A-level Chemistry/OCR/Atoms, Bonds and Groups/Atoms and Reactions/Atoms

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Atomic Structure[edit]

Atoms are made up of three fundamental particles: protons, neutrons, and electrons.

Atomic Structure
Particle Relative Mass Relative Charge
Protons 1 +1
Neutrons 1 0
Electrons 1/2000 -1

Almost all of the mass of an atom is contained in the nucleus, which has an overall positive charge. But the nucleus is tiny compared with the total volume occupied by the whole atom. Overall, an atom is neutral, and contains an equal number of protons and electrons.

Mass number (A) – this represents the number of protons plus the number of neutrons in an atom.
Atomic number (Z) – this is the number of protons in the nucleus of an atom.

Given an atomic number and mass number, you can deduce the number of protons, neutrons (Neutral Charge) and electrons (Negative Charge) in an atom. For example, a lithium atom (Z=3, A=7) contains three protons (found from Z), three electrons (as the number of protons = the number of electrons), and four neutrons (7 – 3 = 4).


Isotopes are atoms of the same element containing different number of neutrons. The most common example of isotopes is chlorine. Chlorine-35 and chlorine-37 are two isotopes of chlorine, which contain 18 and 20 neutrons respectively. Overall, chlorine contains approximately 75% of chlorine-35 and 25% of chlorine-37.

The isotopes of an element can be recorded using a mass spectrometer. The element is vaporised and then bombarded with electrons to create positive ions. The ions are sped up by a known electric field and then refracted by a known magnetic field. The different isotopes can then be collected by a detector and a percentage of each isotopes mass in an element calculated. This can then be used to calculate the Relative Atomic Mass.

Relative Atomic Mass[edit]

The relative atomic mass is the mean mass of an atom of an element relative to Carbon-12 isotope having a value of 12.000.

This is the mass of an element depending on the percentage of the different isotopes of an element. Carbon-12 is used as a standard for the measurement of relative masses. The relative mass can be calculated using the results from a mass spectrometer.

Relative atomic mass = (% of isotope x atomic mass of isotope) ÷ 100

The relative atomic mass of Chlorine is: RAM(Cl) = (75×35)÷100 + (25×37)÷100 = 35.5