# Practice Problems: Net Charge, Mass Number, Atomic Number

Here's an atom. What is the

## atomic

### number

, the### mass

### number

and the net## charge

to this atom? So the## atomic

### number

is the### number

of protons. In this diagram, these little red circles are representing protons here in the nucleus so we’ve got 1, 2, 3, 4, 5 which means my## atomic

### number

is 5. That was easy enough.### Mass

### number

is the### number

of protons plus the### number

of neutrons which are these things in blue so### mass

### number

is 1, 2. 3, 4, 5, 6, 7, 8 protons plus neutrons. So### mass

### number

is 8.Sometimes people think you have to have the
same

### number

of protons and neutrons in the nucleus of an atom, definitely not true, they can differ quite a bit. The net## charge

is how protons and electrons balance each other out. So, electrons are represented by these things here flying around the outside the atom, 1, 2, 3, 4 electrons, e- is a way that we can abbreviate electrons, and 1, 2, 3, 4, 5 p+, protons. So, the protons and the electrons don't quite balance out. There is one more protonthan electrons so
that one extra proton is going to give me a net

## charge

of 1+ because each proton has a positive## charge

have one extra proton. Net## charge

of 1+. This is kind of a basic problem, now we’ll move on to something that's a little trickier. So here, we’ve got and atom with this## atomic

### number

, this### mass

### number

, and net## charge

. From this information, we need to figure out the protons, neutrons, and electrons that are in this atom. Protons, that is going to be easybecause
protons… the

### number

of protons is going to be the same thing as## atomic

### number

. So it's going to be 7. Now what about neutrons?### 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 do 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 positive net## charge

with a certain### number

, that's telling me how manymore 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 that I have to have 5 electrons for me to get a 2+## charge

. Here's a table with various characteristics of a certain atom. We need to fill in the things that are empty. Okay, we don't know how many protons, we don't know how many neutrons, where can we start? Okay, we know the electrons and we know the net## charge

so based on the### number

ofelectrons and the net

## charge

this atom has, we can figure out how many protons there are, okay? Since this is a net## charge

of -3 that means that 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… I'm sorry if I had 10 electrons that means that 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 that is also goingto 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 is going to give me 9 neutrons. Sometimes these table### problems

can be the hardest so we’ll do one more example. Here, we want to find the electrons, protons, and the## atomic

### number

. We know the neutrons, the### mass

### number

, and the net## charge

. Okay, let’s start with protons. We know the### mass

### number

, protons plus neutrons, and we know the### number

of neutrons. So that means we can take 25, which is protons plus neutrons, and subtract the### number

of neutrons, 25 minus 11 is going to give us 14, and that is the### number

of protons alone.## Atomic

### number

is just another word for the protons, so these both are going to be 14. Now that we know the### number

of protons in the nucleus, we can use the net## charge

to figure out the### number

of electrons. So it’s a net## charge

of 1+ which means that thereshould be one more proton because it has a positive