What do you mean by atomic number?

Atomic number is the number of protons in an isotope or element.

Explanation: The atomic number is the same number of protons in an element or isotope and if said isotope is neutral than it is the same number of electrons.

Example: Atomic number = 19 Protons = 19 if neutral Electrons = 19

in the analysis of systems that involve fluid flow, we frequently encounter the combination of properties u and pv.