The periodic table contains a lot of information. The numbers of the neutrons, electrons and protons can all be found using such information.
Proton Number = Atomic number
Neutron Number = Mass number – Atomic Number
Electron Number = Atomic Number – Charge
Once the number of electrons has been established these should be drawn into shells (For information on how to place electrons into orbitals please read more in Atomic Structure).
Each shell can take up a number of different electrons, but for the intents of this chapter the first shell takes in 2 electrons, the second shell takes in 8 electrons, the third shell takes in 8 electrons and the in the fourth shell the maximum number of electrons that will be put in it is going to be 2 electrons.
Oxygen has 8 electrons, therefore these will be placed in shells as 2 in the first shell, and 6 in the second shell, being written as 2, 6.
Calcium has 20 electrons, therefore these will be placed in shells as 2 in the first shell, 8 in the second shell, 8 in the third shell and the final 2 in the fourth shell, being written as 2, 8, 8, 2.
In order to draw these atoms with electrons around them, one should draw the nucleus, either as the letter of the element or the number of protons and neutrons, and shells around it. The electrons are then depicted as either dots or crosses.