Quick Answer
An atom calculator finds the number of protons, neutrons and electrons in any atom or ion using three identities: atomic number Z = protons, mass number A = protons + neutrons, and charge = protons − electrons. Enter any one value and the calculator auto-fills the remaining five fields for all 118 elements.
How to Use This Calculator
Every field is both an input and an output. Type into any one of the six boxes and the other five re-derive themselves, so there is no “calculate” button and no fixed direction to work in.
- Pick an element from the dropdown to auto-fill everything, or leave it on “None” and type the numbers yourself.
- Change the mass number A to switch isotope. Carbon starts at A = 12; type 14 and the neutron count moves from 6 to 8 on its own.
- Change the charge to make an ion. A charge of −1 adds one electron, +3 removes three. The result card relabels itself as a cation or an anion.
- Or work backwards. Type a neutron count and A updates. Type an electron count and the charge updates. Type a proton count and the element itself changes.
- Read the result card for the isotope symbol, the full name with its mass number, and whether you have built a neutral atom or an ion.
Nothing is sent anywhere — the periodic table is built into the page and every calculation happens in your browser.
The Three Identities
The whole tool is three equations. Everything else is presentation:
| Identity | Meaning |
|---|---|
| Z = p | The atomic number is the proton count. Change the protons and you have changed element. |
| A = p + n | The mass number is protons plus neutrons — a whole-number count of nuclear particles. |
| z = p − e | Charge is protons minus electrons. More electrons than protons gives a negative charge. |
Because these are identities rather than a one-way formula, editing any single field leaves exactly one consistent answer for the rest. Typing 8 into the neutron box for carbon can only mean A = 14; the calculator does not have to guess what you meant.
Worked Examples
| What you enter | p | n | e | Result card |
|---|---|---|---|---|
| Z 6, A 12, charge 0 | 6 | 6 | 6 | 12C — Carbon-12, neutral atom |
| Change A to 14 | 6 | 8 | 6 | 14C — Carbon-14, neutral atom |
| Z 17, A 35, charge −1 | 17 | 18 | 18 | 35Cl− — Chlorine-35, anion (−1) |
| Z 26, A 56, charge +3 | 26 | 30 | 23 | 56Fe3+ — Iron-56, cation (+3) |
| Z 92, A 235, charge 0 | 92 | 143 | 92 | 235U — Uranium-235, neutral atom |
The chloride example is the one students most often get backwards. A −1 charge means the atom has gained an electron, so electrons go up to 18 while protons stay at 17. Iron(III) is the mirror image: it has lost three electrons, so e drops to 23, while the result card goes on reporting iron’s standard atomic mass as 55.845 u — losing electrons changes the charge, not the element or its weight.
Uranium-235 shows something else. The tool reports its standard atomic mass as 238.03 u even though you asked for A = 235, and that is not an error — see below.
Reading the Result Card
The large symbol is the element. Underneath it, the isotope notation puts the mass number as a superscript before the symbol and any charge as a superscript after it, which is the standard chemistry convention: 56Fe3+ is iron with 56 nuclear particles and a 3+ charge. A charge of 1 is written as a bare + or −, without the digit, so a chloride ion reads 35Cl− rather than 35Cl1−.
The “Group · Period” line places the element in the table. The period is the row, which tells you how many electron shells the atom uses. The group is the column, and for the main groups it tells you how many outer electrons the atom brings to bonding — which is why group 17 elements like chlorine so readily take one electron to become anions, exactly as the worked example above shows.
What the Warnings Mean
The calculator refuses to quietly produce nonsense. Four checks run on every keystroke, and each one corresponds to a physical impossibility rather than a formatting preference:
| Warning | What you did | Fix |
|---|---|---|
| Protons must be a non-negative whole number | Left the proton box empty or negative | Enter a proton count of 0 or more |
| Neutrons can’t be negative | Set A below Z — asking for more protons than there are particles | Raise A until A ≥ Z |
| Electrons can’t be negative | Set a charge larger than the proton count | Reduce the charge; an atom cannot lose more electrons than it has |
| No element above 118 protons is known | Entered 119 or more | Stay within 1–118 |
The second one catches the most common slip. Entering a mass number smaller than the atomic number is easy to do when you are typing quickly, and without the check the tool would happily report an atom with −3 neutrons.
What the Element Dropdown Actually Fills In
Picking an element sets A to the rounded standard atomic weight. For most elements that lands on the common isotope, and for eleven common ones checked against their real isotope lists — hydrogen, carbon, nitrogen, oxygen, sodium, aluminium, sulfur, chlorine, potassium, calcium and iron — it does exactly the right thing.
For eight elements it does not, because the standard atomic weight is an abundance-weighted average and the average can fall in a gap between real isotopes:
| Element | Standard atomic weight | Dropdown fills A = | Naturally occurring isotopes |
|---|---|---|---|
| Copper | 63.546 | 64 | 63, 65 |
| Gallium | 69.723 | 70 | 69, 71 |
| Bromine | 79.904 | 80 | 79, 81 |
| Silver | 107.87 | 108 | 107, 109 |
| Antimony | 121.76 | 122 | 121, 123 |
| Europium | 151.96 | 152 | 151, 153 |
| Rhenium | 186.21 | 186 | 185, 187 |
| Iridium | 192.22 | 192 | 191, 193 |
Bromine and silver are the clearest cases. Both are two-isotope elements with near-equal abundances, so their averages sit almost exactly halfway between the two and round into a gap that no natural atom occupies. Standard atomic weights are published by IUPAC’s Commission on Isotopic Abundances and Atomic Weights, and the isotopic compositions behind them are tabulated by NIST.
If you pick copper and leave the defaults, you are describing copper-64 — an isotope that does not occur naturally. For homework about ordinary copper, change A to 63. The dropdown is a starting point, not a claim about what is in the bottle.
Mass Number Is a Count, Atomic Mass Is an Average
These two numbers look similar and mean different things. A is an integer: you cannot have 12.011 nuclear particles. The standard atomic mass is what you weigh out in a laboratory, averaged over the isotope mix found on Earth.
| Isotope | Mass number A | Standard atomic mass | Gap |
|---|---|---|---|
| Carbon-12 | 12 | 12.011 u | 0.011 u |
| Chlorine-35 | 35 | 35.45 u | 0.45 u |
| Copper-63 | 63 | 63.546 u | 0.546 u |
| Uranium-235 | 235 | 238.03 u | 3.03 u |
Carbon barely moves because carbon-12 makes up almost all natural carbon — and carbon-12 is the isotope that defines the unit in the first place. Uranium moves by 3.03 u because natural uranium is overwhelmingly uranium-238, so the average sits far from the 235 you asked about. The calculator shows both on purpose: A describes the atom you built, the atomic mass describes the element as it is found.
Use the mass number when you are counting particles or balancing a nuclear equation. Use the standard atomic mass when you are converting to moles or grams — which is the figure the limiting reactant calculator needs when it turns a mass of reagent into a mole count.
Elements Whose “Atomic Weight” Is Really a Mass Number
Thirty-four of the 118 entries carry a whole number rather than a decimal. That is a signal, not a rounding choice: these elements have no stable isotope, so there is no fixed natural mixture to average and the figure quoted is the mass number of a long-lived isotope instead.
Only two of them sit below bismuth — technetium (98) and promethium (145). They are the famous gaps in the periodic table, the two light elements with no stable isotope at all.
Above bismuth there are exactly three exceptions that keep a real decimal weight: thorium (232.04), protactinium (231.04) and uranium (238.03). All three still occur naturally in the Earth’s crust at a consistent isotopic composition, so an average is meaningful even though none of their isotopes is stable. Bismuth itself keeps its decimal weight at 208.98.
Where the Tool Puts Lutetium and Lawrencium
The result card labels 28 elements as “Lanthanide/Actinide” rather than giving them a group number. That is 14 in each series — lanthanum through ytterbium, and actinium through nobelium.
Lutetium and lawrencium are deliberately not in that set. The calculator places both in group 3 alongside scandium and yttrium, which is the arrangement that keeps each f-block series at 14 elements. Textbooks disagree about this boundary, so it is worth knowing which convention a tool follows before you copy an answer out of it.
The calculator stops at 118 because oganesson is the heaviest element that has been confirmed. Type 119 protons and it says so rather than inventing an element name.
Frequently Asked Questions
Neutrons are the mass number minus the proton count: n = A − Z. Enter the element and its mass number and the calculator does the subtraction for you. Carbon with A = 14 gives 14 − 6 = 8 neutrons. Hydrogen-1 is the only ordinary atom with none at all — it is the single element whose default fill-in has a neutron count of zero.
Because charge is protons minus electrons. Electrons carry the negative charge, so gaining one makes the total more negative. Chlorine with a −1 charge has 17 protons and 18 electrons. It is still chlorine — only a proton change would alter the element.
The mass number A is a whole-number count of protons plus neutrons in one specific atom. The standard atomic mass is the average mass of the element as found in nature, weighted by how common each isotope is, and it is almost never a whole number. Chlorine-35 has A = 35 while chlorine’s standard atomic mass is 35.45 u — a gap of 0.45 u, caused by the chlorine-37 in the mix.
The dropdown fills A with the rounded standard atomic weight, and copper’s is 63.546, which rounds to 64. Copper’s naturally occurring isotopes are 63 and 65, so 64 is not one of them. The same happens for gallium, bromine, silver, antimony, europium, rhenium and iridium. Type the mass number you actually want and every other field follows.
Yes — that is the point of the layout. All six fields are editable and none is privileged. Enter 26 protons and 30 neutrons and the calculator reports iron-56. Enter electrons instead and it reports the charge. There is no input side and output side.
Those elements have no stable isotope, so there is no settled natural mixture to average. The figure shown is the mass number of a long-lived isotope instead. Thirty-four of the 118 entries are like this. Technetium (98) and promethium (145) are the only two below bismuth; above it, thorium, protactinium and uranium are the three exceptions that still carry a genuine averaged weight.
No. Only the proton count decides the element, because Z = p. Changing neutrons gives you a different isotope of the same element — carbon-12 and carbon-14 are both carbon. Changing electrons gives you an ion of the same element. Change the protons and the symbol in the result card changes with them.
Oganesson, element 118, is the heaviest element confirmed so far, so 118 is the end of the built-in table. Enter more and the calculator warns you instead of guessing. Elements beyond it have been attempted but none is currently recognised, so there is no symbol, name or mass to report.
