Need up to 30 seconds to load.

in this video I want to tell you how you

can calculate valence electrons but also

to understand it as well so let's look

at an atom of nitrogen so nitrogen has

an atomic number of seven and an average

atomic mass of fourteen point zero one

so an atom of nitrogen has seven protons

and seven electrons atoms are

electrically neutral now let's draw a

simple Bohr model of the nitrogen atom

in the first shell or the first energy

level nitrogen carries two electrons

that's the maximum number of electrons

you could have in a first energy level

in the second energy level you could

have up to eight electrons but what we

have is a total of seven so we need to

place five and the second energy level

so we could have a total of seven the

valence electrons are the electrons in

the outermost energy level so therefore

a nitrogen has five valence electrons so

here are the five valence electrons of

the nitrogen atom now the inner shell

electrons the ones on the inside the

ones that are not valence electrons

these are known as core electrons so

nitrogen contains two core electrons for

a total of seven electrons which is

which goes with the atomic number of

nitrogen now keep in mind for atoms the

number of protons and electrons are the

same but for ions they differ so you

have to adjust the base on a charge now

let's look at another example let's

consider the aluminum atom which has an

atomic number of 13 and an average

atomic mass of 26 point nine eight how

many valence electrons and core

electrons can be found in the aluminum

atom so let's draw a picture

so this is gonna be the first shout and

this is the second energy level and this

one is gonna be the third so in the

first energy level there's gonna be two

electrons now we said the second can

hold a maximum of eight electrons so

right now we have a total of ten but we

need to get up to 13 so that's 13 so

aluminum has three valence electrons

those are the electrons in the highest

energy level now how many core electrons

does have it has two in the first shell

and then eight in the second shell so it

has a total of ten core electrons if you

add ten plus three you're gonna get the

atomic number for the atom thirteen

electrons in total so just make sure you

understand is the valence electrons are

the electrons in the outermost energy

level these are the electrons that

participate in a chemical reaction the

inner core electrons for the most part

really participate in a chemical

reaction

it's the valence electrons that are

involved in reactions that we see in a

day-to-day basis now another way in

which you can identify the number of

valence electrons it is by writing the

electron configuration so let's write it

for nitrogen and for aluminum the

electron configuration for nitrogen I'm

assuming that you already know how to do

this

if not you can check out another video

that have on YouTube it's 1s2 2s2 2p3

so notice that the exponents add up to

seven because nitrogen has a total of

seven electrons now in the highest

energy level that's in the second energy

level we have two sub levels 2s and 2p

but in the second energy level notice

that we have a total of five electrons

so those five are the five valence

electrons

those are the electrons in the outermost

or the highest energy level all the

other electrons are known as core

electrons so you can see that nitrogen

has two core electrons now let's

consider aluminum the electron the

figuration is 1s2 2s2 2p6 3s2 3p2 on the

s sublevel can only hold two electrons P

can hold up to six electrons and D can

hold up to ten but we look at the

highest energy level of aluminum which

is the third energy level we could see

that there are three electrons there two

plus one extreme so aluminum contains

three valence electrons all the others

represent core electrons so we got two

plus two plus six so aluminum contains

ten core electrons so that's how you can

identify the number of valence electrons

in core electrons using the electron

configuration now let me give you

another example let's use chlorine which

has an atomic number of 17 so in an atom

of chlorine write the electron

configuration and then identify the

number of core and valence electrons so

the electrical figuration of chlorine is

1s2 2s2 2p6 3s2 3p5 all the exponents

out of 17 so the valence electrons are

the electrons in the highest energy

level so in the case of chlorine we have

two plus five so seven valence electrons

and the rest our core electrons so we've

got a total of ten core electrons

another way in which you can identify

the number of valence electrons is using

the periodic table you can identify it

based on a group number of

Representative elements so in group

number one we have elements like

hydrogen lithium sodium potassium these

elements all have a valence electron of

one they have one valence electron now

in the second group you have the

alkaline earth metals like beryllium

magnesium calcium so these metals have

two valence electrons they're in group 2

of the periodic now we're gonna skip the

transition metals and move on to group

13 also known as group 3a so these

include aluminum and gallium and I

believe indium is the next one so these

elements have three valence electrons

and I'm forgetting something

can't forget about boron after boron is

aluminum then we have helium so these

elements contain three valence electrons

and then after born you have carbon

silicon germanium and those elements

contain four valence electrons and then

you have nitrogen phosphorus and I

believe arsenic so these they're found

in group five a so they contain five

valence electrons then you have the

chalcogen like oxygen sulfur selenium

these guys they hold up to six valence

electrons and then you have the halogens

like fluorine chlorine bromine iodine

those guys they have seven now be

careful of this one helium is a noble

gas but it only has two valence

electrons because it has a total of two

electrons so here having but the other

ones after it like neon and I think it's

a argon krypton all of these they have

eight valence electrons now I have

another question for you let's focus on

iodine which has an atomic number of 53

and an average atomic mass of 120 6.9

now how many core electrons are found in

an atom of iodine think about it now you

can use any one of the techniques that

we've illustrated in his video you can

draw the Bohr model of the atom which is

gonna take time to draw 53 electrons you

can also write out the electron

configuration which works as well but

because iodine is a relatively large

atom it's going to take some time to

draw the electrical figuration so is

there an efficient way to determine the

number of core electrons what's the

easiest way to

quickly give the answer now we can start

with an equation the total number of

electrons is equal to the sum of the

core electrons plus the valence

electrons keep that in mind

now for an atom of iodine we have 53

electrons in total now in order to find

the core electrons we need to find the

valence electrons first now we could do

so by using the group number iodine is

in group 7a it's a hill agem so it

contains seven valence electrons so the

number of core electrons is simply the

total number of electrons which you can

find a base on the atomic number of an

atom minus the valence electrons found

by the group number so it's gonna be 53

minus seven which is 46 core electrons

because 46 plus seven is 53 and so

that's an efficient way to find the

number of core electrons when you're

dealing with an element that's pretty

large you don't want to draw it out over

at out the electron configuration that's

gonna take time

so just identify the atomic number and

the group number subtract those two and

that will give you the number of core

electrons