Positive and Negative Charges
An atom is made up of electrons going around a nucleus. The nucleus is at the centre, made up of protons and neutrons bonded together.
Each electron has a negative charge, and each proton has a positive charge. They attract each other. Neutrons have no charge, and are not attracted by electrons or protons.
How do we measure the amount of charge?
Charge is a different property from mass. Mass is measured in units of kilograms. The mass of a proton is much larger than the mass of an electron. The charge on a proton, however, has the same size as the charge on an electron, but they have opposite signs. (Mass, on the other hand, do not have opposite signs.)
Charge is measured in units of Coulombs, or C. One electron has a charge of -1.6 x 10-19 C. Notice the minus sign. One proton has a charge of +1.6 x 10-19 C. Notice the plus sign.
When measuring mass of an object using a weighing balance, we really make use of gravity - the attraction between Earth and the object. So to measure charge, we make use of attraction between charges.
For example, we know that the charge of an electron is equal and opposite to that of a proton because an atom does not get attracted or repelled by another electron. This shows that an atom has zero charge overall. Since the number of protons and electrons is equal, the charge of a proton must be equal and opposite to the charge of an electron so they can cancel out exactly.