Now electron capture is when an unstable nucleus captures or absorbs an electron from the inner electron orbital. Now here we're going to say this occurs in nuclei with an excess number of protons, so they would undergo electron capture to help us reduce the number of protons we're as helping to increase the number of neutrons.
Here we're going to say that a proton and electron combined together to help us give us a neutron. So by creating this neutron will actually bring down the number of protons, bringing them more in line with one another in terms of balance. Now here we have cesium 131. It's undergoing electron capture, meaning it's absorbing this electron.
Remember that your mass number on both sides have to be the same, and your total number of protons on both sides have to be the same. Here we have 131 + 0 = 131, 55 - 1 = 54. If we look on the periodic table, the only element with an atomic number of 54 would be xenon. So here we'd say that Susskind 131 undergoes electron capture, and in the process it produces Xenon 131 as a product.