protons and neutrons (together called nucleons) make up almost all of an atom’s mass. Electrons are so light that they barely count.
The numbers you need
- Atomic number (Z) — the number of protons. It defines the element: every carbon atom has 6 protons, every gold atom 79. It’s the whole number shown on every cell of the periodic table.
- Mass number (A) — the number of protons + neutrons. It’s always a whole number, and it describes one specific isotope.
- Charge — tells you whether electrons have been gained or lost.
Isotope notation packs these together: in ¹⁴₆C, the top number (14) is the mass number and the bottom (6) is the atomic number. You’ll also see it written carbon-14 or C-14.
The three formulas
- Protons = atomic number
- Neutrons = mass number − atomic number
- Electrons = atomic number − charge (for a neutral atom, electrons = protons)
Worked examples
Sodium-23 (neutral atom)
- Protons: 11 (atomic number)
- Neutrons: 23 − 11 = 12
- Electrons: 11
Carbon-14
- Protons: 6
- Neutrons: 14 − 6 = 8
- Electrons: 6
Compare carbon-12, which has 6 neutrons. Same element, different number of neutrons — that’s what makes them isotopes.
Uranium-235
- Protons: 92
- Neutrons: 235 − 92 = 143
- Electrons: 92
The ion Mg²⁺ (magnesium-24)
- Protons: 12
- Neutrons: 24 − 12 = 12
- Electrons: 12 − 2 = 10 (a +2 charge means two electrons lost)
The ion Cl⁻ (chlorine-35)
- Protons: 17
- Neutrons: 35 − 17 = 18
- Electrons: 17 − (−1) = 18 (a −1 charge means one electron gained)
The ion O²⁻ (oxygen-16)
- Protons: 8, neutrons: 8, electrons: 8 + 2 = 10
What if you only have the element name?
If no mass number is given, use the most common isotope. The atomic weight on the periodic table, rounded to a whole number, is usually a good guide: iron’s atomic weight is 55.845, and its most abundant isotope is iron-56 (26 protons, 30 neutrons).
Be careful, though — for some elements the rounded atomic weight doesn’t match any abundant isotope. Chlorine’s is 35.45, a mix of about 76% chlorine-35 and 24% chlorine-37; there’s no chlorine-35.5 atom. Every element page on this site lists its isotopes with their natural abundances.
Common mistakes
- Using atomic weight as mass number. Atomic weight (like 35.45) is a weighted average across isotopes and isn’t a whole number. Mass number is always whole. See atomic mass vs. mass number vs. molar mass.
- Changing protons for an ion. Ions gain or lose electrons only. The number of protons never changes in a chemical reaction; if it did, the atom would become a different element.
- Getting the charge sign backwards. Positive ions have fewer electrons than protons; negative ions have more.
Quick answers
What determines which element an atom is? Its number of protons (atomic number) — nothing else.
Can atoms of the same element have different numbers of neutrons? Yes — those are isotopes. See isotopes vs. ions vs. isomers vs. allotropes.
Why is an atom neutral? Because it has equal numbers of protons (+) and electrons (−). For more on ions, see what is an ion.
Advertisement