Mass Number of an Atom: How to Calculate It

This calculator works out the mass number of an atom when you enter the number of protons and neutrons in its nucleus. The mass number is simply the total count of these two types of particles, and it tells you the approximate atomic mass in atomic mass units (u). You get the mass number as a whole number, which is the first step in identifying an isotope.

Formula

Mass number (A) = Number of protons (Z) + Number of neutrons (N)

In symbols: A = Z + N

• A – mass number, a whole number with no unit (often written with the unit u for atomic mass units)
• Z – number of protons in the nucleus (this is the atomic number)
• N – number of neutrons in the nucleus

The mass number is not the same as the atomic mass listed on the periodic table. The periodic table value is a weighted average of all naturally occurring isotopes, so it is usually a decimal. The mass number is always a whole number and applies to a specific isotope.

Worked Example

Using the default values from the calculator above: 6 protons and 6 neutrons.

1. Identify the number of protons: Z = 6
2. Identify the number of neutrons: N = 6
3. Add them together: A = 6 + 6 = 12

The mass number is 12. This is the carbon-12 isotope, the standard for atomic mass units.

Step-by-Step

1. Find the atomic number (Z) – Look up the element on the periodic table. The atomic number is the whole number above the element symbol. It equals the number of protons. For example, oxygen has Z = 8, so it has 8 protons.
2. Determine the number of neutrons (N) – This is not on the periodic table for a specific atom. You need to know which isotope you have. The isotope name or notation usually tells you. For example, carbon-14 has 14 as its mass number, so neutrons = 14 − 6 = 8. If you have a neutral atom, you can also find neutrons by subtracting the atomic number from the mass number given in a nuclide notation like ¹⁴C.
3. Add protons and neutrons – Simply add the two numbers: A = Z + N. The result is the mass number.
4. Write the isotope symbol (optional) – If you need to show the isotope, write the mass number as a superscript before the element symbol, and the atomic number as a subscript. For carbon-14, that is ¹⁴₆C.

Common Mistakes

• Confusing mass number with atomic mass – The atomic mass on the periodic table (e.g., 12.011 for carbon) is an average, not the mass number. The mass number is always a whole number.
• Using electrons – Electrons have almost no mass, so they are never included in the mass number. Only protons and neutrons count.
• Mixing up protons and neutrons – For a neutral atom, the number of protons equals the number of electrons, but neutrons are separate. Always get the neutron count from the isotope notation or a given value.
• Forgetting to subtract for neutrons – If you are given the mass number and the atomic number, neutrons = mass number − atomic number. Do not just use the mass number as the neutron count.

Tips

• Memorize the formula as A = Z + N. It is the same for every atom.
• When reading isotope notation like ²³⁵U, the top number is the mass number, the bottom number (if shown) is the atomic number.
• For a neutral atom, the number of protons equals the number of electrons, but that does not affect the mass number.
• Use the periodic table only for the atomic number; the neutron count varies by isotope.
• If you are unsure about the number of neutrons, look up the specific isotope name (e.g., “carbon-14â€� means mass number 14).

Troubleshooting

• Your result is a decimal? – You probably used the atomic mass from the periodic table instead of whole numbers. Go back and use the atomic number (protons) and the neutron count for the specific isotope.
• You get a negative number of neutrons? – That means the mass number you used is smaller than the atomic number. Check your isotope notation – the mass number must be at least as large as the atomic number.
• You are not sure which isotope you have? – Look for a hyphen notation like “oxygen-18â€� or a superscript in a nuclide symbol. If none is given, assume the most common isotope, but state that assumption.
• Your calculator shows a different answer than your textbook? – Verify that you entered the correct number of protons for the element and the correct neutron count. Also check that you did not accidentally add the atomic mass instead.

Frequently Asked Questions

What is the difference between mass number and atomic number?

The atomic number (Z) is the number of protons, which defines the element. The mass number (A) is the total number of protons plus neutrons. Two atoms of the same element can have different mass numbers because they have different numbers of neutrons (isotopes).

Why is the mass number not on the periodic table?

The periodic table shows the average atomic mass of all naturally occurring isotopes, which is usually a decimal. The mass number is specific to a single isotope, so it cannot be listed for the element as a whole.

Do electrons affect the mass number?

No. Electrons are about 1/1836 the mass of a proton or neutron, so their contribution to the total mass is negligible. The mass number counts only protons and neutrons.

How do I find the number of neutrons if I know the mass number?

Subtract the atomic number from the mass number: N = A − Z. For example, if an isotope has mass number 37 and atomic number 17 (chlorine), then neutrons = 37 − 17 = 20.

Can the mass number be the same for two different elements?

Yes. Atoms with the same mass number but different atomic numbers are called isobars. For example, carbon-14 and nitrogen-14 both have mass number 14, but they are different elements.

Is the mass number equal to the atomic mass in grams?

No. The mass number is a count of particles, so it is dimensionless. The atomic mass in grams is the actual mass of one atom, which is an extremely small number. The mass number is numerically close to the atomic mass in atomic mass units (u), but not exactly equal due to binding energy.

Related

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in p | Protons | - | 6 | 0
in n | Neutrons | - | 6 | 0
out mass | Mass number | u | p + n | 0