Question

1) Why does not the atomic mass of an element represent the actual mass of its atom?
2) “The atomic mass of an element is in fraction.”
What does it mean?

Answer 1

Answer:

The relative atomic mas of hydrogen is u and its corresponding gram atomic mass is 1 . If the atomic mass of an element is in fraction, this means that it exists in the form of isotopes.

Answer 2

1) The atomic weight of an element is actually equal to mass of 1 mole of sample of the element in grams. For example, the at. wt. of sulphur is 32, this means that 1 mole of sulphur, which itself contains 6.023 * 10^23 atoms of sulphur, will have a mass of 32 grams.

So, 1 atom of the element has a mass in grams equal to the atomic weight divided by the Avogadro number or 6.023*10^23.

2) The atomic mass of an element is generally always in fraction as it is the average atomic mass of all isotopes of the element found in nature according to their abundance, for example in the case of chlorine it is 35.5, as chlorine has two isotopes occurring naturally Cl-35 and Cl-37, Cl-35 occurs in greater abundance as compared to Cl-37 (75% and 25%), the average mass= 75/100 * 35 + 25/100*37 => 26.25 + 9.25 => 35.5