 Foundation and higher tiers

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 2nd and 3rd 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. So the carbon atom has 6 protons, 6 electrons and 6 neutrons. The protons and neutrons will go inside the nucleus. The 6 electrons will go into the electron shells, 2e in the first shell leaving 4 for the second shell. This gives carbon an electron arrangement of 2,4. ### Calculating the numbers of protons, electrons and neutrons in ions

You should click the link below to complete the practice questions and test your understanding. The next challenge is calculating the numbers of protons, electrons and neutrons inside ions. An ion is an atom with a charge. Atom have no charge since they contain equal numbers of positively charged protons in their nucleus and negatively charged electrons in the electron shells. However when elements react with each other they transfer electrons. Metal atoms tend to lose electrons and form ions with a positive charge. While non-metals gain electrons and form ions with a negative charge. To calculate the numbers of protons, electrons and neutrons in ions visit the page on "atoms to ions".

### Key points

• The atomic number is the number of protons inside the nucleus. For an atom the number of protons is the same as the number of electrons.
• The mass number is the number of neutron and protons added together.
• To calculate the number of neutrons inside the nucleus subtract the atomic number from the mass number.
• The 1st shell holds a maximum of 2 electrons. The other shells all hold a maximum of 8 electrons.