elements exist in more than one form eg carbon 12 and carbon14
Most atomic masses on the periodic table are decimal numbers because they are weighted averages of the masses of all isotopes of an element, taking into account their relative abundance. Since isotopes have different masses, the atomic mass is typically not a whole number.
You will get the atomic bomb!
The current standard for atomic masses is based on the Carbon-12 isotope. It is defined as exactly 12 atomic mass units (amu), with all other atomic masses determined relative to it. This standard allows for consistency in measurements and comparisons of atomic masses across different elements.
Atomic masses are determined by mass spectrometry. The atomic number is identic with the number of protons in the atom - depends on position in the periodic table.
The average atomic mass of an element is the average of the atomic masses of its isotopes (that is a weighted average). You have to take into account the abundance of each isotope when they do your averaging.
Electrons
Isotopes are atoms with the same number of protons (same atomic number) but different numbers of neutrons, leading to different atomic masses. For example, carbon-12 and carbon-14 are isotopes of carbon with atomic number 6 but atomic masses of 12 and 14 respectively.
Dmitri mendeleev related the chemical properties and atomic masses around 1860
Dmitri arranged the elements in order of increasing atomic masses. He arranged elements in rows and columns according to atomic masses.
1. First cause: the atomic weight is the sum of the weights of protons, neutrons ans electrons.; they don't have masses as integers.2. Second cause: also occurs the so-called mass defect.Not atomic mass for elements; atomic weight is correct.
Isotopes
The isotope used as the reference for atomic masses is carbon-12, with a mass of 12 atomic mass units (amu).