Practice Problems: Net Charge, Mass Number, Atomic Number

Here is an atom. What is the

atomic

number

,

mass

number

, and net

charge

of this atom? So the

atomic

number is the number of protons. In this diagram, these little red circles represent protons here in the nucleus, so we have 1, 2, 3, 4, 5, which means my atomic number is 5. That was pretty easy. The

mass

number is the number of protons plus the number of neutrons which are these things in blue, so the mass number is 1, 2, 3, 4, 5, 6, 7, 8 protons plus neutrons. So the mass number is 8. Sometimes people think that you have to have the same number of protons and neutrons in the nucleus of an atom, but that is definitely not true, they can differ quite a bit.

Net

charge

is how protons and electrons balance each other. So electrons are represented by these things flying around the outside of the atom, 1, 2, 3, 4 electrons, e- is one way we can abbreviate electrons and 1, 2, 3, 4, 5 p+, protons. So the protons and electrons are not quite balanced. There is one more proton than electrons, so an extra proton will give me a net charge of 1+ because each proton has a positive charge and has an extra proton. Net charge of 1+. This is a basic problem, now we'll move on to something that's a little more complicated. So here we have an atom with this atomic number, this mass number and net charge.

From this information, we need to determine the protons, neutrons, and electrons in this atom. Protons, that's going to be easy because protons... the number of protons will be the same as the atomic number. So there will be 7. And now what happens to the neutrons? The mass number is protons plus neutrons, so this 15 here is protons and neutrons together. I already know how many protons there are, so I can make protons plus neutrons, the mass number, minus the number of protons equals 8, the number of neutrons. The net charge here is 2+. If I have a net positive charge with a certain number, that tells me how many more protons I have than electrons.

So in this case, net charge of 2+, there are 2 more protons than elections. Since I have 7 protons, that means I have to have 5 electrons to get a 2+ charge. Below is a table with various characteristics of a certain atom. We need to fill the things that are empty. Well, we don't know how many protons, we don't know how many neutrons, where can we start? Well, we know the electrons and we know the net charge, so based on the number of electrons and the net charge that this atom has, we can calculate how many protons there are, okay? Since this is a net charge of -3, that means there are 3 more electrons than protons because electrons have a negative charge.

So I have 7 electrons that means I'm going to have... Sorry if I had 10 electrons that means I'm going to have 7 protons, 3 more electrons will give me -3. Okay, now I know that my atomic number is the same as my number of protons, so it will also be 7. The mass number here is the number of protons plus the number of neutrons. So to get just the number of neutrons, I take a mass number, protons plus neutrons, minus the number of protons, 7, and that will give me 9 neutrons. Sometimes these table

problems

can be the most difficult, so we will do one more example.

Here we want to find the electrons, protons and atomic number. We know the neutrons, the mass number and the net charge. Well, let's start with the protons. We know the mass number, protons plus neutrons, and we know the number of neutrons. That means we can take 25, which is protons plus neutrons, and subtract the number of neutrons, 25 minus 11 will give us 14, and that's just the number of protons. Atomic number is just another word for protons, so they will both be 14. Now that we know the number of protons in the nucleus, we can use the net charge to calculate the number of electrons.

So it's a net charge of 1+, which means there should be one more proton because it has a positive charge... one more proton than one electron. So 14 protons, that means that in total I will have 13 electrons. Now that you know how to solve

problems

like this, you're ready to move on to watching videos about isotopes.