Mass number and atomic number

The atomic number of an atom is the number of protons in its nucleus. The mass number counts the protons and neutrons together. Subtract the atomic number from the mass number and you get the number of neutrons. The atomic number fixes the element, and atoms of the same element with different mass numbers are called isotopes.

What are mass number and atomic number?

AQA GCSE Physics puts it like this: every atom of a given element has the same proton count, and that count is the atomic number; the total number of protons and neutrons is the mass number. Protons and neutrons are both nucleons, the particles in the nucleus.

Reading nuclear notation

The mass number is written at the top left of the chemical symbol and the atomic number at the bottom left. For sodium, ²³₁₁Na means 11 protons and 23 nucleons in total. The atomic number is sometimes left out, as in Na-23 or sodium-23, because the symbol already tells you the element.

How to find protons, neutrons and electrons

1. Protons = atomic number.
2. Neutrons = mass number − atomic number.
3. Electrons = atomic number in a neutral atom, because a neutral atom carries no overall charge, so its electrons and protons balance.

Nearly all the mass of the atom sits in its nucleus, which is why the mass number counts only protons and neutrons. Build atoms and change their particles in the Atom Builder.

Isotopes

Atoms of the same element can have different numbers of neutrons. These are isotopes of that element: same atomic number, different mass numbers.

All three are carbon because each has 6 protons. Spot isotopes from their notation in this question from the Atomic Structure course.

Ions: when electrons change

Losing or gaining electrons does not change the atomic number or the mass number, only the charge. Atoms become positive ions if they lose one or more outer electrons, and negative ions if they gain electrons. So for an ion, electrons = atomic number − charge. See what is an ion.

Mass number and atomic number in nuclear decay

In a nuclear equation, the mass numbers on each side must add up to the same total, and so must the atomic numbers. AQA GCSE Physics asks you to balance them for alpha and beta decay:

• Alpha decay: the nucleus loses an alpha particle, ⁴₂He, so the mass number falls by 4 and the atomic number by 2.
• Beta decay: a neutron turns into a proton and an electron is emitted, so the atomic number rises by 1 and the mass number stays the same.
• Gamma emission: neither number changes.

Because the atomic number changes in alpha and beta decay, the atom becomes a different element. More in alpha, beta and gamma decay.

Worked examples

Example 1: count the particles

How many protons, neutrons and electrons are in an atom of chlorine-37, atomic number 17?

• Protons = 17.
• Neutrons = 37 − 17 = 20.
• Electrons = 17 in a neutral atom.

Example 2: an ion

Find the numbers of particles in the ion ²⁷₁₃Al³⁺.

• Protons = 13 and neutrons = 27 − 13 = 14.
• The 3+ charge means 3 electrons have been lost: electrons = 13 − 3 = 10.

Example 3: balancing an alpha decay

Thorium-232 (atomic number 90) emits an alpha particle. Find the mass number and atomic number of the new nucleus.

• Mass number: 232 − 4 = 228.
• Atomic number: 90 − 2 = 88.
• Check: 232 = 228 + 4 and 90 = 88 + 2. Atomic number 88 is radium, so the product is radium-228.

Example 4: balancing a beta decay

Sodium-24 (atomic number 11) decays by beta emission. Find the mass number and atomic number of the new nucleus.

• Mass number stays 24; atomic number rises to 11 + 1 = 12.
• The product has 12 protons, so it is magnesium-24. The electron emitted is written ⁰₋₁e, which keeps both totals balanced.

Common mistakes

• Swapping the numbers. The mass number is the larger one (except for hydrogen-1, where both are 1); it goes on top.
• Adding electrons to the mass number. Electrons have very little mass and are not counted.
• Thinking isotopes are different elements. They have the same atomic number, so they are the same element.
• Changing the atomic number for an ion. Only the number of electrons changes.
• Confusing mass number with relative atomic mass. Mass number is a whole number for one atom; relative atomic mass is an average over an element's isotopes.

Practise with a short timed paper

Mass number, atomic number and isotopes are in AQA GCSE Physics (4.4.1.2), Chemistry and Combined Science. The daily GCSE Physics Higher and GCSE Combined Science Higher papers are short timed papers of 8 questions, with a new one every day. For averages across isotopes, see relative atomic mass.

Read next

Chemistry · CoursesAtomic structure course: atoms, isotopes and electron shellsAn interactive atomic structure course in 23 short sections: models of the atom, subatomic particles, isotopes, relative atomic mass, shells and ions.