Chromium has four naturally occuring isotopes. Cr-50 (mass of 49.946 amu and 4.35% abundance); Cr-52 (mass of 51.941 amu and 83.
79% abundance); Cr-53 (mass of 52.941 amu and 9.50% abundance); and Cr-54 (mass of 53.939 amu and 2.36% adundance). Calculate the average atomic mass of chromium. 51.996 amu
Eliminate
B) 52.000 amu
C) 52.191 amu
D) 52.25 amu
An element can have differing numbers of neutrons in its nucleus, but it always has the same number of protons. The versions of an element with different neutrons have different masses and are called isotopes.
The average atomic mass for an element is calculated by summing the masses of the element’s isotopes, each multiplied by its natural abundance on Earth.
Average atomic mass of P = ∑(Isotope mass)(its abundance)
<em>∴ Average atomic mass of Cr = (Cr-50 mass)(its abundance) + (Cr-52 mass)(its abundance) + (Cr-53 mass)(its abundance) + (Cr-54 mass)(its abundance) </em>
Abundance of isotope = % of the isotope / 100.
<em>∴ Average atomic mass of Cr =</em> (49.946 amu)(0.0435) + (51.941 amu)(0.8379) + (52.941 amu)(0.095) + (53.939 amu)(0.0236) = <em>52.000 amu.</em>
<em>So, the right choice is: </em><em>B) 52.000 amu.</em>
