Why is mass number slightly less than actual atomic mass?

Answer 1

Any "stable bound system" is stable because there is a "mass deficit" or "binding energy" that is surrendered to the Universe at large. It takes that much or more energy to break the system apart...

Answer 2

Mass numbers are derived from two sets of data:

• The isotopic ration in nature of the element, and
• The atomic number.

Different elements have different ratios of isotopes, and the different isotopes have different atomic masses. Because of the difficulty in trying to find either pure samples of one isotope or of determining the exact ration in a sample, we assume (reasonably, of course) that a given sample has the natural ration. By using the average mass - determined by finding the percentage of atoms in a sample with given mass, then finding the average mass for the atoms - we make calculations and work much easier.

Also, because of the definition of the units of mass (both the AMU and the gram are determined by properties of carbon-12 isotope) an element with a higher atomic number than carbon will have its mass shifted slightly downward, and for one below carbon the mass is shifted up.

The other factor which affects these numbers is the binding energy: mass must be converted to energy in order to maintain the atom's structure and stability. This results in a slight lowering in each atom and isotope's calculated mass.

Answer 3

Because that's the way it is...

The mass of a proton (1.672622 x 10-27 kg) is slightly less than the mass of a neutron (1.674927 x 10-27 kg), and the reason why is left to the creator of the universe.

Answer 4

That is the result of the binding energy. There is a fairly large force, and therefore also energy, involved in getting the nucleons together (they attract one another strongly). Thus, when they are together, they have less energy (potential energy) than when they are separated. Having less energy, they naturally also have less mass ("mass-energy equivalence").

The same happens in a chemical reaction; however, to a lesser degree. The amount of potential energy involved is so small in a chemical reaction that it is hard to measure the mass difference.

Please note that no mass disappears - when combining the nucleons, energy is released - and this energy also has an equivalent mass.

For more information, check the Wikipedia article on "binding energy".

Answer 5

The electrons (only particle outside of nucleus) are several orders of magnitude less massive than the neutrons and protons which compose the nucleus

Answer 6

Some of the mass is converted into energy, which is also known as nuclear force.