Isotopes, explained with examples
Isotopes are atoms of the same element with different numbers of neutrons. They have the same chemistry but different masses, and some are radioactive. Isotopes appear in both GCSE Physics and GCSE Chemistry.
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Inside the atom
An atom has a tiny, dense nucleus made of protons (positive charge, relative mass 1) and neutrons (no charge, relative mass 1), surrounded by electrons (negative charge, almost no mass) in shells. The nucleus is about 100,000 times smaller than the whole atom, yet it holds almost all of the mass. In a neutral atom, the number of electrons equals the number of protons, so the charges cancel.
Atomic number and mass number
- The atomic number (proton number), Z, is the number of protons. It decides which element an atom is: every carbon atom has 6 protons, every oxygen atom has 8.
- The mass number (nucleon number), A, is the total number of protons and neutrons.
- The number of neutrons = A − Z.
In nuclear notation, the mass number is written at the top left of the element's symbol and the atomic number at the bottom left. Carbon-12 is written with 12 on top and 6 below, before C.
What isotopes are
Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons. So isotopes have the same atomic number but different mass numbers. Because chemical reactions depend on electrons, and isotopes of an element have the same number of electrons, isotopes of an element react in the same way chemically. Their physical properties linked to mass, such as density, differ slightly, and some isotopes have unstable nuclei, which makes them radioactive.
Common examples
- Hydrogen: hydrogen-1 (1 proton, 0 neutrons), deuterium or hydrogen-2 (1 proton, 1 neutron) and tritium or hydrogen-3 (1 proton, 2 neutrons). Tritium is radioactive.
- Carbon: carbon-12 (6 protons, 6 neutrons) and carbon-13 (6 protons, 7 neutrons) are stable; carbon-14 (6 protons, 8 neutrons) is radioactive.
- Chlorine: chlorine-35 and chlorine-37, both stable.
- Uranium: uranium-235 and uranium-238, both radioactive. Uranium-235 is the isotope used as fuel in nuclear reactors.
Worked examples
Example 1: counting particles
An atom of sodium-23 has atomic number 11. How many protons, neutrons and electrons does it have?
- Protons = atomic number = 11.
- Neutrons = 23 − 11 = 12. Electrons = protons in a neutral atom = 11.
Example 2: identifying isotopes
Which two of these are isotopes of the same element? Atom P: 8 protons, 8 neutrons. Atom Q: 8 protons, 10 neutrons. Atom R: 9 protons, 8 neutrons.
- Isotopes have the same number of protons.
- P and Q are isotopes (both oxygen, with 8 protons). R has 9 protons, so it is a different element, fluorine.
Example 3: writing notation
An atom has 26 protons and 30 neutrons. Write its mass number and name it, given that iron has atomic number 26.
- Mass number = 26 + 30 = 56.
- It is iron-56, with 56 written top left and 26 bottom left of Fe.
Example 4: ions
A magnesium-24 atom (atomic number 12) loses two electrons to form an ion. How many protons, neutrons and electrons does the ion have?
- Protons 12, neutrons 24 − 12 = 12, electrons 12 − 2 = 10.
- Losing electrons changes the charge (the ion is 2+), but not the element or the isotope.
Relative atomic mass (Chemistry link)
Most elements exist naturally as a mixture of isotopes. The relative atomic mass on the periodic table is a weighted average of the mass numbers, based on how common each isotope is. That is why chlorine's relative atomic mass is 35.5 rather than a whole number.
Example 5: a weighted average
Natural chlorine is 75% chlorine-35 and 25% chlorine-37. Find its relative atomic mass.
- (75 × 35 + 25 × 37)/100 = (2625 + 925)/100.
- 35.5.
Why some isotopes are unstable
The nucleus is held together by a strong nuclear force between protons and neutrons, while the protons repel each other electrically. A nucleus is stable only when the numbers of protons and neutrons are in the right balance. Too many neutrons, too few, or simply too many particles overall, and the nucleus becomes unstable. It then decays, changing its make-up until it reaches a more stable arrangement. That is why carbon-14, with two extra neutrons, is radioactive while carbon-12 is not.
Radioactive isotopes
Some isotopes have an unstable balance of protons and neutrons. Their nuclei decay randomly, giving out alpha particles, beta particles or gamma rays, and often turning into a different element. These radioisotopes have many uses:
- Carbon dating: living things take in carbon-14 while alive; after death it decays with a half-life of about 5,700 years, so measuring what remains dates ancient remains.
- Medical tracers: technetium-99m emits gamma rays and has a short half-life, so doctors can image organs safely.
- Cancer treatment: cobalt-60 gamma rays can target tumours.
- Smoke alarms: americium-241 emits alpha particles that ionise air in the detector.
- Nuclear power: uranium-235 nuclei split in fission, releasing large amounts of energy.
The half-life guide explains how the activity of a radioisotope falls over time and how to choose an isotope for a particular job.
How the model of the atom developed
GCSE Physics also asks about how scientists discovered atomic structure. Thomson's plum pudding model pictured a ball of positive charge with electrons embedded in it. Rutherford's alpha scattering experiment showed that most alpha particles passed straight through gold foil but a few bounced back, so the positive charge and mass must be concentrated in a tiny nucleus. Bohr added electron shells at fixed distances, and Chadwick later discovered the neutron, which explained why isotopes exist.
Common mistakes
- Saying isotopes have different numbers of protons; it is neutrons that differ.
- Using the mass number as the number of neutrons.
- Thinking isotopes react differently in chemical reactions.
- Forgetting that ions have a different number of electrons, but the same protons and neutrons.
Practice questions
Try each one before opening the answer.
Q1. How many neutrons does uranium-238 have? (atomic number 92)
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Q2. What do carbon-12 and carbon-14 have in common, and how do they differ?
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Q3. An element has isotopes of mass 10 (20%) and 11 (80%). Find its relative atomic mass.
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Q4. Why do isotopes of an element have the same chemical properties?
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Frequently asked questions
Are isotopes in GCSE Physics or Chemistry?
Both. Physics focuses on nuclear structure and radioactive isotopes; Chemistry focuses on atomic structure and relative atomic mass.
Are all isotopes radioactive?
No. Many isotopes, such as carbon-12 and chlorine-35, are stable. Only isotopes with unstable nuclei are radioactive.
Key takeaways
- Atomic number = protons; mass number = protons + neutrons.
- Isotopes: same protons, different neutrons.
- Isotopes of an element have the same chemistry.
- Relative atomic mass is a weighted average of the isotopes.
- Radioactive isotopes have unstable nuclei and many uses.