Why do some heavy elements prefer beta decay over other forms of decay?

[buraqenigma]

Can anybody explain me the conditions for become a beta decay.

for example : Md (Z=101,A=255) --> No (Z=102,A=255) + e (electron) + -Ve (anti-notrino)

can this nuclear reaction be?

 

[Astronuc]

http://www.nndc.bnl.gov/chart/reCenter.jsp?z=101&n=154 and click on Zoom 1 at top right.

Md-101 is more likely to undergo electron capture 92% of time or alpha-decay 8%, with a small fraction of spontaneous fission (SF).

It seems that heaviest actinides have preference for electron capture, alpha decay or SF. Nb-261 apparently undergoes beta-decay, and Es has number of isotopes that undergo beta decay.