An atom has many electron shells, because as electrons orbit the nucleus, they speed up and slow down. As they change speed, the electrons move closer, or father away, from the nucleus creating electron shells. The outermost electrons are called valence electrons. Each atom has a set number of valence electrons, and you can tell how many each atom has by either looking at which column it is in, or by using Bohr's model. Valence electrons live the farthest away from the nucleus, which means that the nucleus has less of an attraction to them. Since this is the case, these electrons are the easiest to share. any atom wants to fill it's outer shell with eight electrons, so it will bond with other atoms to fill that shell. When they bond, an atom will either gain, or lose, electrons depending on how many valence electrons it has. For example, if Sodium (Na) and Chlorine (Cl) bond, Sodium has only one valence electron, and Chlorine has seven, so when they bond is it more likely for Sodium to give up one electron, or Chlorine to give up all seven? Obviously it is more likely for Sodium to give up it's one valence electron, so you would say that Sodium has a +1 charge, because it will almost always give up it's valence electron, losing a negative charge, so now there is a positive one charge. So when you look at the first row on the periodic table, all of the elements, except for hydrogen, have a +1 charge, the second row has a +2 charge, the thirteenth row has a +3 charge (we skipped rows three through twelve, because they are transition metals, and they do not follow this rule) fourteenth row has a +4 charge or a -4 charge (because carbon could either lose or gain four electrons), the fifteenth row has a -5 charge (because it will typically gain more electrons giving it a negative charge), the sixteenth row has a -6 charge, the seventeenth row has a -7 charge, and the eighteenth row of atoms has atoms that do not bond. That is how and why atoms bond, and how to find the charge of the atom ;)