Source: http://ie.lbl.gov/education/glossary/AnimatedDecays/E-Capture.html

During electron capture (also known as K-capture) an electron from generally the lowest energy level (n=1) is pulled into the nucleus. The result is the conversion of a proton into a neutron. This causes the atomic number (Z) to decrease by one while the mass number (A) remains the same. No particle is ejected but as extra-nuclear electrons drop down to fill in the vacancy in n=1, energy is given off.

In the example shown above an isotope of rubidium undergoes electron capture and becomes an isotope of krypton.

The process can be represented in a balanced nuclear reaction using the isotopic symbols:

Notice that in this process the "decay" particle is on the left side of the reaction since it is absorbed into the nucleus rather than ejected.

We can check that the reaction is balanced by comparing the sums of the lower numbers on each side and the upper numbers. There should be no net change.