One mode of radioactive decay is through K electron capture. This occurs when the nucleus of an atom absorbs an electron in the lowest quantum level, changing a proton into a neutron. The change in the nucleus is the same as for positron emission: the mass number stays the same, the atomic number drops by one. For example, ⁸²₃₇Rb decays by electron capture most of the time to form an isotope of krypton. (It can also decay by beta emission.)

⁸²₃₇Rb + ⁰_-1e -> ⁸²₃₆Kr

Example: ¹⁷⁸₇₄W is a radioactive isotope of tungsten. It decays by K electron capture: what is the product?

Solution: We keep the mass number the same (178) but decrease the atomic number by one (74-1 = 73) Atomic number 73 is tantalum, so

¹⁷⁸₇₄W + ⁰_-1e-> ¹⁷⁸₇₃Ta

Information provided by: http://learn.chem.vt.edu