Isotope Notation

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isotope notation isotope notation is a

way that we can write the symbol for an

atom and show the number of protons

neutrons and sometimes electrons that

are in it isotope notation you see it

all over chemistry but it's particularly

important in nuclear chemistry because

in nuclear chemistry it's all about the

nucleus and so when when we write down

an atom we want to be able to show

pretty quickly how many protons and

neutrons are in it so here is what it

looks like to write an atom in isotope

notation we got a couple things going on

here first we got these letters the

letters represent the element of the

atom we're dealing with

so here it's mg so we know which stands

for magnesium so now we're talking about

an atom of magnesium then we've got

these numbers here number down here is

the atomic number it's the number of

protons that are in the nucleus of this

atom so here we have 12 protons in the

nucleus an atomic number of 12 the

number up here is called the mass number

which is the number of protons plus the

number of neutrons don't make the

mistake and think that this is the

number of neutrons because it's not its

protons plus neutrons together then

we've got two plus here and this

represents the net charge of this atom

so we've got protons and electrons and

they balance each other out so we have

the same number of protons electrons in

a net charge of zero but unless we have

the exact number the exact same number

of protons and electrons the atom is

going to have a charge so a little bit

negative or a little bit positive and

that's what the net charge represents

when we write something in isotope

notation it's usually because we want to

be able to find out the number of

protons and neutrons and I said

sometimes electrons if you're adding the

net charge of you let's do this now how

many protons are in this app okay that's


the atomic number here is 12 so 12

protons how many neutrons in this in

this atom okay that's a little bit

harder because we don't have a number

here that's actually neutrons but this

number here is protons plus neutrons

together so if I know protons and


and I know the number of just protons I

can take my protons and neutrons my mass

number and subtract the atomic number

subtract the number of protons and I get

25 minus 12 going to give me 13 neutrons

the mass number minus the atomic number

okay finally electrons here's how I like

to think about it this is a net charge

of positive so means that we have to

have more protons than electrons and

since it's 2 plus it means we have to

have 2 more protons than electrons okay

so if we had 12 electrons these guys

would balance out and they'd be neutral

since I need to have 2 more protons

electrons I want to take the balanced

number of electrons minus 2 is going to

give me 10 electrons now I have 2 more

protons and electrons and that's going

to give me a +2 net charge let's do two

another example here let's swap in

titanium now you'll see something right

away about titanium and that's the

titanium doesn't have a net charge

written here and so if you write an atom

you see an atom written in isotope

notation and it doesn't have a net

charge that means if it does have a net

charge written up here that means the

net charge is zero the protons and

electrons balance perfectly so protons

neutrons and electrons for this guy

protons is the atomic number 22 and


the mass number 48 protons plus neutrons

minus 22 the number of protons and we

get 26 neutrons for electrons they

balance out the protons and as we said

since there's nothing up there it has a

zero net charge so that means I've got

22 protons I've got 22 electrons okay so

now I want to go in the other direction

instead of taking a symbol and finding

out the number

towns neutrons electrons I want to start

with protons neutrons electrons I

already know how many I have I want to

write this symbol here okay so let's

start out with the atomic number 15

number of protons okay I'll write that

in right here

now that the mass number well the mass

number is protons plus neutrons so take

15 add it with 17 and I'm going to get

32 so that's the number if it goes up

here 32 for the mass number okay

what's the letter that goes here what's

a chemical symbol well in order to find

that out I have to look on the periodic

table and I want to look for the element

that has an atomic number 15 if I do

that I find out here that its phosphorus

is I've got this 15 up here so that

means that any atom that has 15 protons

in it has an atomic number 15 its

phosphorus okay so put a big P here that

stands for phosphorus and now the net

charge what's going on here got my

electrons I got my protons in this

example I have more electrons that I

have protons which means that my net

charge is going to be negative I've got

three more electrons and protons so that

means that the net charge is going to be

three - I kind of like to just figure it

out you know think about how many more

electrons or protons I have but if you

want to know how to always calculate the

net charge you can take the number of

protons and subtract the number of

electrons from it and there I get

negative three and that's my net charge

we usually write things three minus when

we're doing the net charge two - you

know that kind of thing let's do another

one of these okay so what's the symbol

of this going to be it's always best to

start with the atomic number because

that's really straightforward 18 18

protons in the nucleus now how about the

mass number I'm going to do protons plus

neutrons 18 plus 22 is going to give me

40 so the 40 goes up here how about the


the symbol is dependent on this number

here 18 I look on the periodic table to

find out that atom that has an atomic

number of 18 and that's argon right here

so AR is a symbol there how about the

net charge how the protons electrons

balance out it turns out that I have 18

protons and 18 electrons so they balance

out perfectly meaning that my net charge

is zero I just leave it blank I don't

write anything in there and that tells

me that I don't have a net charge there

okay so that's how we can take an atom

written in isotope notation and figure

out the number of protons neutrons and

electrons it has in it and then we can

also do the opposite we can take a given

number of protons neutrons electrons and

go ahead and write the symbol for it so

that is that's isotope notation