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# 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?
• 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! :)
• Hey guys, what does l, m, n in the video mean?? Still can't get it.
• At min why is the spin quantum number +1/2 and not -1/2 ?
• 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.
• 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.
• 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.
• What is a quantum number?
• 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?
• I am so confused, Is there a simpler version of this video?
• There are two electrons in Helium atom. Which electron's spin(?) direction is represented by Ms=-1/2?
• It can be any one's spin. But, negative spin is usually given to the second electron.
• 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?
• 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..
• 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)