When you start studying chemistry, everything is about electrons. The nucleus is there to keep the electrons in their orbitals. In ionic bonding, the proton contributes to the ionic charge. But that's because we transferred an electron to create an electrostatic attraction. In covalent bonding, we only care about electronegativity. We don't need the protons. So far, we've assumed that an atom's nucleus remains constant. It turns out that's a poor assumption!
What is Radioactivity
In general, we've considered atoms with stable nuclei. What makes them so long-lasting? The strong force balances the electrostatic repulsion. So, the protons and neutrons hold together. But, that's not true for all atoms. Do you remember nuclear stability? Many of the elements with high atomic numbers were unstable. The cut-off is the transition metal lead at 82. That's why the actinides are radioactive. There are isotopes of tinier elements that are radioactive. For example, oxygen has isotopes from oxygen-11 to oxygen-28. But, only three are stable: 16, 17, and 18. The longest-lasting radioactive isotope is oxygen-15. It has a half-life of 122 seconds.
But, what is radioactivity? It's the emission of a particle or radiation from the nucleus. Why are some elements radioactive? Because they are unstable. Remember, chemistry is all about stability. They want a lower energy. So, they undergo radioactive decay. It's analogous to an excited electron. How do electrons get back to a lower energy level? They release photons. Atomic nuclei have similar mechanisms for reaching a stable state. But, radioactivity has a lot more variety.
Types of Radioactivity
We'll explore each type of radioactivity in future lessons. But, this will give you a broad summary of each type.
Alpha Decay:
An alpha particle is a helium nucleus. It has two protons and two neutrons. An element will undergo alpha decay when it's too large. By reducing the number of nucleons, it reduces its mass. An example of alpha decay is the conversion of Radium-226 to Radon-222.
Beta Decay:
There are two types of beta decay: electron and positron emission. A positron is the electron's anti-particle. So, it has the same mass but the opposite charge. What creates the electron or positron? A proton or neutron! Do you remember how neutrons are heavier than protons? It turns out the difference is the mass of an electron. So, a neutron converts to a proton by releasing an electron. By converting a neutron to a proton, the atom increases its atomic number. It becomes a different element! For example, Carbon-14 converts to Nitrogen-14. Releasing a positron transforms a proton into a neutron. So, the atomic number decreases. An example is Magnesium-23 becoming Sodium-23.
Electron Capture:
Electron capture is technically a subset of beta decay. But I think it's unique enough to have an explanation. In electron capture, the nucleus absorbs one of the atom's electrons. What's the result? A proton becomes a neutron! How can it absorb an electron? We have to remember quantum mechanics. It's all about probability. So, let's say we had an electron in the 1s-orbital. There is a non-zero probability that the electron gets too close to the nucleus. Are you having trouble visualizing it? It might help to look at the electron cloud diagram. What happens when an electron gets too close? In an unstable atom, it may absorb the electron. Why? It generates a more stable nuclear configuration. Remember, nucleons fill shells just like electrons. So, some arrangements have better stability than others.
Gamma Decay:
When a nucleus has undergone alpha or beta emission, its left in a high energy state. How does it release that energy? It emits a photon. The photon is in the gamma region of the electromagnetic spectrum. That's the highest energy. They have unbelievably short wavelengths. They are on the order of 10−12. That's a picometer! So, gamma radiation is the highest energy light.
History of Radioactivity
The history of radioactivity is unique. The first scientist to observe it was Henri Becquerel in 1896. He was trying to determine if uranium produced x-rays when hit by external light. Instead, he realized that the uranium emitted radiation regardless of the external light.
Becquerel's discovery captured the attention of Marie Curie. For her Ph.D. thesis, she studied uranium with an electrometer. It's a device that measures electrical current. She discovered that uranium was generating electricity. Even more interesting, the measured signal only depended on the amount of uranium. She hypothesized that the radiation came from the uranium.
Soon her research turned toward radioactive minerals. She realized that the amount of radiation was more than what uranium could produce. By 1898, she and her husband had isolated two new elements. The first element was polonium. When you discover an element, you get to name it. So, they named it after Poland in honor of Marie's native country. The second element was unbelievably radioactive! So, they decided to call it radium. Eventually, Marie's discoveries would result in two Nobel prizes. The first was in physics in 1903 for her work on radiation. The second was in chemistry in 1911 for her discovery of two new elements.
Meanwhile, Earnst Rutherford was also looking into radiation. He did a similar experiment to Marie Curie. At first, Rutherford measured the current produced by uranium's radiation. But then, he put gold foil sheets on top. The first few sheets reduced the electric current. But suddenly, the current remained constant. He realized there must be two types of radiation! The gold foil blocked some radiation. That's why the current reduced. So, the radiation must be massive. The other must be very small so that it can pass through metals. He was unsure of what they were, so he called them alpha and beta.
But what about gamma radiation? Rutherford had missed gamma radiation! Because they have such high energy, they also went through the gold foil. But, he couldn't distinguish them from beta rays. But, another chemist, Paul Villard, made an important discovery. He started by having the radiation from radium travel through a lead plate. The lead blocked the alpha particles. Then the rest of the radiation traveled through a magnetic field. The beta particles' path bent. But, he detected another type of radiation. The gamma radiation was unaffected by the magnetic field. So it hit the detector straight on.
Do you remember Rutherford's gold foil experiment? He used alpha particles to discover the nucleus. Alpha particles are so large that they bounce off. Occasionally, with a direct collision, they deflected at huge angles. How is that possible? Most of the atom's mass sits at its center. So, he developed the nuclear model.
Uses of Radioactivity
We've all heard of the dangers of radioactivity. The particles have very high energy. So, they can damage our DNA. Chronic radiation exposure is associated with an increased risk of cancer and other diseases. But, we also use radiation to benefit us. For example, do you know how smoke detectors work? There is a small amount of americium inside, which undergoes alpha decay. The alpha particles hit a sensor. What happens when it fills with smoke? The smoke absorbs the alpha particles. So, the sensor is no longer activated. Suddenly, you have an alarm blaring. And you know you need to get out.
We also use radiation in medical applications. For example, we can irradiate cancer cells. Cancer cells die more quickly than regular cells. So, it can be an effective treatment. We also use radioactive isotopes as tracers. As the tracer travels through the body, it releases radiation. We can then track it using specialized detectors. That's the basic premise of PET scans. PET stands for positron emission tomography. So, from our explanation, we know it's from beta decay! Isn't chemistry amazing! We learn so much about the way our world works.
