Sb which is short for stibium is the chemical name for Antimony. Sb has an atomic number 51 and occurs as three stable isotopes. Two of them occur naturally and the third has been prepared synthetically.
The atomic masses of the natural isotopes are 121 and 123. The percentage of the isotope with the atomic mass 121 present in antimony is 57.36 %. The isotope with the atomic mass 123 makes up the rest of 42.64%.
The synthetic isotope with an atomic mass of 125 has a negligible concentration to be considered.
The stable isotopes of antimony (Sb) are 121 Sb and 123 Sb
We'll suppose that the lighter isotope is 121 Sb. Therefore it must be more abundant.
The atomic mass of antimony is 121.760
121.760 = 120.904x + 122.904y (1)
x is fractional abundance of 123 Sb
y is fractional abundance of 121 Sb
y = 1-x
We'll substitute y = 1-x in (1):
121.760 = 120.904x + 122.904(1-x)
We'll remove the brackets:
121.760 = 120.904x + 122.904 - 122.904x
We'll combine like terms and we'll solve for x:
-2x = -1.144
x = 0.5720
The fractional abundance of 121 Sb is 0.5720. That means that the fractional abundance of 121 Sb is 0.4280.