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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?
• what does l=0 and where did the n and me come from?
• What type of notation is this: 140 Ce and 141 Ce?
• It's one way to refer to the isotope an atom is. Those are two particular isotopes of cerium. It is typically written with the atomic mass number as a superscript prior to the symbol like ¹⁴⁰Ce, or written with the atomic mass number after the element name like cerium-140.
• Is it too random to change whether it spin up or down simply by referring to the Pauli exclusion principle? If that's the case, can the helium only be spinning down? which is different from only spinning up for hydrogen.

Also, why spinning up is positive 1/2, whereas spinning up and down is negative 1/2.