This represents the total number of protons in an atom's nucleus. It's similar to your ID number; it identifies who you are (or what element an atom is).
These elements have full electron shells and don't usually react with other elements because they're content just as they are - kind of like someone who prefers staying home instead of going out partying!
Shielding Effect: This describes how core electrons shield or protect the positive charge from the nucleus from reaching the outer (valence) shell. Imagine them as bodyguards protecting a celebrity (the nucleus) from fans (the valance shell).