 Foundation and higher tiers

You could be asked to draw or describe the atomic structure of 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. ## Calculating the number of protons, neutrons and electrons

So for magnesium its atomic number 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 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 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 shell can hold a maximum of 2 electrons, the 2nd, 3rd and 4th 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 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 are confident 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

An ion is a cgarged atom. Atoms 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.

## 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.