Structure and bonding · GCSE Chemistry
Isotopes
GCSE Chemistry revision on isotopes: same proton number, different neutrons, relative atomic mass as a weighted mean, and why chemical properties stay the same.
Same atomic number, different mass number. Chemical behaviour follows the electrons; the extra neutrons only change the mass.
The important bits
What you need to know
- 1
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, so they have different mass numbers.
- 2
Atomic number (Z) is the number of protons and defines the element. Mass number (A) is protons plus neutrons. Neutrons = A − Z.
- 3
Chlorine has isotopes ³⁵Cl and ³⁷Cl. Both have 17 protons and 17 electrons in the atom. ³⁵Cl has 18 neutrons; ³⁷Cl has 20 neutrons.
- 4
Isotopes of an element have the same electronic structure, so they have the same chemical properties and form the same ions and compounds.
- 5
Physical properties that depend on mass can differ slightly, for example density or the rate of diffusion of gases.
- 6
Relative atomic mass, Aᵣ, is the weighted mean mass of the isotopes of an element compared with 1/12 of the mass of a ¹²C atom.
- 7
Aᵣ = (mass₁ × abundance₁ + mass₂ × abundance₂) / 100, when abundances are percentages. That is why chlorine’s Aᵣ is about 35.5, not 35 or 37.
- 8
In a neutral atom, electrons = protons. Making an ion does not change the isotope: ²³Na and ²³Na⁺ have the same 12 neutrons; the ion has 10 electrons, not 11.
Quotations worth analysing
Short evidence. Real method.
“Isotopes are atoms of the same element with different numbers of neutrons.”
Same element means same proton number. Different neutrons change the mass number. Do not say isotopes have different numbers of protons — that would be a different element.
“Aᵣ = (Σ isotope mass × abundance) / 100”
Use percentage abundances. Show two products, add them, divide by 100. The answer is a weighted mean, not a simple average of 35 and 37.
“³⁵₁₇Cl and ³⁷₁₇Cl”
The bottom number is 17 in both: 17 protons, so both are chlorine. The top numbers 35 and 37 are mass numbers. Subtract to get 18 and 20 neutrons.
Go deeper
Count protons first, then neutrons, then electrons
A typical question gives ¹²C and ¹⁴C, or “two atoms of chlorine, mass numbers 35 and 37”. Protons are the identity card: carbon is 6, chlorine is 17. Neutrons fill in the mass: 12 − 6 = 6, 14 − 6 = 8 for carbon; 35 − 17 = 18 and 37 − 17 = 20 for chlorine. Electrons in the atom match the protons. If the question then makes Cl⁻, add one electron, but do not add a proton. Students often change the proton number when they see a different mass number, or they give isotopes different electron arrangements. The chemistry is the same because the outer electrons are the same.
Go deeper
Relative atomic mass is a weighted mean, not a guess
The periodic table does not show a whole number for chlorine because naturally occurring chlorine is a mixture of isotopes. If 75% is ³⁵Cl and 25% is ³⁷Cl, then Aᵣ = (35 × 75 + 37 × 25) / 100 = (2625 + 925) / 100 = 35.5. The mean is closer to 35 because that isotope is more common. Never add 35 and 37 and divide by two unless the abundances are 50:50. On Higher tier, abundances may be given as decimals that add to 1; then you do not divide by 100. Always check that the abundances add to 100% or 1.00 before you calculate.
Go deeper
Same chemistry, different mass — that is the exam contrast
Both ¹²C and ¹⁴C burn in oxygen to make carbon dioxide. Both ¹H and ²H (deuterium) form water, though D₂O is denser. Radioactive isotopes appear more often in Physics, but Chemistry papers still expect you to say chemical properties are the same because electron arrangement is the same. If a question mentions medical tracers or carbon dating, keep the chemistry sentence: same protons and electrons, different neutrons. Do not drift into a half-life essay unless the paper asks for it. A relative-atomic-mass calculation is still Chemistry even when the isotope is radioactive.
See the idea in action
A sample of magnesium is 79% ²⁴Mg, 10% ²⁵Mg and 11% ²⁶Mg. Aᵣ = (24 × 79 + 25 × 10 + 26 × 11) / 100 = (1896 + 250 + 286) / 100 = 24.32. Report 24.3 to three significant figures if the data support it. All three isotopes have 12 protons. Neutrons are 12, 13 and 14. A Mg²⁺ ion from any isotope has 10 electrons; the proton number is still 12.
Exam technique
Turn knowledge into marks
Write protons, neutrons and electrons as three labelled numbers for each isotope. For Aᵣ, show every mass × abundance term, then divide. State that chemical properties are the same because the electron arrangement is the same.
Common mistakes
Do not give these marks away
- 01
Saying isotopes have different numbers of protons, or different chemical properties.
- 02
Averaging mass numbers without using percentage abundance.
- 03
Changing the neutron or proton count when the atom forms an ion.
Atoms of ³⁵Cl and ³⁷Cl are isotopes. What is the same and what is different?
ASame number of neutrons, different number of protons
BSame number of protons, different number of neutrons
CSame mass number, different atomic number
DSame number of electrons in Cl⁻, different number of protons
Show the answer
Same number of protons, different number of neutrons. Both have atomic number 17, so 17 protons. Mass numbers 35 and 37 mean 18 and 20 neutrons. Electron arrangement in the atoms is the same, so chemistry is the same.
Quick questions
If this is the bit you searched
What are isotopes GCSE Chemistry?
Atoms of the same element with the same number of protons but different numbers of neutrons, so they have different mass numbers.
Why is the relative atomic mass of chlorine 35.5?
Chlorine is a mixture of isotopes, mainly ³⁵Cl and ³⁷Cl. Aᵣ is the weighted mean of their masses using percentage abundances, so it is not a whole number.
Do isotopes have the same chemical properties?
Yes. They have the same number of protons and the same electron arrangement, so they take part in the same reactions and form the same type of ion.
How do you calculate relative atomic mass from isotopes?
Multiply each isotope’s mass number by its percentage abundance, add the totals, and divide by 100. If abundances are given as fractions, they should add to 1.