Atoms and isotopes - A level Standard

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The atom is the smallest part of an element that can exist on its own. We can see atoms using powerful and modern microscopes but this is a recent development. We can’t as yet see inside of an atom but experimental evidence has given us models of an atom. Some are better than others!

The excepted model is composed of 3 sub atomic particles called protons, neutrons and electrons.

ParticleChargeRelative mass

The mass of a proton and a neutron is accepted as 1. The mass of an electron is negligible and is therefore ignored.

The protons and neutrons are contained in the nucleus but the electrons orbit the nucleus in shells or orbital’s. The nucleus is tiny but extremely dense as most of the atomic matter is contained within it. Most of the atom though is empty space.

There are 116 known elements all displayed within the periodic table. Each element is distinguished from the number of protons in the nucleus. Each atom of a particular element has the same number of protons in the nucleus. For example all atoms of carbon contain 12 protons. This is known as the atomic number and given the symbol z. Atoms are neutral which means it must contain the same number of positive and negative charges, and therefore the same number of electrons and protons. Therefore the proton number can also tell you the number of electrons orbiting a neutral atom. Note: this is NOT the same for ions.


Like charges will repel each other so the nucleus of an atom is not made solely of positive charges. It also contains neutral neutrons which act as a kind of glue keeping the nucleus together. Unlike the protons the number of neutrons in a nucleus can vary. The different versions of an element are called isotopes. The number of protons stays constant so they are still atoms of the same element. The mass number of an element tells you how many protons and neutrons there are in a nucleus. Therefore different isotopes will have different mass numbers.

Carbon has 3 isotopes – 12C, 13C, 14C. The mass number changes because the neutrons change but the proton number remains at 6 – if it wasn’t 6 it wouldn’t be carbon! Normally we write the mass number like above but don’t bother with the proton number (so not like in the table) as it doesn’t change.

Atomic mass

If you look at the mass numbers of elements in the periodic table you will see that the mass number of some elements is a decimal – how can you have part of a neutron?! The mass numbers given in a periodic table take into account all the isotopes of the element and is based on the relative abundance of them.

The atom is too small to weigh so relative masses have to compared between elements. All scientists compare against carbon-12 so it is known as the international standard measurement of atomic mass. The mass of an atom of carbon-12 is exactly 12 unified atomic mass units (u) therefore 1 unified atomic mass unit is exactly 1/12 the mass of 1 atom of carbon-12. Therefore:

Relative isotopic mass is the mass of an atom of an isotope compared with 1/12 the mass of an atom of carbon-12.

Generally in science we ignore the contribution of the electrons and we take the mass of a proton and a neutron as 1u. Relative isotopic mass is then simple the mass number of the isotope. So 16O has a relative isotopic mass of 16.
Most elements consist of a mixture of isotopes which have different mass numbers. To find the relative atomic mass we must find the weighted average mass of the isotopes present from the natural abundance of the isotopes and the relative isotopic masses.

Relative atomic mass, Ar, is the weighted average mass of an atom of an element compared with 1/12 of the mass of an atom of carbon-12.

Using this theory we can easily calculate the relative atomic mass of an element.

Measuring relative atomic mass

A relative atomic mass can be measured using a mass spectrometer.

  1. The sample is placed into the mass spec and vaporised.
  2. The sample is bombarded with electrons which forms positive ions
  3. The positive ions are accelerated using an electric field
  4. The positive ions are deflected by the magnetic field
  5. The ions are detected to produce a mass spectrum.

Ions of lighter isotopes will be deflected more than ions of heavier ones. Therefore the mass spectrum can be used to identify the masses of different isotopes.

Relative molecular mass

Relative molecular mass is the combination of the atomic masses of a whole compound.

Relative molecular mass, Mr, is the weighted average mass of a molecule of a compound compared with 1/12 the mass of an atom of carbon-12

The problem with relative molecular mass is for giant ionic structures such as SiO2 the molecule is actually comprised of hundreds of atoms bonded together so calculating the molecular mass is complicated.

Relative formula mass

This is the addition of the relative atomic masses for all the atoms contained in the formula. This is not dependent on the number of molecules but on the formula. For giant ionic compounds such as SiO2 the relative molecular mass is the addition of 1 silicon atom and 2 oxygen atoms.

Relative formula mass is the average mass of the formula unit of a compound compared with 1/12 the mass of an atom of carbon-12

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