Chemistry · Basic concepts. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
An atom is the smallest particle of an element that can take part in a chemical reaction. A molecule is the smallest particle of a substance that can exist on its own. An ion is an atom or group of atoms that carries a charge.
Atoms are far too light to weigh one by one, so their masses are compared with carbon-12. This gives the relative atomic mass, measured in amu.
Most elements are mixtures of isotopes: atoms with the same number of protons but different numbers of neutrons. A mass spectrometer separates the isotopes and shows how much of each is present. From this we calculate the average, which is the relative atomic mass of the element.
Students take a simple average of the isotope masses. The average must be weighted: multiply each mass by its percentage abundance before adding.
Answer: 35.5 amu. It is nearer to 35 because chlorine-35 is the more abundant isotope.
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