Protons, Neutrons and Electrons: A Guide

I made a sort of informational post about protons, neutrons and electrons. Here it is:
(note: red asterisks are where I had to do research)

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So first things first, you must know your elementary particles:

• Protons are positive and have an mass of 1
• Neutrons are neutral and have a mass of 1
• Electrons are negative and have a mass of ¹/₁₈₂₂*

By the way, mass is measured in atomic mass units. It doesn't really matter what the mass of poarticles are right now, but for reference, the units used will be at the bottom of this help thing.
Your average atom is made up of these 3 particles. The atoms need a neutral electric charge, so they have the same number of electrons.
This might better be explained with an example:
 
U-235 is an isotope of uranium. Uranium has an atomic number of 92*. This means it has 92 protons. Because, of course, the number of protons and electrons have to be balanced, this means it has 92 electrons too.
Now, we can work out neutrons. To find out the number of neutrons, first we have to look at the isotope number. As you can see, the isotope number is 235.
This isotope number is equal to the sum of the number of neutrons and protons.Since we know nthe number of protons, we can work out the neutrons easily, using the sum 235 (the isotope) - 92 (the protons). This comes to 143 neutrons.
Electron Shells
Now that we know the number of electrons, now to talk about electron shells. Each electron orbits in a 'shell' going around the atom. As more electrons go in, the shells fill up.

In this diagram, you can see the 'shells' in Oxygen. Each shell is made of a type of subshell, which can hold different numbers.

• The s subshell can hold 2 electrons.*
• The p subshell can hold 6 electrons.*
• The d subshell can hold 10 electron.*
• The f subshell can hold 14 electrons.*
• The g subshell can hold 18 electrons.

And here are the shells:

• The first shell is called the 'K' shell*. It is made up of the subshell 1s*.
• The second shell is called the 'L' shell*. It is made up of the subshells 2s and 2p*.
• The third shell is called the 'M' shell*. It is made up of the subshells 3s, 3p and 3d.
• The fourth shell is called the 'N' shell. It is made up of the subshells 4s, 4p, 4d and 4f.
• The fifth shell is called the 'O' shell. It is made up of the subshells 5s, 5p, 5d, 5f and 5g

And so on, until the 'Q' shell.
To tell which order these subshells are filled in, we use the equation n+I. n is the number of the subshell (e.g for 4s, n=4).* I is the letter, starting at s=0 and then going on to p=1 (e.g. for 4s, I = 0).* They are filled in increasing order of n+I.*
If two subshells have the same number of n+I, then they are sorted in increasing order of n. This gives us the subshells filled like this:

• 1s (1+0=1)
• 2s (2+0=2)
• 2p (2+1=3)
• 3s (3+0=3)
• 3p (3+1=4)
• 4s (4+0=4)
• 3d (3+2=5)
• 4p (4+1=5)

and so on. Some elements are exception to this 'rule', but that would be getting into Special Relativity and Hartree-Fock calculation*, which are far too complicated.
In our next lesson, we will talk about Ionic bonds. Yay! !
If you have any questions, just reply to this topic.
PS:
One atomic unit (what the particles mass is measured in) is 1.6726 yoctograms*.
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