In article . com,
Siddhartha Jain wrote:

Given two lenses, one a prime (say 28mm) and the other a zoom (say
28-75mm) and both with an aperture of f2.8 -
- Will the prime be brighter than the zoom because it has fewer lens
elements?
- Will the prime be sharper wide open than the zoom at 28mm?

Ofcourse, a lot will depend on the particular makes and models being
compared but is there a rule of thumb?

F/2.8 lets the same amount of light through regardless of the lens
design. F/2.8 is f/2.8.

Being from the old school, I would expect a fixed focal length lens to
be sharper than a zoom, but you may have to go to laboratory conditions
to prove it.