Atom Calculator

Pick an element, enter a mass number and optional ionic charge, and instantly see its protons, neutrons, electrons, and a Bohr shell diagram.

⚛️ Atom Calculator
Isotope / Ion
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Protons
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Neutrons
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Electrons
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Standard Atomic Mass
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⚛️ What is an Atom Calculator?

An atom calculator finds the number of protons, neutrons, and electrons in any atom or ion once you know the element (which fixes the atomic number Z), the mass number A (protons plus neutrons), and the ionic charge (which determines how many electrons the species actually has compared to a neutral atom). It turns three simple whole-number relationships, protons = Z, neutrons = A - Z, and electrons = Z - charge, into an instant lookup.

This kind of calculation is one of the very first skills built in general chemistry: identifying isotopes from isotope notation like carbon-14, working out the composition of common ions like Na+ or Cl-, checking homework answers on subatomic particle counts, and building intuition for how atomic number, mass number, and charge relate before moving on to electron configuration and bonding.

A frequent point of confusion is mixing up mass number with atomic mass (atomic weight). Mass number is always a whole number specific to one isotope, the literal count of protons plus neutrons in that particular atom's nucleus. Atomic mass, shown on the periodic table, is a decimal number representing the abundance-weighted average across all of that element's naturally occurring isotopes, see the related Average Atomic Mass Calculator for how that average is actually computed.

This calculator handles both neutral atoms and ions in one tool, and adds a simplified Bohr shell diagram so you can see the electron arrangement, not just the raw counts.

📐 Formula

protons = Z     neutrons = A − Z     electrons = Z − charge
Z = atomic number, the fixed number of protons that defines the element
A = mass number, protons plus neutrons in the specific isotope
charge = ionic charge (0 for a neutral atom, positive for a cation, negative for an anion)
Example: for O²⁻ with A = 16, Z = 8: protons = 8, neutrons = 16 − 8 = 8, electrons = 8 − (−2) = 10

📖 How to Use This Calculator

Steps

1
Select the element - choose an element from the dropdown list of the first 40 elements by atomic number.
2
Enter the mass number - enter the mass number A (protons plus neutrons) for the isotope you want, defaults to the rounded standard atomic weight.
3
Enter the ionic charge - enter the ionic charge, use 0 for a neutral atom, a positive number for a cation, or a negative number for an anion.
4
Click Calculate to see protons, neutrons, electrons, isotope notation, and a Bohr shell diagram.

💡 Example Calculations

Example 1 — Neutral oxygen-16

Element = O (Z = 8), mass number A = 16, charge = 0.

1
Protons = Z = 8
2
Neutrons = 16 − 8 = 8. Electrons = 8 − 0 = 8
A neutral oxygen-16 atom has 8 protons, 8 neutrons, and 8 electrons.
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Example 2 — Sodium cation Na+

Element = Na (Z = 11), mass number A = 23, charge = +1.

1
Protons = 11. Neutrons = 23 − 11 = 12
2
Electrons = 11 − 1 = 10
Losing one electron gives sodium-23 a full outer shell and a stable +1 charge, matching common table salt chemistry.
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Example 3 — Chlorine-37 anion

Element = Cl (Z = 17), mass number A = 37, charge = −1.

1
Protons = 17. Neutrons = 37 − 17 = 20
2
Electrons = 17 − (−1) = 18
This heavier chlorine isotope gains one electron to form Cl⁻, the anion found in many chloride salts.
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❓ Frequently Asked Questions

How do you find the number of protons in an atom?+
The number of protons always equals the atomic number Z, which is fixed for each element and found on the periodic table. Every atom of carbon has exactly 6 protons, every atom of oxygen has exactly 8, protons never change without the atom becoming a different element entirely.
How do you find the number of neutrons in an atom?+
Neutrons equal the mass number A minus the atomic number Z, neutrons = A - Z. The mass number is the total count of protons plus neutrons in the nucleus, specific to one isotope, so you need to know or choose a particular isotope's mass number to find its neutron count.
How do you find the number of electrons in an atom or ion?+
For a neutral atom, electrons equal the atomic number Z (protons and electrons balance out to a net zero charge). For an ion, subtract the ionic charge from Z, electrons = Z - charge, so a cation (positive charge) has fewer electrons than protons and an anion (negative charge) has more.
What is the difference between atomic number and mass number?+
Atomic number (Z) counts only protons and identifies the element, it never changes for a given element. Mass number (A) counts protons plus neutrons in one specific atom's nucleus, and can vary between isotopes of the same element, for example carbon-12 and carbon-14 both have Z = 6 but different A.
What is an isotope?+
Isotopes are atoms of the same element (same atomic number Z, same proton count) that have different numbers of neutrons, and therefore different mass numbers. Chlorine-35 and chlorine-37 are both chlorine (17 protons each) but have 18 and 20 neutrons respectively.
Why do cations have fewer electrons than protons?+
A cation forms when an atom loses one or more electrons, leaving more protons (positive charges) than electrons (negative charges), giving the ion a net positive charge. Sodium (11 protons, 11 electrons neutral) commonly loses one electron to become Na+ (11 protons, 10 electrons).
Why do anions have more electrons than protons?+
An anion forms when an atom gains one or more extra electrons, giving it more negative charges (electrons) than positive charges (protons), producing a net negative charge. Chlorine (17 protons, 17 electrons neutral) commonly gains one electron to become Cl- (17 protons, 18 electrons).
What is the Bohr model and why is it simplified here?+
The Bohr model represents electrons as orbiting a nucleus in fixed, concentric shells, an intuitive but not fully accurate picture (the true quantum-mechanical picture uses probability orbitals, not fixed circular paths). This calculator uses a common simplified shell-filling order that works well for main-group elements but does not capture the exact subshell order for transition metals.
Can this calculator handle any element?+
This calculator includes a built-in lookup table for the first 40 elements, hydrogen through zirconium, covering all of the most commonly studied elements in introductory and general chemistry. For elements beyond zirconium, the same neutrons = A - Z and electrons = Z - charge relationships still apply manually.
Does mass number equal atomic mass?+
No, they are related but different. Mass number (A) is always a whole number, the count of protons plus neutrons in one specific atom. Atomic mass (or atomic weight) shown on a periodic table is a decimal number, the abundance-weighted average mass across all of an element's naturally occurring isotopes.
Tags: Atom Calculator Protons Neutrons Electrons Calculator Atomic Structure Calculator Bohr Model Calculator Periodic Table Calculator Chemistry Calculator Free Online

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How do you find the number of protons in an atom?

The number of protons always equals the atomic number Z, which is fixed for each element and found on the periodic table. Every atom of carbon has exactly 6 protons, every atom of oxygen has exactly 8, protons never change without the atom becoming a different element entirely.

How do you find the number of neutrons in an atom?

Neutrons equal the mass number A minus the atomic number Z, neutrons = A - Z. The mass number is the total count of protons plus neutrons in the nucleus, specific to one isotope, so you need to know or choose a particular isotope's mass number to find its neutron count.

How do you find the number of electrons in an atom or ion?

For a neutral atom, electrons equal the atomic number Z (protons and electrons balance out to a net zero charge). For an ion, subtract the ionic charge from Z, electrons = Z - charge, so a cation (positive charge) has fewer electrons than protons and an anion (negative charge) has more.

What is the difference between atomic number and mass number?

Atomic number (Z) counts only protons and identifies the element, it never changes for a given element. Mass number (A) counts protons plus neutrons in one specific atom's nucleus, and can vary between isotopes of the same element, for example carbon-12 and carbon-14 both have Z = 6 but different A.

What is an isotope?

Isotopes are atoms of the same element (same atomic number Z, same proton count) that have different numbers of neutrons, and therefore different mass numbers. Chlorine-35 and chlorine-37 are both chlorine (17 protons each) but have 18 and 20 neutrons respectively.

Why do cations have fewer electrons than protons?

A cation forms when an atom loses one or more electrons, leaving more protons (positive charges) than electrons (negative charges), giving the ion a net positive charge. Sodium (11 protons, 11 electrons neutral) commonly loses one electron to become Na+ (11 protons, 10 electrons).

Why do anions have more electrons than protons?

An anion forms when an atom gains one or more extra electrons, giving it more negative charges (electrons) than positive charges (protons), producing a net negative charge. Chlorine (17 protons, 17 electrons neutral) commonly gains one electron to become Cl- (17 protons, 18 electrons).

What is the Bohr model and why is it simplified here?

The Bohr model represents electrons as orbiting a nucleus in fixed, concentric shells, an intuitive but not fully accurate picture (the true quantum-mechanical picture uses probability orbitals, not fixed circular paths). This calculator uses a common simplified shell-filling order that works well for main-group elements but does not capture the exact subshell order for transition metals.

Can this calculator handle any element?

This calculator includes a built-in lookup table for the first 40 elements, hydrogen through zirconium, covering all of the most commonly studied elements in introductory and general chemistry. For elements beyond zirconium, the same neutrons = A - Z and electrons = Z - charge relationships still apply manually.

Does mass number equal atomic mass?

No, they are related but different. Mass number (A) is always a whole number, the count of protons plus neutrons in one specific atom. Atomic mass (or atomic weight) shown on a periodic table is a decimal number, the abundance-weighted average mass across all of an element's naturally occurring isotopes.

📌 Quick Tips

💡The mass number A is not the same as the atomic weight shown on a periodic table, A is a whole number specific to one isotope, atomic weight is a decimal weighted average across all natural isotopes.
💡Every atom of a given element has the same number of protons, that number, the atomic number Z, is what defines the element.
💡A positive ionic charge removes electrons (cations have fewer electrons than protons), a negative charge adds electrons (anions have more electrons than protons).
💡If you only know an element's standard atomic weight, round it to the nearest whole number for a reasonable default mass number, this calculator does exactly that.
💡The Bohr shell diagram here uses the simplified 2-8-8-18-18-32-32 teaching model, it does not reflect the exact quantum-mechanical subshell filling order for transition elements.