Question

some element don't have whole number atomic mass.why​

Answer 1

Answer:

The atomic mass is not given as a whole number because it is a weighted average taken of all of an atom's isotopes found in nature relative to the mass of carbon-12. The measurement unit used for atomic mass is the unified atomic mass that has the symbol "u."

As Z increases, the nucleus can support different numbers of neutrons, and thus for heavier elements there is an isotopic distribution. The mass quoted on the Periodic Table is the weighted average of the individual isotopes.

other question

Why don't all elements in the periodic table have whole number?

So what you see written on the periodic is actually the atomic mass, weighted by abundance. Originally Answered: Why don't elements in the periodic table have whole numbers? The reason for that is because the mass number that you would find on the Periodic Table of Elements is a sort of average of the different isotopes.

Hydrogen is the element with 1 proton in its nucleus - Iron is the element with 26, and so on. You can’t have half a proton - so the atomic number (Z) is always an integer. But the atomic mass takes into account the neutrons in the nucleus as well. The problem is - you can have different numbers of neutrons in an element.