Protons Neutrons Electrons Calculator
A positive charge means electrons were lost, not gained.
Work out Protons Neutrons Electrons. A positive charge means electrons were lost, not gained. Shows the working, not just the answer.
Protons plus neutrons — the superscript in carbon-14
Positive for a cation, negative for an anion, zero for a neutral atom
Protons, neutrons, electrons
6p · 8n · 6e⁻
Neutral atom, mass number 14
Changing the neutron count gives a different isotope of the same element. Changing the electron count gives an ion. Only changing the proton count gives a different element.
How the Protons Neutrons Electrons Calculator works
Counts protons, neutrons and electrons from an atomic number, a mass number and a charge, and says which of the three each input controls. The charge sign is where this goes wrong for most people, so the result spells out the direction rather than leaving it implied.
Also known as: how many neutrons does an element have · subatomic particle counter · electrons in an ion calculator · isotope proton neutron count
Which number controls which particle
Three inputs, three outputs, and each input controls exactly one thing. The atomic number Z fixes the proton count and nothing else — it is what makes an element that element, so changing it changes the substance entirely. The mass number A fixes the nucleon total, so neutrons are A minus Z. The charge fixes the electron count relative to the protons.
Carbon-14 illustrates all three at once. Z is 6, so there are 6 protons. A is 14, so there are 14 − 6 = 8 neutrons. The charge is 0, so there are 6 electrons. Change A to 12 and it is still carbon, with 6 neutrons instead of 8. Change Z to 7 and it is nitrogen, whatever else you do.
The reason this matters beyond bookkeeping is that the three properties belong to different physics. Chemical behaviour follows the electrons, radioactive behaviour follows the neutron-to-proton ratio, and elemental identity follows the protons. A calculation that mixes them up produces an answer about the wrong property.
The sign error, and why it is so persistent
Electrons equal Z minus the charge. A sodium ion written Na⁺ has 11 protons and 10 electrons, because the plus sign records that an electron left. Chloride, Cl⁻, has 17 protons and 18 electrons, because the minus records one arriving. Written as a formula: electrons = Z − charge, and subtracting a negative number adds.
The error is persistent because the notation reads backwards from the physics. A plus sign looks like an addition, and the instinct is to add. But the charge describes the imbalance the particle already has, not an operation to perform — the sign tells you which particle is in surplus, and protons are what the plus is a surplus of.
A useful check: metals form cations by losing electrons and end up with fewer electrons than protons; non-metals form anions by gaining them and end up with more. If an answer has a metal ion gaining electrons, the sign went the wrong way. Aluminium as Al³⁺ has 13 protons and 10 electrons, matching neon's configuration, which is exactly why it stops at three.
Mass number against atomic mass
Mass number is a whole number, always. It counts nucleons in one specific atom, and you cannot have a fraction of a neutron. Atomic mass, the number printed on a periodic table, is a weighted average across the isotopes found in nature and is almost never a whole number.
Chlorine makes the distinction concrete. Its two stable isotopes are chlorine-35 and chlorine-37, both with whole-number mass numbers. Natural chlorine is about 76% of the lighter and 24% of the heavier, giving an average atomic mass of 35.45 — a value no individual chlorine atom has.
This is why a question asking for the neutron count of chlorine has no single answer without the isotope specified, and why rounding 35.45 to 35 and computing 35 − 17 = 18 gives the neutron count of the most common isotope by accident rather than by reasoning. State the isotope, then subtract.
Where to go next
The Protons Neutrons Electrons question rarely arrives on its own. These are the ones that usually come with it:
- Average Atomic Mass Calculator — Weighted mass from isotope abundances, with a normalisation check.
- Electron Configuration Calculator — Aufbau order, with the elements that break it built in.
- Nuclear Binding Energy Calculator — Mass defect and binding energy per nucleon, from atomic mass.
- Speed Distance Time Calculator — Any one of the three from the other two, in four units.
Frequently asked questions
How do I find the number of neutrons?
Subtract the atomic number from the mass number. Carbon-14 has a mass number of 14 and an atomic number of 6, so it carries 8 neutrons.
Does a positive charge mean more electrons or fewer?
Fewer. A cation formed by losing electrons has more protons than electrons, so electrons = atomic number − charge. Na⁺ has 11 protons and 10 electrons.
Does a negative charge add electrons?
Yes. Cl⁻ has 17 protons and 18 electrons. The same formula still applies — electrons = Z − charge — because subtracting a negative charge adds one electron.
What is the difference between mass number and atomic mass?
Mass number is a whole-number count of protons plus neutrons in one specific atom. Atomic mass is the weighted average over all natural isotopes, which is why chlorine sits at 35.45 rather than 35 or 37.
Can the mass number be smaller than the atomic number?
No. Mass number counts protons and neutrons together, and neutron count cannot be negative, so the mass number is always at least the atomic number. Hydrogen-1 is the only case where they are equal.
What changes when I change each number?
Change the proton count and you have a different element. Change the neutron count and you have a different isotope of the same element. Change the electron count and you have an ion.
How do I read an isotope symbol?
The superscript before the symbol is the mass number and the subscript is the atomic number. In ¹⁴₆C, 14 is the nucleon total and 6 is the proton count, so there are 8 neutrons. The subscript is often left off because the element symbol already implies it.
Why do isotopes behave the same chemically?
Chemistry is governed by the electrons, and isotopes of an element have identical electron configurations. Only the nuclear mass differs, which affects reaction rates slightly — most noticeably for hydrogen, where deuterium is twice the mass of protium.
What makes an isotope radioactive?
An unfavourable neutron-to-proton ratio. Light elements are stable near a 1:1 ratio and heavier ones need progressively more neutrons; nuclei too far from that band of stability decay to reach it. Every element above lead is radioactive in all its isotopes.
How many electrons can each shell hold?
2n², so 2 in the first shell, 8 in the second, 18 in the third and 32 in the fourth. Filling order does not follow shell order strictly, though — 4s fills before 3d, which is why the transition metals appear where they do.
Why do ions form the charges they do?
Atoms gain or lose electrons to reach a noble gas configuration. Sodium loses one to look like neon; chlorine gains one to look like argon; aluminium loses three. It is why group 1 forms 1+ and group 17 forms 1− with such regularity.
Does the calculator work for any element?
Yes — it works from the atomic number rather than a lookup table, so any Z from 1 to 118 and beyond is handled. The mass number just has to be at least equal to the atomic number, since neutron counts cannot be negative.
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