I am using Quantities (units) in Mathematica 11.3 and would like to express the numerical portion of the output in scientific notation. The command
ScientificForm[quantity] does not work.
When the argument to
Quantity is in arbitrary precision (e.g. an exact integer), it does not get converted automatically to scientific notation. You can force the conversion if you numericize the input, converting it e.g. to machine precision:
Quantity[2000000, "Kilograms"] N @ Quantity[2000000, "Kilograms"]
Note that, however, even machine precision numbers whose exponents are "small" according to some preset threshold do not get converted to scientific notation.
I have not (yet) found a way to force that conversion at will.