What does the atomic mass or mass number tell you about an atom?

Answer 1

As a first approximation it tells us simply the total number of protons and neutrons in the nucleus. The electrons and binding energy of the nucleus contribute a very much smaller amount to the mass.

Since the number of protons in a given element is constant (the atomic number), we can also tell the number of neutrons in an atom by subtracting the atomic number from the Atomic Mass (approximately).

Since different isotopes of each element have different numbers of neutrons (but the same number of protons), they will have different atomic masses. The given atomic weight for a particular element is usually based on the relative abundance of the isotopes as they occur in nature.

It is also true that elements with very high elemental weights tend to be unstable, that is, radioactive.

(Referring to the Wikipedia entry on "atomic mass" I see that what I have defined here may include some of the sense of "atomic weight", however the difference is subtle. See that entry for further clarification.)

Answer 2

The atomic mass number tells you the number of total number of protons and neutrons in an atom. It can also tell you how many grams one mole of the substance will weigh. For instance, carbon 12 has a total of 12 protons and neutrons. Also one mole of this carbon isotope weighs 12 grams.

Answer 3

The atomic weight (not mass) of a chemical element is the ratio between the average mass of the atoms of this element to 1/12 from the atomic mass of carbon-12.

The atomic mass is a term applied only to specific isotopes; the unit is the same as above. Is a value denoting the total mass of all the protons, neutrons, and electrons in an isotope.

The mass number tells us the number (the sum) of protons and neutrons in the nucleus of an atom.

The atomic number, on the other hand, tells us how many protons are in the nucleus of an atom.

IUPAC publish periodically tables of atomic weights - the last edition is from 2009-2010.

For the atomic masses of isotopes the last published edition is The AME 2003 atomic mass evaluation, edited by Audi, Wapstra and Thibault.

Answer 4

Atomic mass = mass of protons + mass of neutrons

Mass number = Number of protons + number of neutrons

Answer 5

The atomic number tells us how many protons are in the atom and therefore the species of atom. The atomic mass tells us how many neutrons and protons are in the nucleus of the atom.

Answer 6

The mass number tells us the number (the sum) of protons and neutrons in the nucleus of an atom.