what is real meaning of atomic mass and how it is calculated?

Question

According to me atomic mass is number of neutron+number of proton of an atom.it means that atomic mass should be in simple whole number.
But in perodic table for some elements it is given in decimal why?

Answer 1

The number of protons plus neutrons gives the atomic mass number of the isotope. The mixture of isotopes in nature gives the atomic weight. Chlorine has 17p + 18n = 35 or 17p + 20n = 37. There is about 75% Cl-35 in nature and 25% Cl-37. So the atomic weight of chlorine is given as 35.5.

Answer 2

Simple answer: because an element is defined by the number of protons in the nucleus, regardless of the number of neutrons. Nuclei with the same number of protons but different numbers of neutrons are called isotopes, and in many cases there are several stable isotopes (usually multiples of two different).

Atomic mass is defined as 1/12 of the mass of a 12C atom. (having 6 protons, neutrons and electrons each). You will have heard about "radiocarbon" 14C, and there is a stable isotope 13C which makes up about 1% of natural carbon on earth (and possibly across the inner solar system outside of the sun).

Less relevant answer: While neutrons and the combo proton/electron have roughly the same mass, it is not exactly the same, because there is an energy difference that results from beta radioactivity (a neutron emitting an electron at some speed - the kinetic energy which makes up for the mass difference - and leaving behind a proton). Assembling free protons and neutrons into a single nucleus ought to result in an energy difference (even after eliminating the individual kinetic energies of the constituents), also resulting a mass difference.

Answer 3

The atomic mass (ma) is the mass of an atom at rest, most often expressed in unified atomic mass units.
The atomic weight is the average atomic mass of an element, expressed in amu.
The amu (atomic mass unit) is the mass unit that equals 1/12 of the mass of carbon-12.

Answer 4

it means that 6.022*10^23 atoms has mass equal to its atomic mass in grams.
they just calculated avogadro number...(I think)