What this calculator does
Three numbers describe an atom completely at this level. Protons equal the atomic number, neutrons are the mass number minus it, and electrons are the atomic number minus the charge. A chloride ion with Z = 17 and A = 35 has 17 protons, 18 neutrons and 18 electrons.
The sign convention on charge trips people up. A negative ion has gained electrons, so subtracting a negative charge adds them: 17 minus −1 gives 18. A positive ion has lost them, so iron(III) with Z = 26 has 26 minus 3, or 23 electrons.
The formula
Protons come straight from the atomic number, which defines the element. Neutrons are the mass number minus the atomic number, since the mass number counts both. Electrons are the atomic number minus the charge, because charge is the excess of protons over electrons.
| Term | Meaning |
|---|---|
| Atomic number (Z) | The proton count, which defines which element it is. |
| Mass number (A) | Protons plus neutrons. It is a whole number, unlike the atomic mass. |
| Isotope | Same element, different neutron count and therefore different mass number. |
| Ion | An atom with unequal protons and electrons, carrying a net charge. |
The inputs explained
| Field | What to enter |
|---|---|
| Atomic number Z | Atomic number, the proton count. Chlorine is 17, iron 26. |
| Mass number A | Mass number, the total of protons and neutrons. |
| Ion charge | Charge. Negative for an anion that has gained electrons, positive for a cation that has lost them. |
When to use it
Reading isotope notation
Nuclear notation gives Z and A, and this unpacks them into particle counts.
Understanding ions
Seeing that an ion differs from its atom only in electron count clarifies what ionisation actually changes.
Comparing isotopes
Two isotopes differ only in neutrons, which the calculation makes explicit.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What do these atomic numbers give?
A range of atomic numbers at fixed mass number and charge.
Questions
How do I find the number of neutrons?
Subtract the atomic number from the mass number. Chlorine-35 has 35 minus 17, or 18 neutrons. Note that the mass number is a whole-number count, not the atomic mass from the periodic table, which is a weighted average across isotopes.
Why does a negative charge mean more electrons?
Because charge is the excess of protons over electrons. An ion with a charge of −1 has one more electron than it has protons. Subtracting a negative number adds, which is why the arithmetic works out as 17 minus −1 equals 18.
What is the difference between mass number and atomic mass?
Mass number is the whole-number count of protons plus neutrons for one specific isotope. Atomic mass is the weighted average across all natural isotopes, in amu, and is rarely a whole number. Chlorine has mass numbers 35 and 37 and an atomic mass of 35.45.
Does changing the neutron count change the element?
No. The element is defined entirely by the proton count. Changing neutrons gives a different isotope of the same element, with the same chemistry but a different mass and possibly different nuclear stability.
For the weighted average across isotopes, see the average atomic mass calculator. For electron shielding, see the effective nuclear charge calculator.