4 · Isotopes and relative atomic mass

✏️ The relative atomic mass calculation in the second half goes beyond the printed textbook pages — but it is worked through in your own notes on page 73, so it is treated as core here.

Neutrons are the one that can vary

Unlike the number of electrons, the number of neutrons in an atom is not always equal to the number of protons. Some types of atom have different numbers of neutrons.

Atom	Protons	Electrons	Neutrons
carbon	6	6	6, 7 or 8
nitrogen	7	7	7 or 8
uranium	92	92	143 or 146

Definition — straight from your notes Isotopes are atoms of the same element with the same number of protons and electrons, but a different number of neutrons.

Nitrogen-14 and nitrogen-15 are isotopes of nitrogen. Uranium-235 and uranium-238 are isotopes of uranium.

Same element, different neutrons

Two columns never move; one always does. That is the whole definition, in a table.

Nucleon number

The number below the symbol Nucleon number = number of protons + number of neutrons. ("Nucleons" just means the particles in the nucleus.)

Rearranged, that gives the version you will actually use in questions:
number of neutrons = nucleon number − proton number

So carbon-14 has 6 protons (it is carbon) and 14 − 6 = 8 neutrons. Chlorine-37 has 17 protons and 37 − 17 = 20 neutrons.

Why chlorine's mass is 35.5

Every nucleon number is a whole number — you cannot have half a proton. Yet the periodic table gives chlorine a relative atomic mass of 35.5. That is not a rounding error; it is telling you something real.

Chlorine exists as two isotopes: about 75% chlorine-35 and 25% chlorine-37. The relative atomic mass is the weighted average of the two.

Relative atomic mass from abundance

Abundance of the lighter isotope

Lighter isotope—

Its abundance—

Heavier isotope—

Its abundance—

Relative atomic mass = —

Set chlorine back to 75% and the answer lands exactly on 35.5 — the number printed on the periodic table.

The method, in the form your notes use

Multiply each isotope's nucleon number by its percentage abundance over 100, then add:

( ⁷⁵⁄₁₀₀ × 35 ) + ( ²⁵⁄₁₀₀ × 37 ) = 26.25 + 9.25 = 35.5

Two checks worth doing every time. The abundances must add to 100%. And the answer must land between the two nucleon numbers — closer to whichever isotope is more abundant. Chlorine's 35.5 sits nearer 35 than 37, which fits 75% being the lighter one.