This topic accounts for approximately 9% of your exam marks.
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Stable9%
Subatomic particles and electronic configuration appear in every exam series.
Isotopes are atoms of the same element with the same number of protons but a different number of neutrons
All isotopes of an element share:
the same atomic number (so the same number of protons and electrons)
the same chemical properties (chemistry is set by the electron count, which equals the proton count)
Isotopes differ in:
mass number (different neutron count)
some physical properties like density and rate of diffusion
Isotopes are written with the mass number as a left-superscript on the element symbol:
¹²C, ¹³C, ¹⁴C are three isotopes of carbon
All three have 6 protons; they have 6, 7 and 8 neutrons respectively
Why isotopes matter
Most elements in nature are a mixture of their isotopes in fixed natural proportions
This is why most relative atomic masses on the periodic table are not whole numbers — they are weighted averages over the natural isotope mix (see section 5)