You could be asked to draw or describe an atomic structure diagram for any element up to element 20; calcium in your GCSE exam. Drawing atomic structure diagrams for atoms is very easy once you get the hang of it e.g. Find the element magnesium on the periodic table. The two numbers beside the element symbol are called the atomic number and the **mass number. These two numbers allow you to calculate the number of protons, neutrons and electrons inside the atom; this is shown below:

Calculating the number of protons, neutrons and electrons inside an atom

The atomic number of magnesium is 12; this means it has 12 protons in its nucleus. In an atom the number of protons always equals the number of electrons so magnesium also has 12 electrons. To calculate the number of neutrons inside the atom we simply take the atomic number from the mass number.....
Since the mass number of magnesium is 24 and the atomic number is 12 we simply subtract 12 from 24, to leave 12 neutrons. So an atom of magnesium has 12 protons, 12 electrons and 12 neutrons.

• The 12 protons and 12 neutrons will go inside the nucleus
• The electrons will go into the electron shells. The first electron shell can hold a maximum of 2 electrons; the 2^nd and 3^rd shells can hold a maximum of 8 electrons.
• The 12 electrons inside a magnesium atom will occupy the electron shells, 2 electrons in the first shell, 8 electrons in the second shell. For more help with electron arrangements for any elements click here or on the link above. This leaves 2 electrons to go into the third shell. This gives magnesium an electron arrangement of 2,8,2. An atomic structure diagram is shown below for magnesium.

Below is one more example just to make sure you are confident in calculating the numbers of protons, neutrons and electrons inside atoms as well as drawing out atomic structure diagrams.