IsotopesOxford AQA IGCSE Chemistry: Revision notes

Isotopes are atoms of the same element that have different numbers of neutrons.

• Because they are the same element, isotopes always have the same number of protons (same atomic number)
• Because they have different numbers of neutrons, isotopes have different mass numbers
• Isotopes have identical chemical properties, since chemical behaviour depends on electron arrangement, which is unaffected by neutron number

Atoms (and isotopes) are represented using standard notation, with the mass number above and the atomic number below the element symbol:

Mass number (top) — total protons + neutrons Atomic number (bottom) — total protons

For example, an isotope of sodium is written with mass number 23 and atomic number 11 (23 above, 11 below the symbol Na).

For any isotope, you can calculate the number of subatomic particles:

1. Number of protons = atomic number
2. Number of electrons = atomic number (in a neutral atom)
3. Number of neutrons = mass number − atomic number

Different isotopes of the same element will give a different neutron count in step 3, but protons and electrons stay the same.

Most elements exist as a mixture of two or more isotopes in fixed proportions in nature. The relative atomic mass of an element is an average value that takes into account the different masses and relative abundances of its isotopes — this is why relative atomic masses are often not whole numbers (e.g. chlorine's relative atomic mass is 35.5).