Blog of Veikko M.O.T. Nyfors, Hybrid Quantum ICT consultant

Quantum Mechanics demystified, a try

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What’s keeping things from falling just apart as plasma

From quarks, through atoms and molecules, to planets and glaxies, what’s holding these things together. Forming solid objects or liquids or gases instead of just being plain plasma of particles.

In short, it is a question of four fundamental forces of nature: Strong nuclear force, weak nuclear force, electromagnetic force and gravity. These forces build up bonds between subatomic particles, atoms, ions, molecules larger and objects built up of molecules.

In this article I’m trying to summarize and clarify it to myself.


On atomic and subatomic scale, having a look on Subatomic particles might be clarifying.

Atom consist’s of it’s nucleus and electrons ‘orbiting’ it.


Nucleus consists of one or more protons and zero or more neutrons.

Protons and neutrons in the nucleus are held together by residual nuclear force. This force vastly overcomes the electromagnetic repelling force between protons.
Residual nuclear force is mediated by gluons. In this case, I suppose, gluons are exchanged between quarks in distinct protons and/or neutrons.
Earlier understanding was that Pions, or quark-antiquark pairs, were mediating this force. If huge enough amount of energy is proposed, a quark can stretch out from the proton. Once this happens, the quark immediately couples with an anti-quark to form a Pi-meson. The anti-quark has been formed, I guess, by the huge amount of energy around. This Pion is very unstable and decomposes back to a quark and anti-quark pair very shortly. When this is close to another proton, the anti-quark combines with the quark inside the proton making them both disappear, causing the left around quark to join the proton. Also gluons were involved in this process, when quark and anti-quark pairs were formed and decayed.

There is another force that takes effect in nucleus, weak nuclear force. Several kinds of it exist, including beta decaying, electron capture. Hoping to be able to go through all of these some day.

Proton and Neutron

Protons and neutrons are built of quarks, specifically Up and Down quarks.
Proton has UP+Up+Down (UUD) quarks, neutron has DDU.

Quarks within proton and neutron are held together by strong nuclear force.
Strong nuclear force is caused by quarks exchanging gluons inside the proton or neutron.


In equilibrium state there is as many electrons as there is protons in nucleus.
Atom can loose or gain an electron making it negatively or positively charged atom, an ion.

Electrons surround nucleus, not by orbiting it, but each one taking up a specific volume around the nucleus. There somewhere it can be found with quantum mechanical probability.
Traditionally we talk electrons would orbit nucleus as a point particle at certain distance. But as electrons are actually waves, they act quantum mechanically, not being at a specific spot with specific momentum at any time. Instead they fill out a specific volume where it can be found somewhere with some probability. It is much easier to talk about this using orbiting conceit, though.

Electrons position and energy is described with three quantum numbers.

Quantum principal number n, n = 1, 2, 3, …
Describes the energy level of the electron. Called shell or orbit. The higher the number, the further away from nucleus and the higher the energy.

Angular Momentum Quantum Number l, l = 0 - 2^n-1 Characterizes the shape of the volume. This is called a subshell and is synonymous (kind of :-) to atomic orbital. Subshells are named s, p, d and f. Each subshell can have max 2 electrons, with distinct magnetic quantum numbers.
Subshells are organized on each shell as below.

First shell n=1 has one s subshell -> max 2 electrons.
Shell n=2 has 1 s and 3 p subshells -> max 8 electrons.
Shell n=3 has 1 s, 3 p and 5 d subshells -> max 18 electrons.
Shell n=4 has 1 s, 3 p, 5 d and 7 f subshells -> max 32 electrons.

Theoretically there is also subshell g, but current periodic table does not describe any atoms having electrons on g subshell.

As a sample, Neon with 10 protons and 10 neutrons $^{20}_{10}\text{Ne}$ has electron configuration of 1s² 2s² 2p⁶.
I.e. it has 2 electrons on first shell’s s subshell, 2 electrons on second shell’s s subshell and the remaining 6 electrons on 2nd shell’s 3 p subshells, two at each.

Electron configuration of Xenon, $^{131}_{77}\text{Xe}$ is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶

Magnetic Quantum Number (m_l or m) specifies spatial orientation of the subshell. This feature is the basis for magnetism. More of that in Magnetism

Finally to the point, what keeps the electron from not falling to the positively charged nucleus? Due to electromagnetic attraction it really should, shouldn’t it?
Or, what keeps the electron not from shooting out of nucleus’ reach.

For the latter first.
Actually an atom can loose one or more of it’s electrons if energetic enough photon excites an electron so that it is kicked out. Involved electron is most likely from the outermost subshell, as it would require least energy. But surely electron could be from lower subshells as well, if photon’s energy was big enough. In that case, remaining electrons on upper subshells would drop downwards filling up the slot of the runaway electron, releasing photon(s) of appropriate energy/energies in this process.

And for the first then.
Electrons position or volume around the nucleus is expressed by appropriate wavefunction. Nucleus has it’s own wavefunction describing it’s volume. These volumes overlap with some extent, being minor though, I think. So in a way, electron and nucleus are kind of overlapping each other really.
But they do not react themselves under normal circumstances. Only with specific unstable atoms an electron capture, which is one form of radioactive decay, can take place.