If fluorine 20 undergoes beta decay , what will it become ?

Fluorine 20 (F - Atomic number 9 and atomic mass 20). Firstly we need to know what is beta decay. Beta decay occurs when one neutron changes into a proton and an electron therefore the atomic mass will remain the same as even though we loose a neutron it is replaced by a proton, the atomic number is always raised by 1 when one beta decay occurs. The produced electron is shot out of the nucleus at an incredible speed. This speedy electron we call a beta particle.

Ok now the reaction.

 20       20        0
F  -> Ne    +     e
 9         10       -1

Remember the atomic number determines the nature of the element ( i.e what elemnt it is).
Hope this helps :).