Explain Please

The atomic number (Z) represents the number of protons in an atom.
The mass number (A) represents the number of both protons and neutrons in an atom, this is because neutrons and protons are the heaviest particles in an atom, and so the atom's mass is equal to the mass of the protons and neutrons it contains.
If you have the total number of "protons + neutrons " which is the mass number (A), and you also have the number of protons (the atomic number (Z)), then you can deduce the number of neutrons present.

A = Z + N(number of neutrons).   ===> N = A - Z

Wow, how did I miss that? Clearly I was not thinking along that line. Thanks!

Another thing I'm not seeing is how to find the valence like  of hydrogen or carbon. I know hydrogen is 1 and carbon is 4.

In the first shell, you have 2 electrons, second shell you can have a maximum of 8, third max of 8, and so on.

So carbon has 6 electrons. So you will have 2 in the first, and 4 in second.

In upper year chemistry, it can also be written like this:

Using the 1s, 2s, 2p notation for atomic orbitals, the electrons are divided up as follows:

The 1s shell has two electrons

The 2s shell has two electrons

The 2p shell has two electrons.