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Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Free GCSE Physics notes and 14 FAQs.
Isotopes and Ion Formation is part of Atoms and isotopes in GCSE AQA Higher Physics.
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Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Free GCSE Physics notes and 14 FAQs.
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Use this lesson to stay inside Atoms and isotopes while connecting the detail back to the wider Atomic structure topic.
Read the core explanations from Isotopes and Ion Formation before testing yourself from memory.
Here's something that puzzled scientists for decades: why do some carbon atoms weigh more than others? All carbon atoms have 6 protons - that's what makes them carbon. But some carbon atoms are heavier than others, even though they're still definitely carbon.
The answer lies in the nucleus. While all atoms of the same element must have the same number of protons, they can have different numbers of neutrons. These different versions of the same element are called isotopes.
Understanding isotopes is crucial for explaining radioactivity, dating ancient objects, and even medical treatments. But first, we need to master the basics of atomic structure and how atoms can change.
Think of isotopes like different models of the same car. A Ford Focus and a Ford Focus with a bigger engine are both Ford Focus cars - they have the same basic identity. But one weighs more because it has extra components.
Similarly, carbon-12 and carbon-14 are both carbon atoms. They both have 6 protons (that's what makes them carbon). But carbon-12 has 6 neutrons whilst carbon-14 has 8 neutrons. The extra neutrons make carbon-14 heavier and radioactive.
**Trap**: Many students think isotopes are different elements. They're not - isotopes are different versions of the same element. The number of protons never changes within an element.
Scientists use a special notation to show isotopes clearly. You'll see this written as $^{14}C$ or $^{14}_{6}C$. The big number at the top is the mass number (protons + neutrons). The small number at the bottom is the atomic number (just protons).
Let's decode $^{235}_{92}U$ (uranium-235). The atomic number is 92, so it has 92 protons. The mass number is 235, so it has 235 - 92 = 143 neutrons. In a neutral atom, it also has 92 electrons to balance the 92 protons.
Worked example: Finding Particles in Isotopes
How many protons, neutrons and electrons are in $^{16}_{8}O$?
Answer: 8 protons, 8 neutrons, 8 electrons
Atoms don't always stay neutral. When they gain or lose electrons, they become charged particles called ions. This happens because electrons are on the outside of atoms and can be removed or added relatively easily.
Think of it like a person carrying bags. If you pick up extra bags (gain electrons), you become negatively 'charged' with more stuff. If you put bags down (lose electrons), you become positively 'charged' because you have less than you started with.
Metals typically lose electrons to form positive ions. For example, sodium loses one electron to become Na⁺. Non-metals typically gain electrons to form negative ions. Chlorine gains one electron to become Cl⁻.
**Trap**: Students often think ions have different numbers of protons. Wrong! The number of protons never changes - that would make it a different element. Only electrons are gained or lost.
Now we can combine our knowledge. An isotope can also be an ion - it can have a different number of neutrons AND a different number of electrons from the neutral atom. The key is tracking each type of particle separately.
Worked example: Analysing an Isotope Ion
How many protons, neutrons and electrons are in $^{24}_{12}Mg^{2+}$?
Answer: 12 protons, 12 neutrons, 10 electrons
Notice how the mass number and atomic number tell us about the nucleus (protons and neutrons), whilst the charge tells us about the electrons. These are independent - you can have any combination.
Isotope and ion questions are mark-winners if you know the method, but they catch many students out. The key is being systematic and not rushing.
Always start by identifying what you know from the notation. The atomic number (bottom) gives protons. The mass number (top) gives protons plus neutrons. Any charge tells you about electrons gained or lost.
Remember that isotopes of the same element have identical chemical properties because they have the same electron arrangement. They only differ in mass and nuclear properties like radioactivity.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. They have the same atomic number but different mass numbers; carbon-12 has 6 neutrons whilst carbon-14 has 8.
Metals typically lose electrons to form positive ions; for example, sodium loses one electron to become Na⁺. Non-metals typically gain electrons to form negative ions, such as chlorine becoming Cl⁻.
Atoms become ions by gaining or losing electrons, giving them an overall electric charge. Positive ions have lost electrons, such as Na⁺, and negative ions have gained electrons, such as Cl⁻; the number of protons never changes.
Isotopes of the same element have identical chemical properties because they have the same electron arrangement. They only differ in mass and nuclear properties like radioactivity; for example, the extra neutrons make carbon-14 heavier and radioactive.
In isotope notation, the big number at the top is the mass number (protons + neutrons) and the small number at the bottom is the atomic number (protons). Uranium-235 has 92 protons and 235 − 92 = 143 neutrons.
Uranium-235 has 143 neutrons. Its atomic number is 92, so it has 92 protons, and its mass number is 235, so it has 235 − 92 = 143 neutrons; a neutral atom also has 92 electrons.
An oxygen-16 atom has 8 protons, 8 neutrons and 8 electrons. The atomic number is 8, the mass number is 16, so neutrons = 16 − 8 = 8, and a neutral atom has as many electrons as protons.
A positive ion has lost electrons, and a negative ion has gained electrons. Metals typically lose electrons to form positive ions, such as Na⁺, while non-metals typically gain electrons to form negative ions, such as Cl⁻.
An isotope can also be an ion, with a different number of neutrons and a different number of electrons from the neutral atom. The mass number and atomic number tell us about the nucleus, while the charge tells us about the electrons, and these are independent.
Isotopes of the same element differ only in mass and nuclear properties like radioactivity. They have identical chemical properties because they have the same electron arrangement.
In an ion, protons equal the atomic number, neutrons equal the mass number minus the atomic number, and the charge shows how many electrons were gained or lost. Mg²⁺ with mass number 24 and atomic number 12 has 12 protons, 12 neutrons and 12 − 2 = 10 electrons.
Carbon-12 and carbon-14 are isotopes of carbon: both have 6 protons, but carbon-12 has 6 neutrons and carbon-14 has 8 neutrons. The extra neutrons make carbon-14 heavier and radioactive.
Ions do not have a different number of protons from their atoms; only electrons are gained or lost. Changing the number of protons would make it a different element.
Carbon-14 is not a different element from carbon-12; isotopes are different versions of the same element. They have the same number of protons, and so the same atomic number, but different numbers of neutrons and therefore different mass numbers.