Why are there less electrons on the level nearest to nucleus?

A maximum of two electrons can be held on the first level, compared to many more on the next levels. Why is this? Because there is a smaller circumference?

Reply 1

AN.0

Pls explain in simple terms- no orbitals etc required

Reply 2

_NMcC_

I can't explain this without taking about Orbitals to some extent, I won't go into the maths...

The Pauli Exclusion Principle (sometimes 'Aufbau') forbids this; Basically, Two fermions (in this case - 2 electrons) cannot have the same 'Quantum state', which is the same as saying it can't have the same 'Quantum Numbers'. (Quantum State = 4 Quantum Numbers).

At A level these quantum numbers are the designations 1s2 2s2 etc. (Term Symbols are a more accurate description though.)

1.) '1'/'2' = Is the principle quantum number and represents the energy level of the 'shell' = n

2.) S/P/D/F = Are the angular momentum quantum numbers and represent the shapes of the different spatial wavefunctions (orbitals) = l

S = 0
P = 1
D = 2
F = 3

There are two others;

3. ) The 'magnetic quantum number' (ml) is present to distinguish the individual orbitals within Space, usually best described with cartesian axis.

S can only have a value of 0 (it's a sphere so It has infinite symmetry)

P can have 3 values, -1, 0, +1 which = Px, Py Pz (Each P orbital lies along an axis)

D has 5 five values, -2, -1, 0, +1, 2 (Dxy, Dxz, Dyz, Dz^2, Dx^2-y^2)

F has 7....(pointless for most Chemists unless you're looking at superheavy elements).

4.) Finally the Spin Quantum number represents the weird 'spin' property of the electron and can only be +1/2 or -1/2 = Ms

.....Essentially The Pauli Exclusion Principle prevents two electrons having the same 4 of these values i.e You can't have 2 electrons that have.

1.) Energy level = 1
2.) Angular Q.N = S
3.) Magnetic Q.N = 0
4.) Spin = both spin +1/2 ( or both spin -1/2)

So you can see that there are only two electrons allowed in the first energy level (n=1) as electrons can only have spin of +1/2 or -1/2. So a third electron (like in lithium) has to jump to a higher energy level (n = 2). Whereas on the second energy level, from experimental observation, there are more possible combinations of those 4 quantum numbers (i.e You have the 3 P orbitals as well, different numbers; -1,0,+1 along with S = 0). So the Pauli Exclusion principle is probably the best answer to your question, it's an empirically derived law (from experimental observation) - So ultimately it seems P and S orbitals naturally lie at higher energies.

I should mention also, Hund's rule basically means that the ground state filling of shells has to be done so that the electrons occupy singly first, rather than doubly. So with 2p, 3 orbitals and if you had 3 electrons. You fill; 2px (1), 2py(1) and 2pz(1) rather than e.g 2px(2), 2py(1). Only with >3 electrons would you then start to doubly occupy e.g 4 electrons 2px(2), 2py (1), 2pz (1).

All of this theory arose due to weird experimental results, like the Stern-Gerlach Experiment.

Spin is by far the weirdest one of the lot though, it can't be modelled classically (as in like a 'spinning top') but it basically causes things like Scalar-Coupling in Nuclear Magnetic Resonance (''Fermi contact interaction''). So it's a very real and very unimaginable thing!

Another key point to remember is an orbital is a probability space, so it's a place where you're likely to find an electron within an atom but not actually 100% certain.....

The thing I like about Quantum Theory is, despite it's weirdness, it's verifiable by experiment and the Chemistry agrees with it. Often Theoretical Physics is too reliant in Maths and completely lacks experimental backing. So take things like 'String Theory' and 'Cosmology' with a pinch of salt...should really be called 'String Hypothesis'.

Why are there less electrons on the level nearest to nucleus?

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