Hi Shahd,

In electron capture, an atom "captures" an electron. This electron (neg charged particle) binds with a proton (pos charged) to form a neutron (neutral charged particle, because -1 + 1 = 0).

So, initially we lose a proton because the captured electron binds to it (decreasing the atomic number, hence converting Xe-125 to Te-124). But, a neutron is also formed which compensates for the decreased mass due to the proton loss (hence, Te-124 turns to Te-125).

Hope these help!

Me again!

I want to correct something fundamental here:

A proton + an electron DOES NOT make a neutron. If this were true, our universe would look very different!

In electron capture, you are correct that the atom "captures" an electron. An electron has a mass of 9.11 x 10^-31 kg, while protons and neutrons have masses of approximately 1.67 x 10^-27 kg. So, a proton or neutron is approximately 2000 times MORE massive than an electron. If we add an electron or two to an atom to make it more stable... well that's negligible!

It's like if you're SUPER hungry and you eat two peas. Wooooow what a huge meal. 🙄 This is why the answer states that the change in molar mass is negligible, and the mass at the end is the same as the mass at the beginning.