Using the periodic table as a guide, picking a number of atoms, write down the number of Protons, Neutrons and Electrons that those elements have


Each of the atoms is defined by the number of protons that the atom has in it nucleus. This number is called the atomic number

The very 1st atom in the periodic table is Hydrogen it only has one proton, therefore it must also have 1 electron, the following is the bohr diagram of Hydrogen.

The Atomic number is equal to the number of protons in an atom.

The next element in the Periodic table is Helium

As you can see the Mass number is 4 and the atomic number is 2, therefore there are 2 protons (the atomic number) and 2 neutrons (mass number - atomic number (4 - 2 = 2)), so the bohr diagram looks like

If we look at the periodic table we will see that there are only 2 element on the first period (row) of the table, this is because electrons only put 2 electrons in their 1st shell (orbit), the next elements electrons have to occupy a new shell.


Look how this element now incorporates the 3rd electron on a new shell, the 2nd shell.

This 2nd shell can hold up to 8 electrons, this can be easily remembered by looking at the periodic table and counting the number of element on the 2nd row.

So as we go across the period we fill the extra electrons in the 2nd shell until we get to ...... ??

Then we move to the next shell, again filling one by one until we get to ..... ??

How to draw a Bohr Diagram of a particular element ...

  • Draw a box or cloud in the middle of your page, inside write the name and atomic number of the element you wish to draw.
  • Look at the Periodic Table, check which row (↔) the element is on, this is the number of shells,

number of shells = ______

  • All the shells inside the last shell are FULL, 1st shell 2 electrons, 2nd & 3rd have 8 electrons.

2, 8, 8, 18, 18

  • Now count along the row, the number of atoms across = number of electrons on the outer shell

number of electrons on outer shell = ______

this is VERY similiar to the rules above, I suppose great minds ......

Electronic Configuration

This is very like the Bohr Diagrams except we convey the information in written numbers and not in diagram format. Each shell is separated by a comma (,), so if we want to write out the Electronic Configuration of an element we need to follow the same rules above for the diagram but we write and don't draw it out


He Electronic configuration = 2.

This is because He only has 2 electrons and both of them live in the 1st shell

O Electronic Configuration = 2, 6

This is because O has 8 electrons and only 2 can live in the 1st shell, so the rest must live on the next shell, as there are 6 more electrons on the next shell we get and electronic configuration, 2, 6

Ca Calcium Electronic Configuration = 2, 8, 8, 2.

This is because Ca is on the 4th row therefore has 4 shells, the inner ones are full, 2, 8, 8 this adds up to 18 electrons, Calcium has 20 electrons therefore 2 more live on the 4th shell.

The mass number is defined as the number of protons + the number of neutrons together.

To find the number of Neutrons

# neutrons = Mass Number - Atomic Number

Further knowledge

Electron Orbits

In truth the number of electrons on each shell is not as simple as the 2,8,8,18 as we have said before, but it does seem like it is. This is due to each energy level actually being subdivided into other discrete energy levels. The real number of electrons on each shell is found by giving each shell a number n, i.e. the first shell n = 1, next n = 2 and so on so the formula is