Atom Calculator

Explore the building blocks of matter instantly

Calculate protons, neutrons, electrons, and more with our smart chemistry tool

Last updated: December 3, 2025

CreatorFrank Zhao

⚛️ Atomic Properties

Atomic Number (Z)

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Mass Number (A)

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Charge (z)

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─── AND / OR ───

🧬 Atomic Composition

Number of Protons (p)

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Number of Neutrons (n)

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Number of Electrons (e)

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Introduction / Overview

This Atom Calculator helps you quickly connect the “core numbers” that describe an atom: atomic number, mass number, protons, neutrons, electrons, and ionic charge. It’s made for learning, homework checks, and quick sanity checks in lab notes.

What problems does it solve?

Find neutrons when you know mass number and atomic number.
Determine ionic charge when you know protons and electrons (and vice versa).
Cross-check that your numbers are physically consistent (for example, mass number can’t be below atomic number).

If you’re working specifically with isotopes and average atomic weights from the periodic table, our Atomic Mass Calculator is a helpful companion.

How to Use / Quick Start Guide

You don’t need to fill every box. Enter the values you know, and the calculator fills the rest. For the best results, start with the two numbers you’re most confident about.

Enter atomic number (or protons)

Atomic number is the count of protons in the nucleus. If you type one, the other should match.

Add mass number or neutrons

Mass number is protons + neutrons. Enter either one, and the calculator can derive the remaining value.

Optionally enter electrons (for ions)

If electrons don’t equal protons, you have an ion and the calculator will show the net charge.

Example 1: Find neutrons

Suppose you’re working with Sodium-23:

Mass number $A$ = 23, atomic number $Z$ = 11.

N = A - Z

=

23 - 11

=

12

Result: neutrons $N$ = 12.

Example 2: Find ionic charge

Suppose an atom has 11 protons and 10 electrons:

q = Z - e

=

11 - 10

=

+1

Result: charge $q$ = +1, which you can write as $\mathrm{Na}^+$ .

How to read the results

If electrons equal protons, the atom is neutral and $q = 0$ .
If electrons are fewer than protons, the charge is positive (a cation).
If mass number is below atomic number, something is off — double-check your inputs.

Real-World Examples / Use Cases

Here are a few “this actually comes up” scenarios. Each one includes the kind of numbers people usually have on hand.

1) Homework: isotope neutron count

Background: identify the isotope composition for Chlorine-37.

Inputs: $A = 37$ , $Z = 17$ .

Result: $N = 37 - 17 = 20$ .

How to use it: write the isotope as ${}^{37}\mathrm{Cl}$ .

2) Chemistry: ion charge check

Background: you’re told an oxygen ion has 10 electrons.

Inputs: $Z = 8$ , $e = 10$ .

Result: $q = 8 - 10 = -2$ → $\mathrm{O}^{2-}$ .

How to use it: match common ionic charges in compounds.

3) Lab notes: consistency sanity check

Background: you copied an isotope label and want to confirm it’s possible.

Inputs: $A = 12$ , $Z = 13$ .

Result: impossible because $A < Z$ .

How to use it: catch transcription mistakes before turning in work.

4) Quick study: neutral vs ion

Background: you’re reviewing why atoms become ions.

Inputs: $Z = 12$ , $e = 12$ .

Result: $q = 0$ (neutral magnesium atom).

How to use it: compare to $\mathrm{Mg}^{2+}$ where $e = 10$ .

After you’ve nailed the structure numbers, you may also want the average atomic weight used on periodic tables. That’s exactly what our Atomic Mass Calculator focuses on.

Common Scenarios / When to Use

Especially useful when…

You have two values and need the rest fast.
You’re checking whether an ion label makes sense.
You want a quick “did I copy this right?” sanity check.

May be less appropriate when…

You need the periodic-table “average atomic weight” (a decimal) instead of mass number.
You’re doing nuclear binding-energy / mass defect calculations (this tool keeps it introductory).

Pair it with: if your class asks for average atomic weight and isotopic abundance, try the Atomic Mass Calculator.

Tips & Best Practices

Avoid common mistakes

Don’t confuse “atomic mass” (periodic table average) with mass number $A$ (an integer).
Make sure $A$ is at least $Z$ .
For neutral atoms, electrons should match protons, so $q = 0$ .

Get the most accurate result

Prefer entering the values you’re certain about (often atomic number and mass number).
If you’re calculating ions, double-check the electron count — that’s where sign errors happen.
Use integer counts; if you see decimals, you probably meant average atomic weight instead.

Calculation Method / Formula Explanation

The calculator is based on a few simple relationships. Think of them as a tiny system of equations — once you know enough values, the rest are determined.

Core relationships

A = Z + N

q = Z - e

$A$ : mass number (protons + neutrons)

$Z$ : atomic number (protons)

$N$ : neutron count

$e$ : electron count

$q$ : net charge (positive for cations, negative for anions)

Worked multi-step example (wrapped for mobile)

Given $A = 23$ and $Z = 11$ , find neutrons.

N

=

A - Z

=

23 - 11

=

12

So $N = 12$ .

Related Concepts / Background Info

Mass number vs. atomic mass

Mass number $A$ is a whole number for a specific isotope. Atomic mass (periodic table value) is often a decimal because it’s a weighted average across naturally occurring isotopes.

Why neutrons matter

Neutrons help stabilize the nucleus. Change the neutron count and you change the isotope — sometimes from stable to radioactive.

What “charge” really means

Charge is just the imbalance between protons and electrons. If $e < Z$ , the ion is positive; if $e > Z$ , it’s negative.

Frequently Asked Questions (FAQs)

Is atomic number the same as protons?

Yes. Atomic number is the proton count, so $Z = p$ .

What’s the difference between mass number and atomic mass?

Mass number $A$ is an integer for a single isotope. Atomic mass is typically a decimal average shown on periodic tables.

Can neutrons be negative?

No. If you compute $N = A - Z$ and get a negative number, one of the inputs is wrong.

If electrons change, does the element change?

No. The element is defined by protons (atomic number). Changing electrons changes charge, not the element.

Why does my ion show a plus sign?

Because $q = Z - e$ is positive when protons outnumber electrons.

Does electron mass matter in mass number?

For mass number calculations, we treat electron mass as negligible; mass number is defined as nucleons: $A = p + N$ .

Limitations / Disclaimers

This calculator is for education and quick checks. It does not replace a textbook, lab manual, or professional guidance.

Uses introductory relationships like $A = Z + N$ and $q = Z - e$ .
Does not model nuclear binding energy, mass defect, or relativistic effects.
If you need periodic-table averages and isotopic abundance, use the Atomic Mass calculator instead.

External References / Sources

If you want to go deeper (or cite something for a report), these are solid starting points:

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