Atom Particle Calculator
Work out the number of protons, neutrons and electrons in an atom or ion from its atomic number (Z), mass number (A) and charge. Protons equal Z, neutrons equal A − Z, and electrons equal Z minus the net charge. Enter the numbers for any element to see its subatomic particle counts.
Formula
Protons = Z, Neutrons = A - Z, Electrons = Z - charge
- The atomic number Z equals the number of protons and defines which element the atom is.
- Neutrons = mass number A minus atomic number Z, because the mass number counts protons plus neutrons.
- In a neutral atom electrons equal protons (Z). For an ion, electrons = Z minus the net charge: a +2 cation has two fewer electrons, a −1 anion has one more.
- The mass number must be at least the atomic number, since an atom cannot have a negative number of neutrons.
Example: iron-56 (Z = 26, A = 56)
Inputs
- Atomic number (Z): 26
- Mass number (A): 56
- Charge (optional): 0
Iron-56 has Z = 26 and A = 56, so it has 26 protons, 56 − 26 = 30 neutrons, and (being neutral) 26 electrons.
Frequently asked questions
How do I find the number of neutrons?
Subtract the atomic number from the mass number: neutrons = A − Z. For example, iron-56 has 56 − 26 = 30 neutrons.
How many electrons does an ion have?
Take the atomic number and subtract the charge. A neutral atom has electrons = Z; a +2 ion has two fewer, and a −1 ion has one more.
What is the difference between atomic number and mass number?
The atomic number (Z) is the proton count and identifies the element; the mass number (A) is the total of protons and neutrons in that particular isotope.
Why must the mass number be at least the atomic number?
Because neutrons = A − Z cannot be negative — an atom can have zero neutrons (like hydrogen-1) but never fewer than zero.
Do isotopes change the number of protons?
No. Isotopes of an element share the same number of protons (same Z) but differ in neutrons, which changes the mass number A.