Isotopes — same protons, different neutrons
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. The proton number is unchanged, so a neutral atom keeps the same electrons and configuration and all isotopes of an element are chemically identical. Only the neutron number differs, so the nucleon (mass) number and the atom mass differ. Carbon exists as C-12, C-13 and C-14: all have 6 protons and 6 electrons but 6, 7 and 8 neutrons.
(Extended) Relative atomic mass = weighted mean
The relative atomic mass () is the weighted mean mass of an atom of an element relative to 1/12 of the mass of a carbon-12 atom, allowing for the abundance of each isotope. It need not be a whole number: individual isotopes have whole-number masses, but their average in unequal amounts usually is not. Chlorine is 75% Cl and 25% Cl, so , matching the Periodic Table.
Same chemical, different physical properties
Isotopes react identically because chemical behaviour depends on the outer electrons, and all isotopes of an element share the same proton number, electron number and configuration; they cannot be separated chemically. Their physical properties differ because the neutron number, and so the mass, differs: heavier isotopes are denser and diffuse more slowly. This mass difference is what physical methods such as mass spectrometry exploit to separate them.
Drawn from real examiner reports.
Isotope definition needs the word atom
A definition must say isotopes are atoms of the same element with different numbers of neutrons. Answers that call them particles or species, or that leave out one half, lose the mark. The safe full answer names both parts: atoms of the same element with the same number of protons but different numbers of neutrons.
S23 Paper 41 Q2(b)(i); S22 Paper 42 Q3(a)(i) — definitions that omitted the word atom received no credit.
Say neutrons, not different masses
Saying isotopes have different relative masses is not accepted. Relative atomic mass () is a property of the element as a whole — a weighted average — not of one atom, so it does not describe what makes two atoms isotopes. The atomic-level cause is a different number of neutrons, so state different numbers of neutrons rather than different masses or mass numbers.
S23 Paper 41 Q2(b)(i); S22 Paper 42 Q3(a)(i) — stating different relative masses instead of different numbers of neutrons lost credit.
Ar 35.5 is an average, no atom has it
The relative atomic mass of an element is a weighted average, so no single atom actually has that mass. Every chlorine atom is either Cl (mass 35) or Cl (mass 37); none has mass 35.5. The value 35.5 reflects the roughly 3:1 mix of the two isotopes. Do not describe an individual atom as having the non-whole — it belongs to the element.
Weighted mean, not a simple average
Weight each isotope by its abundance — do not average the masses equally. For chlorine, is wrong because it ignores the 3:1 ratio; the weighted value is . Multiply each mass by its percentage abundance, add the products, then divide by 100.
Isotopes share chemical properties
It is wrong to say heavier isotopes are less reactive or react more slowly. Chemical properties are fixed by the electron arrangement, and isotopes of an element have identical electron arrangements, so they react in exactly the same way. Mass differences change only physical properties, such as density and diffusion rate; these are physical, not chemical, differences.
Isotopes vs allotropes
Do not confuse isotopes with allotropes. Isotopes are atoms of one element that differ in neutron number, such as C and C. Allotropes are different structural forms of one element in the same state, such as diamond and graphite, where the atoms are arranged differently. Neutron number points to isotopes; structure or arrangement points to allotropes.
Ar calculation: weight, sum, divide
Multiply each isotope mass by its percentage abundance, add the products, then divide by 100. Write the multiplication line even if the arithmetic then goes wrong — it earns the method mark. Sense-check that the answer lies between the lightest and heaviest isotope mass.
Neutrons = nucleon − proton
To find the number of neutrons, subtract the proton number (bottom) from the nucleon number (top): for Cl, neutrons . A common slip is to quote the nucleon number (37) as the neutron count. Protons and electrons both equal the bottom number.
Answer the isotope definition in full
State both halves explicitly: same element (same proton number) and a different number of neutrons. Use the word atoms, not particles. Where the mark covers two elements of the definition, make each its own clear point rather than one vague phrase about mass.
Isotopes: Atoms of the same element with the same number of protons but different numbers of neutrons.
Both parts of this definition are required for full marks in a Cambridge 0620 "state" or "describe" question:
What stays the same in all isotopes of an element:
What differs between isotopes:
Standard notation: The isotope of element X with nucleon number and proton number is written . The number of neutrons .
(Extended) Relative atomic mass (): The weighted mean mass of an atom of an element relative to 1/12 of the mass of a carbon-12 atom, taking into account the relative abundances of all naturally occurring isotopes. Because most elements have more than one naturally occurring isotope, is often not a whole number.
State the meaning of the term isotope.
The symbols below represent three atoms.
(a) State the meaning of the term isotope.
(b) State which two atoms are isotopes of each other. Justify your answer.
(c) Calculate the number of neutrons in .
