Thanks so much for your question regarding electron capture. This is one of the means that a nuclear change can take place. When a nuclear change takes place, an atom changes its identity and is now an atom of a different element.
Electron capture occurs when an inner-orbital electron (negatively charged) is captured by the nucleus (positively charged). The result is that a proton will combine with this electron and a neutron is formed. This process will reduce the atomic number by one and not changed the atom's mass.
And example of this would be is Be-7 undergoes electron capture. Be-7 has four protons and 3 neutrons. Your atomic number will drop by one, thus a lithium atom is formed and the mass will remain the same. Li-7 is the result.
Also, note that gamma radiation is emitted along with electron capture to release excess energy.
Electron capture can occur when an electron comes too close to a proton. The negative electron and the positive proton are attracted to each other. If the electron and proton do combine, a neutron will be formed.