Basic Information on Atoms

Atoms are very small particles which form the basic building blocks of everything around us.

The ‘magic numbers’ in chemistry are 2, 8, 8 which are the maximum numbers of electrons that can fit into the first, second and third shell.

Tip – When you draw the electrons on a dot and cross diagram try to draw them in pairs as it makes it easier for us to count how many electrons are in each shell. If you do not do this, then it becomes harder to know how many electrons you have drawn and you run the risk of miscounting the electrons and putting more electrons than you should be. (Electrons also move around in pairs, so by drawing them like this it makes the diagram more realistic).


We can work out how many electrons would be in the outer shell of a particular element by looking at the periodic table and by looking at the proton number. With atoms (not ions) the number of protons in the nucleus is always equal to number of electrons in that particular atom.

Lithium has a proton number of 3 so this means there are 3 protons in the nucleus and 3 electrons in the atom’s shells.

The element’s aim is to become stable during its life. An element becomes stable when its outer shell is full of electrons. (Elements want full outer shell, we need to keep this in mind when we draw our dot and cross diagrams).

Covalent molecules – A covalent molecule is when we have 2 non-metal atoms. We can tell if an element is a metal or non-metal by looking at the periodic table and which side it is on of the baron step.

If we need to draw a dot and cross diagram of CH4 we would need to think about the electron configurations (2, 8, 8) of the elements in that bond.

So carbon has 6 electrons (2 in its first shell and 4 in its outer shell, this means that it needs 8 electrons to have a full outer shell, so it will need 4 more electrons). A non-metal element would gain electrons by making a bond and each time the atom forms a bond it gets an additional electron. This means that carbon needs to make 4 bonds in order to get a complete outer shell. As carbon always will have 4 electrons it will always need to create 4 additional bonds. This means that we can determine how many bonds an element will need to make by simply looking at how many electrons that element needs in order to get a full outer shell.

atom 2

Tip – When drawing covalent bonds do not draw the electrons in pairs, instead draw them separately as these electrons will overlap the orbit of a different electron. When a carbon atom and a hydrogen atom stick together (bond together) their orbits will overlap and both of these atoms will share an electron. Hydrogen requires 2 electrons to be stable so it shares an electron with carbon. The formula CH4 has the number 4 because it needs to make 4 bonds to become stable.

Another example would be CO2 the carbon atom needs to make 4 bonds, but the oxygen only needs to create 2 as it has 6 electrons in its outer shell. Each carbon atom needs to make 4 bonds while oxygen needs to make 2 bonds. We would be making a double if two oxygen atoms bond together. In a single bond each element shares a single electron. In a double bond each element shares 2 electrons.
The electrons that do not overlap in a covalent bond are known as ‘unbonded pairs’ or ‘lone pairs’.

Ionic bonds – An ionic bond is when a non-metal atom bonds with a metal atom. Remember – Always a non-metal atom will take electrons from a metal atom. For example NaCl – a chlorine atom will take an electron from the sodium atom so they will both have a full outer shell, which means they are stable.

An electron has a charge of -1 so if a chlorine atom gains an electron it becomes negatively charged. If the sodium atom loses an electron it becomes positively charged.
Another example would be MgO. A magnesium atom needs to lose 2 electrons to become stable while oxygen needs to gain 2 electrons to become stable.

An interesting fact to finish off with… Did you know there are more atoms in a single glass of water than all the glasses of water from the whole of the Earth’s oceans.

Thank you for reading.