Isotopes

“Define the term isotopes” has appeared on 0620 theory papers so regularly that examiner reports now sound resigned about it: candidates write “same element, different mass” and lose a mark they could have banked in Year 9. The full definition has two precise halves. Beyond the definition, this subtopic explains why chlorine’s Ar is 35.5, the bridge between atomic structure and the mass calculations in stoichiometry.

The definition, exactly (Core)

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Equivalently: same proton number, different nucleon number. Because the proton number is unchanged, isotopes occupy the same place in the Periodic Table and are the same element; because the neutron count differs, their masses differ.

Chlorine illustrates it. Chlorine-35 has 17 protons and 18 neutrons; chlorine-37 has 17 protons and 20 neutrons. Both have 17 electrons arranged 2,8,7. Natural chlorine is roughly 75% Cl-35 and 25% Cl-37.

	Chlorine-35	Chlorine-37
Protons	17	17
Neutrons	18	20
Electrons	17	17
Nucleon number	35	37

Same chemistry, different mass

Isotopes of an element have the same chemical properties. The Supplement-level reason: chemical properties depend on electrons, and isotopes have the same number of electrons and the same electronic configuration. Physical properties that depend on mass, such as density and rate of diffusion, differ slightly, with heavier isotopes diffusing more slowly.

The syllabus distinguishes non-radioactive isotopes from radioactive ones and expects one medical and one industrial use of radioactive isotopes: treating cancer (cobalt-60 radiotherapy) and generating electricity in nuclear power stations (uranium-235) are the standard pair. Carbon-14 dating is also accepted as a use.

Relative atomic mass from abundance (Core calculation, routinely set at Extended)

Relative atomic mass, Ar, is the average mass of the atoms of an element on a scale where one atom of carbon-12 is exactly 12 (strictly, relative to 1/12 of the mass of a carbon-12 atom). “Average” is in the definition precisely because of isotopes. The weighted-mean calculation:

Ar = (mass1 × %1 + mass2 × %2) ÷ 100

For chlorine: (35 × 75 + 37 × 25) ÷ 100 = (2625 + 925) ÷ 100 = 35.5. Sanity-check every answer: the Ar must land between the lightest and heaviest isotope masses, closer to the more abundant one. An answer of 36 from a 75:25 chlorine mix has the weighting backwards. The Ar values you compute here feed every Mr in relative atomic and molecular mass.

Worked exam question

Boron exists as two isotopes, boron-10 and boron-11. A sample of boron contains 20% boron-10 and 80% boron-11. (a) State the number of protons and neutrons in an atom of boron-11. [Proton number of boron = 5] (2) (b) Explain why both isotopes react identically with oxygen. (2) (c) Calculate the relative atomic mass of this sample of boron. (2)

Model answer: (a) 5 protons (1); 11 − 5 = 6 neutrons (1). (b) Both isotopes have the same number of electrons / the same electronic configuration (1); chemical reactions involve only the electrons (1). (c) Ar = (10 × 20 + 11 × 80) ÷ 100 (1) = (200 + 880) ÷ 100 = 10.8 (1).

Mark-by-mark: (a) gives one mark per particle count. (b) needs the electron statement and the link to reactions; “they are the same element” scores zero because it restates the question. (c) awards the method line and the answer separately; show the weighted sum even if your arithmetic wobbles, because the method mark survives.

The mistakes that cost marks

1. Half a definition. “Atoms with different masses” or “same element, different atoms” misses the protons-same, neutrons-different structure that carries the marks.
2. Saying isotopes have different numbers of electrons. Neutral isotopes have identical electron counts, which is the whole reason their chemistry matches.
3. Ar answers outside the isotope range, from multiplying by the wrong percentages or forgetting to divide by 100. Check the answer sits nearest the abundant isotope.
4. Calling Cl-35 and Cl-37 “different elements” or “different compounds”. Same proton number means same element, full stop.

How examiners want it phrased

Typical student wording	Accepted mark-scheme wording
”Isotopes are versions of an element"	"Atoms of the same element with the same number of protons but different numbers of neutrons"
"They react the same because they’re the same thing"	"They have the same electronic configuration, and chemical properties depend on electrons"
"Chlorine’s mass is 35.5 because it’s average"	"Ar is the weighted average of the isotopic masses: (35 × 75 + 37 × 25) ÷ 100 = 35.5"
"Radioactive isotopes are used in hospitals"	"Cobalt-60 is used in radiotherapy to treat cancer”

The Malaysia note

Malaysian candidates sit 0620 in May/June or Oct/Nov, and in both series the isotope definition behaves like free marks for prepared students. It is one of a short list of definitions worth memorising word-perfect (mole, isotope, Ar, diffusion among them). Students switching from SPM-aligned classes know “isotop” already; what they tend to lack is the electron-based explanation, which is Supplement. We rehearse that exact definitions list inside the free 1-hour trial, so you leave knowing which ones you would have dropped.

Test yourself

Do these three from memory first. Each answer stays hidden until you click it.

Q1 (2 marks). Define the term isotopes.

Show answer

• Atoms of the same element with the same number of protons [1] • But different numbers of neutrons (accept: same proton number, different nucleon number) [1]

Q2 (2 marks). Copper exists as two isotopes: 69% copper-63 and 31% copper-65. Calculate the relative atomic mass of copper. Give your answer to 3 significant figures.

Show answer

• Ar = (63 × 69 + 65 × 31) ÷ 100 [1] • = (4347 + 2015) ÷ 100 = 63.6 [1]

Q3 (2 marks). State one medical use and one industrial use of radioactive isotopes.

Show answer

• Medical: cobalt-60 used in radiotherapy to treat cancer [1] • Industrial: uranium-235 used to generate electricity in nuclear power stations [1]