1this happens because of unstability of those nuclides
a nucleus of any atom becomes unstable when no of neutrons(N) becomes unequal to no of protons(A)
ie |A - N|>0 (higher the value of |A - N|,higher will be the unstability)
now K-capture is a process which happens when no of protons becomes higher than no of neutrons (ie A - N > 0)
so nucleus captures electrons from outside which combine with a proton and coverts into a neutron thus decreasing the value of (A - N).
all atoms don't have [no of protons > no of neutrons]
so this doesn't happen in all nuclides.....it happens only in those nuclides where no of protons is higher than no of neutrons.
also there's a decreasing tendency to undergo K-capture as the value of (A - N)decreases
1ok now k capture though seems to be alluring bt it isn't a magic solution for atom's quest to stability for two reasons
first the n/p ratio should be exceedingly high
second there should be insufficient energy for positron emission
though the first condition is generally met the second is not. For a highly unstable nucleus {i.e., one with a high n/p ratio} it would be unusual to have a energy less than reqd. for positron emission and if this is the case then nucleus prefer to do positron emission as it reduces a much better amount of energy and thus stabilizes the nucleus to much better extent than k capture.
Moral : low energy path isn't always advisable :P
6so wud it be safe to say that those elements of heavier nuclei or more atomic no. undergo this process
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