Question

Is Atomic Mass and Atomic Weight the same terms? Or is Atomic mass and Mass Number the same terms?

On the MCAT Kaplan books it says that atomic mass and mass number are the same thing but online and other chemistry textbooks, atomic mass and atomic weight are the same thing.

Can someone please clarify which two terms are the same thing or are they all different? I really need help to make sure I understand this for the MCAT.

Answer 1

Answer.

Atomic mass and Atomic weight are the same terms.These are the weighted average mass of an atom based on its relative natural abundance of its isotopes..

Mass number is also equal to the atomic weight (atomic mass) for a pure element without any mixing of its isotopes.Here the atomic mass is the mass number expressed as atomic mass unit. 1 proton/neutron = 1 amu. So the mass number and atomic mass are equal.

For this we must know about isotopes. isotopes are the same element with a difference in mass numbers. mass number is the sum of protons and neutrons. For the same element, the number of protons must be equal ( because it is the atomic number). Therefore isotopes differ in their number of neutrons.

For example magnesium

magnesium has three isotopes Mg-24, Mg-25 and Mg - 26. Here 24, 25 and 26 represents mass number. So their atomic masses are 24 ( 12 P + 12 N), 25 ( 12 P + 13 N) and 26 ( 12 P + 14 N)

The magnesium element is available in nature as a mixture of these three isotopes with the natural abundance of

Mg - 24 = 79 %

Mg -25 = 10 %

and Mg - 26 = 11 %

Therefore

Atomic weight of Mg = [79 * 24 + 10 * 25 + 11 * 26] / 100 = [1896 + 250 + 286]/100 = 24.32

Thus the element magnesium is available in three isotopic forms of which the isotopic masses are 24,25,26 available in the natural abundance of 79 %, 10%, 11% respectively and the weighted average of these isotopic masses is 24.32 is its atomic mass or atomic weight.

If we consider the major isotope Mg-24 only, then the mass number and atomic mass are the same.