Subatomic Particles

Remember that the basic functional unit in Chemistry is the atom itself. Now, if we want to go deeper into the atom, remember it's composed of three subatomic particles. We're going to say in the center of our atom, the very center of our atom is what we have, that's called the nucleus.
If we take a look at this illustration on the right—that right there what I circled is the nucleus. We're going to say inside the nucleus, we have two out of the three subatomic particles. We have our protons and our neutrons. We're going to say spinning around it, we find the third subatomic particles, so we have the small green negatively charge balls, subatomic particles flying around our nucleus. These are the electrons.
We're going to say that the protons are positively charge. We're going to say that the electrons which are spinning around their nucleus are negatively charge and then we're going to say that the neutrons are non charge subatomic particles. They're neutral, they don't have a charge.
Inside the nucleus we have protons which are positive and neutrons which are neutral. The nucleus overall has a positive charge and then the electrons are spinning around it, the electrons are negatively charged. 

Now, what we're going to say here is we're dealing with two types of information on any given element. We're going to say we have the atomic number and we're going to have the atomic mass, oftentimes called the mass number.
We're going to say that the atomic number uses the variable Z and it equals the number of protons. So, the atomic number tells us the number of protons and also we determine the identity of an element. If you know the number of protons, then you know the element we're talking about, because each element has a specific number of protons. The atomic number kind of, is like the fingerprint for that element.
For example, if you look at periodic table at carbon, you'll see that carbon has associated with it, the number six. That six tells us that carbon has six protons. Carbon is the only element on the periodic table with six protons. Again, remember the atomic number, not only tells us the number of protons, but from that the identity of the element.
Now, we also have the atomic mass, also called the mass number at times. We're going to say it uses the variable A or we're going to say, 'This equals the number of protons and neutrons in an element.' So basically it gives us everything inside of the nucleus. Because remember the nucleus also has protons and neutrons.
Now, if we bring all of this together, we're going to say our element is, we usually use the variable X to represent our element, then we're going to have the atomic mass is A and then right below it is our atomic number. We usually see this kind of notation in class and on your exams.