Mass Number, Atomic Number and Isotopes

This lesson covers mass number, atomic number and isotopes as required by the AQA GCSE Physics specification (4.4.1). Understanding these concepts is essential for interpreting the periodic table, writing nuclear equations, and explaining radioactive decay processes.

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Atomic Number (Z)

The atomic number (also called the proton number) is the number of protons in the nucleus of an atom. It is given the symbol Z.

Key facts:

• The atomic number defines the element. All atoms of the same element have the same number of protons.
• For example, every carbon atom has 6 protons (Z = 6). Every uranium atom has 92 protons (Z = 92).
• If you change the number of protons, you change the element.
• In a neutral atom, the atomic number also tells you the number of electrons, because the number of protons equals the number of electrons.
• On the periodic table, elements are arranged in order of increasing atomic number.

Exam Tip: The atomic number is the smaller of the two numbers shown for each element on the periodic table. It is always a whole number because you cannot have a fraction of a proton.

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Mass Number (A)

The mass number is the total number of protons and neutrons in the nucleus of an atom. It is given the symbol A.

The formula to remember is:

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

This can be rearranged to find the number of neutrons:

Number of neutrons = Mass number (A) - Atomic number (Z)

Examples

Element	Symbol	Atomic Number (Z)	Mass Number (A)	Protons	Neutrons	Electrons
Hydrogen	H	1	1	1	0	1
Helium	He	2	4	2	2	2
Carbon	C	6	12	6	6	6
Nitrogen	N	7	14	7	7	7
Oxygen	O	8	16	8	8	8
Iron	Fe	26	56	26	30	26
Uranium	U	92	238	92	146	92

Exam Tip: Always use the formula: neutrons = mass number - atomic number. This is one of the most commonly examined calculations, and it is easy to get full marks if you show your working clearly.

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Standard Notation

Atoms are often written using standard notation (also called nuclide notation), which shows both the mass number and atomic number:

The notation places the mass number (A) as a superscript and the atomic number (Z) as a subscript to the left of the element symbol.

For example, carbon-12 is written with mass number 12 above and atomic number 6 below, to the left of the symbol C.

Some important examples:

Nuclide	Mass Number (A)	Atomic Number (Z)	Protons	Neutrons
Hydrogen-1	1	1	1	0
Carbon-12	12	6	6	6
Carbon-14	14	6	6	8
Uranium-235	235	92	92	143
Uranium-238	238	92	92	146

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Isotopes

Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons. This means they have the same atomic number but different mass numbers.

Key facts about isotopes:

• Isotopes of the same element have identical chemical properties because they have the same number of electrons and the same electron configuration.
• Isotopes have different physical properties (such as density and rate of diffusion) because they have different masses.
• Some isotopes are stable (they do not change over time), while others are unstable and undergo radioactive decay.

Examples of Isotopes

Isotope	Protons	Neutrons	Mass Number	Stable?
Carbon-12	6	6	12	Yes
Carbon-13	6	7	13	Yes
Carbon-14	6	8	14	No (radioactive)
Hydrogen-1 (protium)	1	0	1	Yes
Hydrogen-2 (deuterium)	1	1	2	Yes
Hydrogen-3 (tritium)	1	2	3	No (radioactive)
Uranium-235	92	143	235	No (radioactive)
Uranium-238	92	146	238	No (radioactive)

graph LR
    A["Carbon Isotopes"] --> B["Carbon-12<br>6 protons<br>6 neutrons"]
    A --> C["Carbon-13<br>6 protons<br>7 neutrons"]
    A --> D["Carbon-14<br>6 protons<br>8 neutrons"]

    B --> E["Stable"]
    C --> F["Stable"]
    D --> G["Radioactive<br>(used in carbon dating)"]

    style A fill:#2c3e50,color:#fff
    style B fill:#27ae60,color:#fff
    style C fill:#27ae60,color:#fff
    style D fill:#e74c3c,color:#fff
    style E fill:#27ae60,color:#fff
    style F fill:#27ae60,color:#fff
    style G fill:#e74c3c,color:#fff

Exam Tip: A very common exam question asks you to define isotopes. The correct definition is: "Atoms of the same element with the same number of protons but a different number of neutrons." You MUST mention both "same protons" and "different neutrons" for full marks. Saying "different mass" is not enough — you need to specify that it is the neutrons that differ.

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Why Some Isotopes Are Unstable

The stability of a nucleus depends on the balance between the number of protons and neutrons. If the nucleus has too many or too few neutrons relative to protons, it becomes unstable.

An unstable nucleus will undergo radioactive decay to become more stable. During decay, the nucleus emits radiation in the form of alpha particles, beta particles, or gamma rays. You will learn more about these types of radiation in later lessons.

The key point is that not all isotopes are radioactive — only those with unstable nuclei.

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Relative Atomic Mass and Isotopes

Because most elements exist as a mixture of isotopes, the relative atomic mass shown on the periodic table is a weighted average of the masses of all the naturally occurring isotopes, taking into account their relative abundance.

For example:

• Chlorine has two main isotopes: chlorine-35 (75% abundance) and chlorine-37 (25% abundance).
• The relative atomic mass of chlorine = (35 x 75 + 37 x 25) / 100 = (2625 + 925) / 100 = 3550 / 100 = 35.5
• This is why the periodic table shows chlorine's relative atomic mass as 35.5, even though no single chlorine atom has a mass of 35.5.