How can I write the electron capture equation?

Hint: Electron capture is a nuclear process. A nucleus rich in protons absorbs an inner electron from the atom. This process involves emission of high energy gamma rays from the nucleus.

Complete step by step answer:
Electron capture is a nuclear process. A nucleus undergoes electron capture, if it contains more number of protons compared to that of neutrons. The nucleus absorbs an electron from the K-shell or L-shell of the atom. Then the absorbed electron and one proton combines to form a neutron, thereby balancing the number of protons and neutrons in the nucleus. The energy released during this process comes out as gamma rays. Electron capture reactions are also called K-electron capture or L-electron capture, because electrons are absorbed from the K or L shell of the atom.
The equation for electron capture can be written as,
${}_1^1p + {}_{ - 1}^0e \to {}_0^1n + \gamma $
When a nucleus undergoes electron capture reaction, its mass number doesn’t change but atomic number decreases by one. Mass number is the total number of protons and neutrons inside the nucleus and atomic number is the number of protons or number of electrons in an atom. In an electron capture reaction, the electron is captured by the nucleus. Hence the number of electrons decreases and thus atomic number decreases. Number of protons also decreases but it is compensated by the increase in the number of neutrons. Hence mass numbers do not change.
Electron capture reaction of potassium is given below.
${}_{19}^{40}K + {}_{ - 1}^0e \to {}_{18}^{40}Ar + \gamma $
Mass number of atoms remains in $40$ . But potassium (K) is converted into argon (Ar) as the atomic number changes by one unit.

Note:
Equation electron capture reaction is similar to that of positron decay. In positron decay, the mass number of an atom doesn’t change, but the atomic number decreases by one. In this process, a positron will be emitted from the nucleus.