Main content

## Class 11 Chemistry (India)

### Unit 2: Lesson 9

Electronic configuration of atoms- Introduction to electron configurations
- Electron configurations article
- Noble gas configuration
- Electron configurations for the first period
- Electron configurations for the second period
- Electron configurations for the third and fourth periods
- Electron configurations of the 3d transition metals
- Electron configurations
- Electron configurations of ions
- Atomic structure and electron configuration

© 2023 Khan AcademyTerms of usePrivacy PolicyCookie Notice

# Electron configurations for the first period

Introduces aufbau principle, Pauli exclusion principle, and orbital notation. Writes out the quantum numbers for the electrons in H and He. Created by Jay.

## Want to join the conversation?

- What does the spin of an electron even mean in general?(12 votes)
- There isn't any very good analogy between quantum spin and a non-quantum motion, so it is most often compared to angular momentum. Angular momentum in the non-quantum world means rotation - for instance, a wheel on a moving car is rotating, and therefore has angular momentum.

When you rotate an electrically charged object, a magnetic field is produced. Given the size, charge, and rotational speed of an object, you can calculate the magnitude of the field produced. We know the size, charge, and magnetic field of electrons - the only problem is, according to classical (non-quantum) formulas, the rotational speed of the electron necessary to generate their magnetic field is faster than the speed of light - which obviously isn't possible.

So while electrons don't actually rotate in the conventional sense, we still call the quality that produces their magnetic field "spin."

Let me know if that helped! :)(20 votes)

- Hey guys, what does l, m, n in the video mean?? Still can't get it.(9 votes)
- At2:50min why is the spin quantum number +1/2 and not -1/2 ?(5 votes)
- Electrons that are not required to have a particular spin by Hund's Rules can freely change between the +½ and −½. Since monatomic hydrogen has a single electron it is just as likely to be +½ and −½. So, in a given sample of monatomic hydrogen (which is not stable in the conditions found on Earth, BTW) we would expect approximately half the sample to have a +½ spin and the other half −½ spin.(8 votes)

- I'm not sure if I skipped a video or what but, I don't understand what he means by "spin up or spin down". That just confused me.(2 votes)
- Electrons have a quantum state usually called "spin" (it has nothing to do with spinning around, this is just what we call this state). Have two possible spins, +½ or −½. These are often depicted using up arrows or down arrows, so "up" and "down" are just unofficial ways of referencing the two possible spin states.

For two electrons to exist in the same orbital, they must have opposite spins.(8 votes)

- What is a quantum number?(2 votes)
- A quantum number is a number that occurs in the theoretical expression for the value of some quantized property of a subatomic particle, atom, or molecule and can only have certain integral or half-integral values. Confusing definition right?(5 votes)

- I am so confused, Is there a simpler version of this video?(4 votes)
- Might want to start with these two:

https://www.khanacademy.org/science/chemistry/electronic-structure-of-atoms/electron-configurations-jay-sal/v/orbitals

https://www.khanacademy.org/science/chemistry/electronic-structure-of-atoms/electron-configurations-jay-sal/v/more-on-orbitals-and-electron-configuration(2 votes)

- There are two electrons in Helium atom. Which electron's spin(?) direction is represented by Ms=-1/2?(2 votes)
- It can be any one's spin. But, negative spin is usually given to the second electron.(2 votes)

- Hey, Can anyone tell me how was Pauli exclusion principle found?

I mean according to Pauli exclusion principle why no two electrons in an atom have same 4 quantum number?What is the reason behind that?(2 votes)- this is actually a good and important question............

as they must have opposite spins to have a repulsion force existing between them. this helps us to distinguish between all electrons in a atom. if an electron's spin is given an value of +1/2 then we give a value of -1/2 to the electron which is opposite to it in their shell. basically any two electrons that are found in the same orbital, and same sub-shell and in the same shell are given different "spins"! hope u got it......

you can get more information regarding your question by simply searching the word Pauli exclusion principal itself..(2 votes)

- Does the first electron always have to have an up-spin? For example, can a Hydrogen atom have only a down-spin for its electron?(1 vote)
- It does not matter which electron is up-spin and which is down-spin. These are just arbitrary terms used to represent the two different possible values for an electron's intrinsic angular momentum. :)(1 vote)

- I'm having some trouble with understanding how to calculate certain quantum numbers. I know what they are, but even after I watch the Q. Numbers video, I'm not really able to figure out how to calculate
**l**and**ml**(1 vote)- For any value of n, possible values of l are integers from 0 to n-1

Each possible value of l represents a type of orbital.

When n=1 the only possible value of l is 0, so the first (n=1) shell only has an s orbital

When n=2 the possible values of l are 0 and 1, so the second (n=2) shell has s and p orbitals

When n=3 the possible values of l are 0, 1 and 2, so the third (n=3) shell has s, p and d orbitals

See where this is going?

For any value of l, possible values of ml are integers from -l to +l

The number of possible ml values tell us how many of that orbital there are.

When l=0 the only possible value of ml is 0, this tells us there is only one s orbital per shell

When l=1 the possible values of ml are -1, 0 and +1, this tells us there are three p orbitals per shell (from the second shell onwards)

When l=2 the possible values of ml are -2, -1, 0, +1 and +2, this tells us there are five d orbitals per shell (from the third shell onwards)(2 votes)

## Video transcript

- [Voiceover] Let's look at how to write electron configurations
for the first period. And so here's the first
period in the periodic table, and we have only two
elements to worry about. We have hydrogen and then
over here we have helium. So let's start with hydrogen, atomic number of one. So if there's an atomic number of one, that means there's one
proton for hydrogen. In a neutral atom, the number of protons is equal to the number of electrons. So if there's one proton
there must be one electron. So our goal is to write
an electron configuration for that one electron of hydrogen. And we're gonna use the Aufbau principle. Aufbau is German for "building up". Because as you write
electron configurations you're thinking about the
best way to build up an atom. So you're thinking about
where to put your electrons. Here we have only one
electron to worry about. So where's the best place to put the one electron for hydrogen? Well, we wanna put that electron as close to the nucleus as possible, in order to maximize the attractive force between the positive charge
and the negative charge. So therefore, the electron goes into the lowest energy level possible. And that's when n is equal to one. So we talked about
quantum numbers earlier, if n is equal to one there's
only one allowed value for l, and that's equal to zero. If l is equal to zero, there's only one allowed
value for ml right? So the magnetic quantum number, that's equal to zero. So l is equal to zero tells us we're talking about an s orbital, and this tells us how many orientations. Only one value so only
orientation for an s orbital. An s orbital is shaped
like a sphere right? So in this sphere, in this three dimensional volume here, this is the most likely place, the most likely region we're going to find this one electron. And so the electron for hydrogen is going to go into an s orbital. An s orbital in the first energy level. So let's go ahead and write
the electron configuration. We write the electron
configuration as one s one. Let's talk about what those mean here. So this first one, this is talking about
the energy level right? The shell, n is equal to one. S says the electron for hydrogen goes into an s orbital. And this superscript one here, this is telling us how many electrons are in that orbital. And here of course we're
talking about only one electron. So one s one means one electron in an s orbital in the first energy level. There's another way to write
an electron configuration, or to draw one out, it's called orbital notation. So you draw a line here, which represents an orbital. We're talking about an s orbital in the first energy level, so we could label this orbital
as being the one s orbital. And we put the one electron of hydrogen into that one s orbital. And let's say the electron
enters the orbital spin up. So this arrow pointing up is representing one electron with an up spin. So the fourth quantum number ms we could say that's positive 1/2 spin. So here are two ways to write
the electron configuration. One s one, or we could draw orbital notation like that for hydrogen. Alright, and so we're done
with hydrogen's one electron. Let's move on to helium now, so two electrons to worry about. So atomic number of two, so two protons and two electrons. So two electrons to worry about. We're still in the first shell, we're still in the first energy level. So n is equal to one. If n is equal to one, l
must be equal to zero. Ml must be equal to zero, and so we're still
talking about an s orbital in the first energy level right? So we're still talking about an s orbital in the first energy level. So for helium right? An s orbital in the first
energy level, like that. Alright, let's think about
orbital notation for helium here. So we have two electrons, so an s orbital in the first energy level. So we could draw the first
electron for helium as spin up. And the second electron for helium, we would have to do that spin down. So we have to pair the spins, one spin up and one spin down. So why do we have to do that? So let me go ahead and write, I'm gonna write negative
1/2 here for the spin. The reason we have to pair the spins is because of the Pauli
exclusion principle, which says that, "No two electrons in an atom can have "the same four quantum numbers." So this first electron that we put in, the one s orbital right? So this one right here is spin up, that would be these same
four quantum numbers as, that would be these four
quantum numbers up here. So instead of rewriting them, I'll just circle them for hydrogen. And so for this second electron here, the one that I put in
the orbital spin down, that can't have the same
set of quantum numbers. So n is equal to one, l is equal to zero, ml is equal to zero, all those have to be the same, but the last one here is different. That's why we make it negative 1/2 so it's spin down. And so the two electrons in helium have a different set of
four quantum numbers right? They differ by the last quantum number. And so that's the idea of the
Pauli exclusion principle. As a consequence of the
Pauli exclusion principle, an orbital can contain a
maximum of two electrons, because you've exhausted
all of the possible combinations of quantum numbers. We've used them up completely. And so the one s orbital
is completely full. So we could also write
the electron configuration for helium right, as one s two. And once again what that means, is we're talking about an s orbital, s orbital in the first energy level, and there are two electrons
in that s orbital. So one s two is the electron
configuration for helium. And since we have two
electrons in the one s orbital, we can't fit in any more electrons. And so the first shell is closed. We have a closed shell. There are no more orbitals
in the first energy level. If you wanna add another electron, you have to move on to the next shell. And so that takes us
into the second period on the periodic table.