What is relative atomic mass of an element and why is it a decimal?

The atomic mass is an average of the sum of protons and neutrons in an atom. For example, chlorine always has 17 protons but it's atomic mass is 35.45. That means it usually has 18 or 19 neutrons.

Both models are effective because they show the valence electrons, and these valence electrons indicate the reactivity of an element.

Ametal with a large number of free flowing electrons most likely has good conductivity