Find the building blocks of any atom or ion. Enter the values you know — atomic number, mass number, charge, protons, neutrons, or electrons — and the atom calculator fills in the rest and identifies the element. The three core relationships are Z = protons, mass number = protons + neutrons, and charge = protons − electrons.
The atom calculator is a fast, reliable way to work out the composition of any atom or ion. Give it the values you already know — the atomic number, the mass number, the charge, or the raw counts of protons, neutrons, and electrons — and it calculates everything else and tells you which element you are looking at. It is built for students learning atomic structure, teachers preparing examples, and anyone who needs to check the particle count of an atom quickly and correctly.
Behind the scenes the calculator relies on just three simple relationships that connect the six quantities. Once you understand them, you can reason about any atom by hand — and this page explains each one in plain language, with worked examples you can reproduce, a tour of atomic structure, and answers to the questions that come up most often.
Every atom is built from three kinds of subatomic particle. Protons are positively charged particles found in the nucleus; the number of protons defines the element and never changes without changing the element itself. Neutrons are neutral particles that also sit in the nucleus, adding mass and helping to hold it together. Electrons are tiny, negatively charged particles that occupy the space around the nucleus in a cloud of orbitals. Protons and neutrons are roughly equal in mass and account for almost all of an atom's weight, while electrons are nearly 2,000 times lighter and contribute very little mass but everything to chemistry, because it is the electrons that form bonds.
The atom calculator connects six quantities using three equations. The quantities are the atomic number (Z), the mass number (A), the charge, and the counts of protons (p), neutrons (n), and electrons (e).
Because these equations work in both directions, you can supply whichever three values you know and the calculator solves for the other three. Enter protons, neutrons, and electrons to get the atomic number, mass number, and charge — or go the other way.
Students are most often asked to find the three particle counts from an element's place on the periodic table. Here is the reliable recipe the calculator automates:
Sodium has an atomic number of 11 and a mass number of 23, and in its neutral form it has no charge. Applying the formulas:
So neutral sodium has 11 protons, 12 neutrons, and 11 electrons. These are the calculator's default values — press Calculate to see the full breakdown, including the isotope notation 23Na.
Now suppose you know the particle counts and want the summary numbers. A sulfide ion has 16 protons, 16 neutrons, and 18 electrons. The calculator works backward:
The two extra electrons give the ion a 2− charge, written S²⁻. This is exactly how the calculator distinguishes an ion from a neutral atom.
Helium has 2 protons and 2 neutrons. Its mass number is simply p + n = 2 + 2 = 4, and because it is neutral it has 2 electrons and a charge of 0. Helium-4 is the most common isotope in the universe and the gas that makes balloons float.
These two numbers are easy to mix up but mean very different things. The atomic number counts only protons and defines the element — it is a whole number that never changes for a given element. The mass number counts protons plus neutrons and can differ between atoms of the same element, because the neutron count can vary. On the periodic table the atomic number is the small integer usually shown above the symbol, while the atomic mass (a decimal average of all natural isotopes) appears below it. To find neutrons you round that atomic mass to the nearest whole number to get a typical mass number, then subtract the atomic number.
Atoms of the same element always have the same number of protons, but they can have different numbers of neutrons. These variants are called isotopes. Carbon, for example, exists mainly as carbon-12 (6 neutrons) but also as carbon-13 (7 neutrons) and the radioactive carbon-14 (8 neutrons) used in radiocarbon dating. Isotopes of an element behave almost identically in chemical reactions, since chemistry is governed by electrons, but they differ in mass and sometimes in nuclear stability. The atom calculator makes exploring isotopes easy: keep the atomic number fixed and change the mass number to see how the neutron count shifts.
An atom is electrically neutral when it has the same number of protons and electrons. When it gains or loses electrons it becomes an ion with a net charge. Losing electrons creates a positively charged cation (for example, a sodium atom that loses one electron becomes Na⁺), while gaining electrons creates a negatively charged anion (a chlorine atom that gains one electron becomes Cl⁻). The number of protons never changes when an ion forms — only the electron count does — so the element stays the same. The calculator reports the charge automatically whenever the electron count differs from the proton count, and writes the ion in standard notation such as Na⁺ or S²⁻.
Because no two elements share the same number of protons, the atomic number alone is enough to name the element. The calculator contains the full periodic table from hydrogen (Z = 1) to oganesson (Z = 118) and simply looks up the symbol and name that correspond to the number of protons you provide. That is why entering just the protons — or anything that lets the calculator work out the protons — is enough to identify the atom. If you enter an atomic number above 118, no known element matches, and the calculator will still report the particle counts while noting that the element is not on the current periodic table.
Understanding what an atom is made of is the foundation of all of chemistry and much of physics. The number of protons decides which element you have and therefore its basic character. The arrangement of electrons determines how atoms bond, which shapes every molecule from water to DNA. The number of neutrons affects mass and nuclear stability, underpinning everything from radiocarbon dating to nuclear power and medical imaging. Whether you are balancing equations, predicting reactions, or studying radioactivity, it all starts with counting these three particles correctly — and that is exactly what this calculator is designed to help you do.
Chemists have a compact shorthand for describing an atom, and the calculator writes its answer in exactly that form. In full isotope notation the element symbol carries two numbers on its left: the mass number as a superscript on top and the atomic number as a subscript below, like 2311Na for sodium-23. Because the symbol already tells you the element — and therefore the atomic number — the subscript is often dropped, giving the shorter "sodium-23" or 23Na. When the atom is an ion, the charge is written as a superscript on the right: a single positive charge is shown as a plus sign (Na⁺), a double negative as 2− (S²⁻), and so on. Learning to read this notation lets you decode any atom at a glance: the top-left number is the total nucleon count, the bottom-left is the proton count, and the top-right, if present, is the net charge. The calculator assembles all of these automatically so you can see the standard scientific representation of whatever atom you enter.
A few errors trip up students again and again. The most common is confusing the mass number with the atomic mass shown on the periodic table: the atomic mass is a decimal average across all natural isotopes, while the mass number is a whole-number count of nucleons for one specific atom, so you must round before subtracting to find neutrons. Another frequent slip is forgetting that ions change the electron count but never the proton count — adding or removing electrons makes an ion, but the element stays the same because its protons are unchanged. People also sometimes assume that a neutral atom must have equal numbers of protons, neutrons, and electrons; in fact only the protons and electrons are equal in a neutral atom, while the neutron count is independent and often different. Finally, watch the sign of the charge: gaining electrons makes the charge more negative, which feels backwards until you remember that electrons themselves are negative. Keeping these distinctions straight — and letting the calculator check your arithmetic — makes atomic bookkeeping far less error-prone.
Subtract the atomic number from the mass number: neutrons = mass number − atomic number. For sodium-23 that is 23 − 11 = 12 neutrons.
A neutral atom has the same number of electrons as protons. For an ion, subtract the charge from the number of protons: electrons = protons − charge.
The atomic number is the number of protons and defines the element. The mass number is protons plus neutrons and can vary between isotopes of the same element.
Yes. The number of protons is the element's identity — change it and you have a different element. Changing neutrons gives an isotope; changing electrons gives an ion, but both keep the element the same.
A negative charge means the atom has more electrons than protons — it has gained electrons to become a negative ion (anion). A positive charge means it has lost electrons to become a positive ion (cation).
Yes. The calculator is fully bidirectional. Enter any consistent set of values — for example protons, neutrons, and electrons — and it will compute the atomic number, mass number, and charge and identify the element.
This Atom Calculator is provided for general educational purposes. It uses the standard definitions relating atomic number, mass number, charge, protons, neutrons, and electrons, and identifies elements by atomic number using the current periodic table. Mass number here is the whole-number count of nucleons, not the decimal atomic mass shown on the periodic table.