Questions:
1. why are atomic masses on the periodic table decimal numbers?

The atomic mass on the periodic table is a decimal number because it is an average of the different isotopes of an element.

P.S.

An isotope is each of two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei, and differ in relative atomic mass but not in chemical properties; in particular, a radioactive form of an element.