List two major problems we face with nuclear reactors A. the giving off of dangerous rays B. air pollution C. heat pollution D. water pollution

You use the atomic number to find out how many protons and neutrons there are in the atom. you use the atomic mass to find out how many protons and electrons there are in the atom. 

Neither model is able to show the reactivity of potassium.

☆ionisation energy increases across a period from na -> ar☆ionisation energy decreases down a group from li -> fr☆ionic radius decreases across a period from na -> cl☆ionic radius increases down a groupfrom f -> ii hope it clarifies your doubts