How this instrument works
Every atom is built from three particles, and two of the three counts are fixed by definition. The atomic number, Z, is the number of protons in the nucleus — it's what makes an element that element; change the proton count and you've made a different element entirely. The mass number, A, is the total count of protons plus neutrons in that particular nucleus, so subtracting one from the other, A − Z, hands you the neutron count directly.
Electrons are the flexible piece. A neutral atom carries exactly as many electrons as protons, so its overall charge cancels to zero. Strip an electron away or add one and you've made an ion, with a charge equal to protons minus electrons. That means electron count runs backward from charge: electrons = Z − charge. A positive charge (a cation) means electrons were removed; a negative charge (an anion) means electrons were added.
This is also the definition behind isotopes: every isotope of an element shares the same atomic number (same element, same proton count) but can carry a different mass number, because it has a different number of neutrons riding along in the nucleus. Carbon-12 and carbon-14 are both carbon — six protons each — but carbon-14 carries two extra neutrons, which is exactly what makes it useful for radiocarbon dating.
- Enter the atomic number (Z) — the element's position on the periodic table.
- Enter the mass number (A) — protons plus neutrons for the specific isotope you're describing.
- Enter the ionic charge — leave it at 0 for a neutral atom, or enter a positive or negative integer for an ion.
- Read off protons, neutrons, and electrons — each computed directly from the three inputs above.
Worked example — sodium-23, the only stable sodium isotope
Sodium's atomic number is 11, and its one naturally occurring stable isotope has a mass number of 23. For the neutral atom (charge = 0): protons = 11, neutrons = 23 − 11 = 12, and electrons = 11 − 0 = 11. That's sodium metal, silvery and reactive, with eleven electrons arranged as 2, 8, 1 — a single lonely electron in the outer shell that sodium gives up eagerly, which is exactly why sodium reacts so violently with water.
Ionize it — sodium almost always shows up as Na+ in compounds like table salt — and only the electron count changes: electrons = 11 − 1 = 10, matching the stable electron arrangement of neon. The proton and neutron counts stay exactly as they were, because losing an electron doesn't touch the nucleus at all.
Questions
How do you find the number of neutrons in an atom?
Subtract the atomic number from the mass number: neutrons = A − Z. The mass number already counts protons and neutrons together, and the atomic number is just the proton count, so what's left over after subtracting is the neutron count. For sodium-23 (A=23, Z=11), that's 23 − 11 = 12 neutrons.
Why do protons equal electrons only in a neutral atom?
Because electric charge has to balance to zero for an atom to be neutral overall, and each proton carries exactly the same magnitude of charge as each electron, just opposite in sign. Add or remove electrons without touching the nucleus and the atom becomes charged — an ion — with the imbalance between protons and electrons equal to the ionic charge.
What's the difference between atomic number and mass number?
Atomic number (Z) counts only protons and defines which element you're looking at — it never changes for a given element. Mass number (A) counts protons plus neutrons for one specific nucleus, and it does change between isotopes of the same element, since isotopes differ only in neutron count. Every carbon atom has Z=6; carbon-12 and carbon-14 differ in A because they carry 6 and 8 neutrons respectively.
How does isotope notation like carbon-14 relate to atomic and mass number?
The number after the element name is the mass number, A. Carbon-14 means an atom with atomic number Z=6 (it's still carbon) and mass number A=14, so it has 14 − 6 = 8 neutrons rather than the more common 6 neutrons in carbon-12. Chemists also write this as a superscript before the symbol, ¹⁴C, with atomic number as a smaller subscript when both need to be shown.
Does a negative ionic charge mean the atom lost neutrons?
No — ionic charge is entirely about electrons, never neutrons. A negative charge means the atom gained extra electrons (more electrons than protons), which is what makes anions like Cl⁻ or S²⁻. Neutron count is set independently by which isotope you're dealing with, and it stays fixed regardless of how many electrons the atom picks up or loses to become an ion.