Question

Why initially oxygen was taken as a standard for defining various atomic mass units?

Answer 1

Because of two reasons:
1) oxygen reacted with large number of elements and formed compounds
2) this atomic number gave masses of most elements as whole numbers

Answer 2

Reason why oxygen was taken as a standard for defining various atomic mass units is:

1. Firstly, oxygen react with most of the elements on the periodic table forming oxides with them, thereby determination of element’s atomic mass in an oxide was easily possible.

2. During  $17^{th}$ century, when oxygen was discovered, the weight  of the oxygen was 8 times of hydrogen, which was considered the most lightest element during those times.

Hence, the atomic weight of the oxygen was considered 16, thereby making it possible to calculate atomic mass of the other elements.