 You may recall from the first video that every atom is made up of subatomic particles called protons, neutrons and electrons. The protons are positively charged and they have a mass of about one atomic mass unit, an AMU. Now if you google atomic mass unit you'll find out how many kilos that is, it's a pretty small number. For now we're just going to stick with this, we can discuss the unit in more detail in class. A neutron has a neutral charge, it has no charge and it also has a mass of approximately one atomic mass unit, so protons and neutrons are roughly the same size. The electron is negatively charged and it only has a mass of about one-eighteen-hundredth of an AMU. So an electron is much much lighter than a proton and a neutron and that means that the mass of an atom is almost entirely due to its protons and neutrons. In a normal uncharged atom the number of electrons equals the number of protons and this means that the negative charges of the electrons cancel out the positive charges of the protons and overall the atom is neutral. So how are the particles arranged? Well the protons and neutrons in an atom are always located in a tiny and incredibly dense nucleus right in the center of the atom. This nucleus takes up a ridiculously small amount of the total volume of the atom, there's no way I can draw it to scale on this page and still have you be able to see the nucleus. And we're going to explore that in more detail later in the course. For now let's draw six protons and seven neutrons in this nucleus just for the sake of argument. That's six protons and the neutrons have seven of them. The rest of the volume of the atom is mostly empty space with electrons whizzing around it. Electrons are usually depicted orbiting the nucleus like planets with some orbits close into the nucleus and others further out. The first orbit or electron level can hold two electrons, a maximum of two electrons and the next can hold eight. So we put our first two in the first level and our remaining four in the next level. Now why do we have a total of six? We said that this atom had six protons and if it's a neutral atom then the number of protons equals the number of electrons. So we've got six electrons to play with, we've put two in the first level and we have a remaining four to distribute in the second level which can hold up to eight. The third level which in this atom is empty could hold up to 18 electrons. Sometimes you'll hear people talk about the third level being able to hold eight electrons and there's a good reason for this simplification but it is a simplification as are the orbits. We're going to look at this in more depth when we get onto the section on atomic structure so let's leave that controversy for now.