Question

When potassium-40 undergoes decay by electron capture the product is?

Answer 1

Electron capture decay is a decay process. In this process electron is captured and a proton is converted into a neutron. Hence, mass number remains same but atomic number is lowering by 1 in daughter atom. 

K has atomic number 19. Hence daughter atom should have 19 - 1 = 18 as the atomic number. Hence, produced particle is Ar. but the mass number of Ar remains as same as 40.

The equation is,
₁₉⁴⁰K + ₋₁⁰e → ₁₈⁴⁰Ar