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How To Calculate The Number of Protons, Neutrons, and Electrons - Chemistry



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in this lesson we're going to talk about

how to calculate the number of protons

neutrons and electrons in an atom and in

an ion so let's use sodium as an example

and on a periodic table you might see

something that looks like this the

smaller of the two numbers is the atomic

number which is also equal to the number

of protons the larger of the two numbers

is known as the average atomic mass now

sometimes you might see a problem that's

written this way they may tell you to

find the number of protons neutrons and

electrons for this particular element

now in this format you can see that the

atomic number is on the bottom the

number on the top noticed that it's not

the same as twenty two point nine nine

in fact it's rounded this is not the

average atomic mass but rather it is the

mass number so this is a specific

isotope of sodium if you average all of

the isotopes of sodium on earth you're

gonna get this number so because that

number is close to 23 this is the most

abundant isotope of sodium on the earth

so in addition to sodium 23 you also

have sodium 24 and is also sodium 22 but

as we said before because the average

atomic mass was twenty-two point nine

nine this is the most abundant isotope

which is also the most stable one on the

earth the other forms of sodium they're

not stable and they're not very common

but these are other isotopes of sodium

now what you need to know is that the

atomic number is associated with

the identity of the element so sodium

always has an atomic number of 11 now

the mass number can change and it will

change for different isotopes so that

number can vary but the identity of the

element is dependent on an atomic number

now let's talk about the formulas that

we need in order to calculate the number

of protons and neutrons and electrons so

let's start with the number of protons

this is always equal to the atomic

number that's it so sodium always has 11

protons now the number of neutrons is

the difference between the mass number

and the atomic number and then the

number of electrons is equal to the

atomic number minus the charge of the

species so for an atom let's say like

aluminum which doesn't have a charge the

charge is zero the electrons will thus

be equal to the atomic number so for an

atom the number of electrons and protons

are the same because the atom is neutral

but if we have in an ion let's say a

positively charged cation then the

number of electrons will change in this

case there's gonna be three more protons

than electrons or if we have an anion

like the phosphide ra9 there's gonna be

three more electrons than protons so for

ions the number of electrons and protons

are different but for atoms which are

electrically neutral the number of

protons and electrons are the same so

let's start with this example go ahead

and calculate the number of protons

electrons and neutrons for this element

so the number of protons is equal to the

atomic number which is

the smaller of the two values so that's

gonna be 11 the number of neutrons is

the difference between the mass number

which is 23 and the atomic number which

is 11 so it's 23 minus 11 and that gives

us 12 so we have 11 protons 12 neutrons

now for the number of electrons it's

equal to the atomic number which is 11

minus the charge now there's no charge

for this particular element it's an atom

not an ion so the charge is 0 so we have

11 electrons so as we can see for

electrically neutral atoms the number of

protons and electrons are the same now

let's consider another example aluminum

27 with a 3 plus charge so here we have

an ion and go ahead and calculate the

number of protons and neutrons and

electrons in this ion so we can see that

the atomic number is 13 and the mass

number is 27 so the number of protons is

always equal to the atomic number which

is 13 in this example now the number of

neutrons is the difference between the

mass number and the atomic number so

it's 27 minus 13 which gives us 14 now

for the number of electrons it's going

to be the atomic number which is 13

minus the charge on the ion which is

positive 3 so 13 minus 3 is 10 so 4

positively charged cations there's going

to be more protons and then electrons so

because it has a positive 3 charge

there's 3 more protons and then

electrons now let's try a negatively

charged anion so this is the sulfide ion

and let's calculate the number of

Tod's neutrons and electrons so we have

an atomic number of 16 and a mass number

of 33 so the number of protons just like

before

is equal to the atomic number so that's

16 the number of neutrons is a

difference between the mass number and

atomic number so 33 minus 16 so we have

17 for this example and finally the

number of electrons is the atomic number

which is 16 minus the charge which is

negative 2 so 16 minus negative 2 is the

same as 16 plus 2 so we get 8 and

electrons thus 4 negatively charged ions

or anions you're going to have more

electrons than protons for an ion with a

negative 2 charge there's two more

electrons than protons now let's try a

different example or at least one that's

presented in a different way find the

number of protons neutrons and electrons

in silicon 29 so in this example we need

to write this in a format that we're

familiar with so silicon 29 that tells

us that the mass number is 29 but what

about the atomic number what we need to

use the periodic table so if you go to

it if you don't have one you can go to

Google Images you'll find that silicon

is the 14th element in the table so it

has an atomic number of 14 now there was

no charge indicated in this problem so

we're gonna assume that we're dealing

with an atom and not an ion thus we have

14 protons because that's the atomic

number and the number of neutrons is

going to be the mass number

- atomic number so that's 29 - 14 which

gives us 15 finally the number of

electrons is going to be the atomic

number minus the charge so it's 14 so

for all atoms the number of protons and

electrons are the same now let's work on

a challenge problem so let's see if you

truly understand the lesson in this

video so we have a cation with a 4 plus

charge and it has 26 neutrons and 18

electrons what is the identity of the

element feel free to pause the video if

you want to try it by the way for those

of you who haven't done so already feel

free to subscribe to this channel and

don't forget to click on that

notification bell and now let's begin

working on this problem so right now

we're going to say that the unknown

element is element X so we know it has a

4 plus charge and the number of neutrons

is 26 the number of electrons is 18 in

order to determine the identity of the

element what do we need to know remember

the atomic number identifies the element

so if we can determine the number of

protons we can identify the unknown

element and so that's what we need to do

in this example now which of the three

formulas that we wrote in a beginning

can help us here is it this one the

number of protons is equal to the atomic

number now this is not going to help us

because we know neither of these could

we use this the number of neutrons is

equal to the mass number minus the

atomic number now we have the number of

neutrons but we don't know the mass

number or the atomic number

so the formula that is going to be

helpful is the third one the number of

electrons is equal to the atomic number

minus the charge because we know the

charge and the number of electrons with

that we can calculate the atomic number

so there's 18 electrons and I'm just

gonna write a four atomic number and the

charge is positive four so this becomes

a minus four so we need to add four to

both sides and so the atomic number is

22 and the number of protons is equal to

the atomic number which is 22 now if we

wish to calculate the mass number we can

the number of neutrons was the

difference between the mass number and

the atomic number if you rearrange that

equation you'll find that the mass

number is the neutrons plus the atomic

number or you could say is the neutrons

plus the protons because the protons is

the same as the atomic number so it's 26

plus 22 which is 48

now granted that last step wasn't really

necessary but I decided to do it anyway

for those of you who were curious on how

to get it but let's focus on this number

so if you go to the periodic table and

look for element 22

let me get mines real quick you'll find

that this corresponds to titanium so

this is the titanium 4 plus ion and this

is the answer

so titanium is the unknown element and

that's how you can identify any unknown

element for these types of problems once

you identify the number of protons you

could use a product able to identify

them as an element