Illustrated · 1 min
Electron Capture
Electron capture is a phenomenon that causes decay similar to beta-plus decay. It is caused by the weak nuclear force, where an inner electron is absorbed into the nucleus. This is a very rare occurrence and happens much less frequently than beta-plus decay.
by T. Gibbons
by T. Gibbons
Read · 4 min
We've been exploring atomic nuclei. For the most part, chemists only care about electrons. But, nuclear chemists prefer to study the nucleus. Why? Because the elements beyond the metal lead are unstable. They're so large that the protons' electrostatic repulsion overcomes the strong nuclear force. So beyond the isotope lead-208, nuclear stability decreases. How do we know this? We see a decrease in nuclear binding energy! It requires less energy to rip apart the nucleus.
How does an atom become more stable? It lowers its energy via radioactive decay! For massive nuclei, there are a couple of options. The easiest way to lose mass is alpha decay. The nucleus ejects two protons and neutrons (helium). So, it loses four atomic mass units. The second option is to try and balance the forces. So, it undergoes beta decay. But, there are two types. In beta-minus, a neutron becomes a proton by radiating an electron. In beta-plus, a proton converts to a neutron by emitting a positron. What happens in enormous atoms, like the actinides? Beta-decay is one step in a decay series. Sometimes there are ten or more steps to stability. What about unstable isotopes with small atomic numbers? For example, we could consider the alkali metal sodium-22. It undergoes beta-plus decay to become Neon-22, a stable isotope. So, beta decay is a direct path to stability for tiny elements. Lastly, we studied gamma radiation. Sometimes, a nucleus has excess energy after alpha or beta decay. It's excited! So, it releases a high-energy photon to return to a normal state.
In previous explanations, we've hinted at one more option. Today we'll study electron capture!
We've seen that a proton can convert to a neutron by releasing an electron. But what happens if it absorbs an electron? Are you wondering how that's even possible? Remember, quantum mechanics is all about probability. For the 1s and 2s orbitals, there is a non-zero chance that an electron is within the nucleus. To help you visualize this, look at the electron cloud diagram. So, if an electron is too close, it gets absorbed. The absorption converts a proton to a neutron. The goal is similar to beta decay. By converting a proton to a neutron, we reduce the electrostatic repulsion. So, we balance the forces.
If we've changed the forces, we've done work. But, we need to obey the conservation of energy. Often, the nucleus is in an excited state. So, often we'll observe gamma decay. But what about the actual absorption of an electron? That should need to release a lot of energy! To do so, it discharges a neutrino. They're an interesting subatomic particle. So, we'll explore them more below.
But what about the electron shells. We've violated the octet rule by absorbing an inner electron. It forms an electron hole. Most often, an outer shell electron fills the void. It's like an excited state electron. How do they release energy? They emit a photon. The same thing happens in electron capture. But sometimes, the photon has a ton of energy. It can even be in the x-ray region of the electromagnetic spectrum! If that hits another electron, it overcomes the ionization energy. But, there is another possibility. The falling electron can collide with another electron. The collision also exceeds the ionization energy. In both cases, an electron gets ejected. So, we end up creating an ion. Both processes are named the Auger effect.
The history of the neutrino is fascinating! Early on, chemists had been studying beta decay. They calculated the theoretical energy of the emitted electron. But, the experimental values were completely different. There was an immense gap between the pure and applied chemistry! They had no idea what to do! Neil's Bohr was even ready to concede that energy and momentum are not conserved. Do you remember him? He's famous for the Bohr atomic model. He's renowned for his contributions to quantum mechanics.
Thankfully, Wolfgang Pauli proposed a new particle. Do you remember him? He created the Pauli Exclusion principle! These are some very famous scientists. Originally, Pauli named it the neutron. That's because James Chadwick hadn't developed the neutron yet! They were still working with Rutherford's nuclear model. Scientists discovered the neutron first. It was also way more massive than a neutrino. So, the name neutrino is actually a pun. In Italian, the "-ino" ending makes something tiny. So, a neutrino is a miniscule neutron. So, what is a neutrino? It is a subatomic particle that's electrically neutral. It is the carrier of beta decay and electron capture energy.
Video
by Tyler DeWitt
In this video, Tyler DeWitt explains electron capture. He starts with a definition. Then, he shows you three example problems. So, you'll really understand how it works. This is an excellent and concise explanation!