The atomic number is the number of protons in the nucleus of an atom.
In a neutral atom the numbers of protons (+) and electrons (-) are the same.
The mass number is the total number of protons and neutrons in the nucleus of an atom.
But wot about the electrons?
Electrons are arranged in cloud like shells around the nucleus.
Shells closest to the nucleus fill up with electrons first.
The first shell is full with two electrons.The second and third shells are full with eight electrons.
For the first twenty elements remaining electrons go in the fourth shell.
The sodium atom
the atomic number is 11. There are 11 protons in the nucleus and eleven electrons in the shells around the nucleus.
Two electrons go in the first shell, eight in the second and the remaining electron goes in the third shell.
The electron configuration for sodium is Na(2,8,1). This is enough information to understand how the first 20 elements in the periodic table react with each other,
