How can work out the number of protons, neutrons and electrons in this atom of an element?

Each element has its own individual atomic number which can be found on the periodic table. This number is the number of protons in each atom of this element . As protons have each have a charge of +1 and atoms have an overall neutral charge there is an equivalent number of electrons that each have a charge of -1 to balance it out.
To work out how many neutrons there are in the atom you need to use its mass number (This will be given to you.) The mass number is the relative weight of an atom. Electrons do not contribute to this weight as they are so light, this means the mass number is made up from protons and neutrons which both have a mass of 1 each. Therefore you can calculate the number of neutrons by subtracting the number of protons (the atomic number) from the mass.
To summarise:Number of protons = the atomic numberNumber of electrons = the number of protonsNumber of protons = the mass number - the number of protons