The element antimony has two stable isotopes, antimony-121 with a mass of 120.90 amu and antimony-123 with a mass of 122.90 amu. From the atomic weight of Sb = 121.76 one can conclude that: antimony-123 has the highest percent natural abundance
most antimony atoms have a mass of 121.76 amu
antimony-121 has the highest percent natural abundance
both isotopes have the same percent natural abundance

antimony-121 has the highest percent natural abundance

Explanation:

percent natural abundance;

121.76 = 120.90 x + 122.90 (1 - x)

121.76 = 120.90 x + 122.90 - 122.90x

121.76 = -2x + 122.90

121.76 - 122.90 = -2x

x= 121.76 - 122.90/ -2

x= 0.57

Where x and 1 - x refers to the relative abundance of each of the isotopes

Percent natural abundance of antimony-121 = 57 %

Percent natural abundance of antimony-123 = (1 - 0.57) = 43%

Let us remember that isotopy refers to a phenomenon in which atoms of the same element have the same atomic number but different mass numbers. This results from differences in the number of neutrons in atoms of the same element.

We can clearly see that antimony-121 has the highest percent natural abundance.

the answer is 303.15 or 303