 Now let's bring in a few useful terms. The atomic number of an atom is simply the number of protons that it has. It also happens to be the way in which atoms are arranged in the periodic table. They're ordered by atomic number, starting with hydrogen, with one proton in its nucleus, and going up to the much larger atoms. So the atomic number of our atom here is six. It has six protons. If we were to look at a periodic table, you would find that atomic number six corresponds with carbon. So what we've got is a carbon atom. The mass number of an atom is the total number of protons and neutrons. Remember we said that most of the mass of an atom is the protons and the neutrons? By totaling them up, we get a feel for how heavy the atom is without getting picky about the small extra mass of the electrons. So the mass number of this atom is six plus seven, which is thirteen. So let's say for this atom we have an atomic number of six, that's six protons. And the mass number is the protons plus the neutrons, which is six plus seven, which is thirteen. Next, the charge. We said for a normal neutral atom, the number of electrons equals the number of protons. But it is possible for an atom to gain or lose electrons. And this will give the atom an electrical charge, either positive or negative. To calculate the charge on an atom, look at the total positive charge from the protons and the total negative charge on the electrons and add them together. If there are more protons, the overall charge will be positive. And if there are more electrons, the overall charge will be negative. In this case here they're equal. We've got six protons, so that's a charge of plus six. And we have six electrons, so that's a charge of minus six. Plus minus six, so our charge is zero. So it's a neutral atom. If an atom does have a charge, whether it's positive or negative, we call it an ion. This means an atom with a charge. So only use the term atom if you're referring to a neutral atom. And use ion instead if the atom has a charge. Finally, here's a shorthand way of summarizing the information about an atom, using its chemical symbol. You write the chemical symbol of the element that you're talking about and you put the atomic number as a subscript on the left and the mass number as a superscript also on the left. If the atom has a charge, you put it at the top right here. But our carbon atom is neutral, so we're going to leave that blank. From this symbol you can get all of the information that you need to characterize an atom.