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Practice Problems: Net Charge, Mass Number, Atomic Number

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.
practice problems net charge mass number atomic number
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 proton
than 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 easy
because 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 many
practice problems net charge mass number atomic number
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 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

of
electrons 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 going
to 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

practice problems net charge mass number atomic number

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 there
should be one more proton because it has a positive

charge

… one more proton than electron. So 14 protons that means that total I'm going to have 13 electrons. So now that you know how to solve

problems

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