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Atom Calculator

Created by Anna Pawlik, PhD candidate
Reviewed by Bogna Szyk and Adena Benn
Last updated: Jan 18, 2024

The atom calculator is a tool for calculating the atomic number and the mass number based on the number of atom components - protons, neutrons, and electrons (or vice versa). In addition, you can define the charge of ions with known numbers of protons and electrons. This article will provide you with the following:

  • Definitions of:
    • Atom;
    • Atom components;
    • Atomic number; and
    • Mass number.
  • The equations used to calculate the number of:
    • Protons;
    • Neutrons;
    • Electrons;
    • Atomic number;
    • Atomic mass and
    • Atomic charge.
  • Lastly, a short step-by-step tutorial on how to calculate:
    • Atomic number;
    • Atomic mass; and
    • Charge.

What is an atom?

🙋 An atom is the smallest constituent unit of matter that retains the properties of an element.

A typical atom consists of a nucleus and electron cloud. Atom components are positively-charged protons and electrically neutral neutrons in the nucleus with negatively-charged electrons orbiting this nucleus. Protons and neutrons are called nucleons.

Atoms are electrically neutral because they possess an equal number of electrons and protons. However, when an atom has an unequal number of protons and electrons, it has an electrical charge and creates ions.

🙋 If an ion has more protons than electrons, it is positively charged and is called a cation. Conversely, if an ion has more electrons than protons, it is negatively charged and is called an anion.

Some atoms can split into smaller parts (lighter nuclei). You can use our half-life calculator to calculate the number of nuclei remaining after a particular time.

Atomic number, atomic mass

Atom is uniquely identified with the atomic symbol Z. The atomic number is the number of protons present in the nucleus. In the case of uncharged atoms, the atomic number is equal to the number of electrons.

In contrast, in ions, the number of electrons is the difference between the atomic number and the charge of ions.

In cations, there are fewer electrons than protons, while in anions, there are more electrons than protons.

The number of neutrons in a given element, however, can vary. The variants of the same chemical element with a different number of neutrons are called isotopes. Protons and neutrons determine the mass of an atom. Mass number symbol A is the sum of the number of protons and neutrons.

It is vital to know the number of atoms that make up a molecule, as it serves as the base for many estimations such as degree of unsaturation (see the degree of unsaturation calculator). It provides information about the structure of a molecule.

Equations used to calculate the numbers of protons, neutrons, electrons, atomic mass, and atomic charge

  • If you know the atomic number (Z), atomic mass (A), and charge (z), you can calculate the numbers of protons (p), neutrons (n), and electrons (e) using the given mathematical equations 1-3:

    1. p = Z

    2. n = A - Z

    3. e = Z - z

  • On the other hand, if you know the number of protons (p), neutrons (n), and electrons (e), you can calculate the atomic number (Z), atomic mass (A), and charge (z) using equations 4-6:

    1. Z = p

    2. A = p + n

    3. z = p - e

How to calculate the atomic number, mass and charge

  1. Choose your element. Let's assume that it is the atom of sodium (Na). The charge is 0.

  2. Find the atomic number (Z) and mass number (A). They are equal to 11 and 23, respectively.

  3. Calculate numbers of protons, neutrons, and electrons by using mathematical expressions (1-3):
    p = 11 (1)

    n = 23 - 11 = 12 (2)

    e = 11 - 0 = 11 (3)

  4. Alternatively, you can also calculate the atomic number, atomic mass, and charge.

  5. Choose your element. Let's assume that it is the sulfide anion.

  6. Find the numbers of protons, neutrons, and electrons. They are equal to 16, 16, and 18, respectively.

  7. Calculate atomic number, atomic mass, and charge by using mathematical expressions (4-6):

    Z = 16 (4)

    A = 16 + 16 = 32 (5)

    z = 16 - 18 = -2 (6)

Of course, you don't have to perform all these calculations by hand! Type any three values into this atom calculator and watch it do all the work for you.

Did you find this calculator helpful? We also have a mass percent calculator that may interest you.


What are atoms made of?

Atoms are made of three kinds of particles: neutrons, protons, and electrons. Protons and neutrons form the nucleus of the atom, and electrons circulate around the nucleus. Electrons are negatively charged, and protons are positively charged. Normally, an atom is electrically neutral because the number of protons and electrons are equal.

How can I find an atomic mass?

If you want to find the atomic mass, try this trick:

  1. Find the number of protons (atomic number).

  2. Find the number of neutrons by subtracting the number of protons from the mass number.

  3. Add the protons and neutrons.

  4. Omit the electrons, as their combined mass is very small compared to the mass of the nucleus.

  5. Congratulations! Now you have the atomic mass of that atom.

What is the mass number of an atom of helium that contains 2 neutrons?

4. The number of neutrons is 2, and the helium atom always has 2 protons (check the atomic number 2 of helium). Thus, the mass number is the sum of protons and neutrons in the nucleus: 2 + 2 = 4.

Which elementary particle gives an atom its identity?

The number of protons gives an atom its identity. Protons are positively charged particles that are found in the nucleus of an atom. No two elements have the same number of protons. An atom with 1 proton is a hydrogen atom; an atom with 11 protons is a sodium atom, and so on.

What is the atomic number for 6 protons, 7 neutrons, and 6 electrons?

6. The atomic number is the number of protons in the nucleus. You can identify an element by its atomic number, so in this case, it would be carbon – C. Summing up protons and neutrons 6 + 7 = 13, you will receive the isotope 13C (carbon-13).

Who discovered atom?

Atoms were discovered by John Dalton in 1808. This English chemist and physicist developed the atomic theory, which states that all the atoms of an element are identical in size and mass. He determined that the elements were made up of smaller atoms and that the basic unit of mass was the mass of one hydrogen atom.

Where do chemical reactions mainly occur in atoms?

Chemical reactions usually take place between the electrons furthest from the nucleus. The nucleus of an atom usually remains intact during these reactions. The characteristic of a chemical reaction is that it changes the configuration of atoms, which can result, for example, in the formation of new chemical compounds.

Which elementary particle gives an atom its charge?

The answer is the protons and the electrons. The protons have a positive charge, while electrons are negatively charged. If you remove one or more electrons from the atom, you will receive a positive ion called a cation as you will leave more protons than electrons. Similarly, by adding electrons, you will gain a negative ion, called an anion. Normally, you cannot add or remove protons and neutrons in the nucleus.

Anna Pawlik, PhD candidate
Related calculators
Atomic number
Mass number
Atomic composition
Number of protons
Number of neutrons
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