Atomic structure

The mass number of an atom is its total number of protons and neutrons. Isotopes are atoms of an element with different numbers of neutrons. The relative formula mass of a compound is found by adding together the relative atomic masses of all the atoms in the formula of the compound. The relative formula mass of a substance in grams is one mole of that substance.

Subatomic particles

the proton and neutron are within the nucleus which is within the centre of the atom, the elctrons are on the edges of the atom

Structure of the atom

Each atom consists of a nucleus [nucleus: The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom. ] containing protonsproton: A proton is a small particle with a positive charge found in the nucleus of the atom. and neutronsneutron: ###TODO###, with electrons [electron: An electron is a very small negatively-charged particle found in an atom in the space surrounding the nucleus. ] arranged around it in energy levels.

Relative masses of subatomic particles

Name of particleRelative mass
ElectronVery small (1/1836)

Mass number and atomic number

The mass number of an atom is never smaller than the atomic number. It can be the same, but is usually bigger:

  • The mass number of an atom is the total number of protons and neutrons it contains
  • The atomic number of an atom is the number of protons it contains

Notice that most of the mass of an atom is found in the nucleus.


The atoms of a particular element [element: A substance made of one type of atom only. ] will all have the same number of protons. Their atomic number will be the same. However, the atoms of an element can have different numbers of neutrons - so their mass numbers will be different.

Atoms of the same element with different numbers of neutrons are called isotopes. The different isotopes of an element have identical chemical properties. However, some isotopes are radioactiveradioactive: A substance that emits radiation is said to be radioactive..

Chemical symbols

The full chemical symbol for an element shows its mass number at the top, and atomic number at the bottom. Here is the full symbol for carbon:

C 12,6

It tells us that a carbon atom has six protons. It will also have six electrons, because the number of protons and electrons in an atom is the same.

The symbol also tells us that the total number of protons and neutrons in a carbon atom is 12. Note that you can work out the number of neutrons from the mass number and atomic number. In this example, it is 12 – 6 = 6 neutrons.

Isotopes of hydrogen

Most hydrogen atoms consist of just one proton and one electron, but some also have one or two neutrons. The table summarises these isotopes.



H 1,1

111 - 1 = 0

H 2,1

112 - 1 = 1

H 3,1

113 - 1 = 2

Relative formula mass

For your exam, you will need to know what relative formula mass is. You should also be able to work out the relative formula mass of a substance when given its formula [formula: A formula is a combination of symbols that indicates the chemical composition of a substance. ].

The symbol for relative formula mass is Mr. The symbol for relative atomic mass is Ar. You will be given any Ar values you need in the examination. The table shows some of these values:


ElementRelative atomic mass (Ar)

Working out Mr

To find the relative formula mass of a substance, you just add together the relative atomic mass values for all the atoms in its formula. Here are three examples:

Example 1

  • Find the Mr of carbon monoxide, CO

  • Mr = 12 + 16 = 28

Example 2

  • Find the Mr of sodium oxide, Na2O

  • Mr = (23 × 2) + 16 = 46 + 16 = 62

Example 3

  • Find the Mr of magnesium hydroxide, Mg(OH)2

  • Mr = 24 + 2 × (16+1) = 24 + 34 = 58

  • (Remember that there are two of each atom inside the brackets)


The relative formula mass of a substance - shown in grams - is called one mole of that substance. For example, the Mr of carbon monoxide (CO) is 28. This means that one mole of carbon monoxide has a mass of 28 g. You should be able to see that:

  • 14 g of carbon monoxide contains 14 ÷ 28 = 0.5 moles
  • 56 g of carbon monoxide contains 56 ÷ 28 = 2 moles

Read on if you're taking the higher paper.

Relative atomic mass - Higher tier

The relative atomic mass of an element (Ar) is an average value for the isotopesisotope: Atoms of an element with the same number of protons and electrons but different numbers of neutrons. of the element. For example, the Ar for chlorine is 35.5 because it contains two different isotopes.


Chlorine has two isotopes:



Cl 35, 17

171735 - 17 = 18

Cl 37,17

171737 - 17 = 20

75 per cent of chlorine atoms are 35Cl and 25 per cent of chlorine atoms are 37Cl.

This means that in 100 chlorine atoms, 75 will be 35Cl and 25 will be 37Cl.

The total Ar for these chlorine atoms will be (75 × 35) + (25 × 37) = 2625 + 925 = 3550.

So the average Ar for chlorine is 3550 ÷ 100 = 35.5.

A standard atom

The massmass: The amount of matter an object contains. Mass is measured in 'kg'. of the 12C isotope is the 'standard atom' that the masses of other atoms are compared to. The Ar of 12C is defined as 12. This means, for example, that one 24Mg atom has twice the mass of a 12C atom.

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