why is the atomic mass of technitium usually in brackets?
Because its a Synthetic Element!!!!!!!!
Because its a synthetic element. The number that appears in brackets is the most stable isotope of technetium.
The standardized atomic weight of technetium is  amu.
The brackets are used for isotopic elements--those with variable numbers of neutrons. The atomic weight of the most stable isotope is the one in brackets. You know, this is a terrible way to be getting your chemistry coursework done. Here you go: www.lenntech.com/periodic-chart-elements/tc-en.htm There you go.
Atomic Relative Mass is 51.996
relative Atomic Mass
relative atomic mass
The relative atomic mass of carbon is 12.
Yes, relative atomic masses are expressed in atomic mass units
Relative atomic mass of chlorine (Cl) is 35.45
the relative atomic mass for the element potassium is 39.0983