Electron capture usually occurs in which type of radioactive elements?

Electron capture is a specific type of radioactive decay process where an electron from an inner shell is captured by the nucleus, combining with a proton to form a neutron and producing a neutrino. This process primarily occurs in elements that have higher atomic numbers, especially those with a proton-to-neutron ratio that is not optimal for stability.

As atomic number increases, the balance of protons and neutrons becomes more critical for maintaining stability. High Z elements often have excess protons, leading to an unstable nucleus. To alleviate instability, such elements can undergo electron capture as a means to transform a proton into a neutron, thus moving closer to a more stable ratio.

This correlation between higher atomic number and the preference for electron capture is why the correct choice is attributed to high Z elements. The unique characteristics of their nuclear structure make them more prone to utilizing this decay mode compared to lower atomic number elements, which typically undergo different types of decay processes such as beta decay or alpha decay.