You need to know about the valence electrons, Valence electrons are electrons in the outer most shell or level, as you know electrons are always orbiting around the nucleus of the atom, they are arranged in energy levels, the electrons are held by the positive nucleus ( which is the proton ) so the nearer to the nucleus the stronger its tendency to be attracted, if it is far away it will be easier to loose or gain electrons, since the forces holding the electrons is lower as it gets far away from the nucleus
Basically, an atom consists of a nucleus which contains protons and neutrons. This nucleus is surrounded by outer shells which contain electrons.
Protons have a positive charge (+1), neutrons have a neutral charge (0) and electrons have a negative charge (-1). Therefore, the nucleus is positively charged because neutrons have no charge and protons are positive. Overall, an atom has a neutral charge because it has the same amount of electrons as protons and therefore the charge is neutral. (+1 -1 = 0)
The first shell of electrons contains 2 electrons, the second shell can contain 8 and the third can contain 8.
I hope this helps and I would definitely look at the bbc bitesize link as that website is really good :) Good luck!!