Welcome to Topic 6: Radioactivity!
Radioactivity often sounds like something out of a science fiction movie, but it is a natural process that happens all around us. In this chapter, we are going to explore the tiny world of the atom, learn why some atoms are "unstable," and see how we use radiation in everything from smoke alarms to generating electricity. Don't worry if it seems a bit small and invisible at first—we'll use plenty of analogies to bring it to life!
1. The Structure of the Atom
Before we can understand radioactivity, we need to know what an atom looks like. Every atom is made of three subatomic particles:
- Protons: Found in the nucleus. They have a positive charge (+1) and a relative mass of 1.
- Neutrons: Found in the nucleus. They have no charge (0) and a relative mass of 1.
- Electrons: Orbit the nucleus in shells. They have a negative charge (-1) and a tiny relative mass (almost 0).
The Nucleus: The center of the atom is called the nucleus. It contains almost all of the atom's mass, but it is incredibly tiny.
Analogy: If an atom was the size of a football stadium, the nucleus would be like a small marble in the center of the pitch!
Key Terms to Remember:
Atomic Number: The number of protons in an atom. This tells you what element it is (e.g., Carbon always has 6 protons).
Mass Number: The total number of protons plus neutrons.
Isotopes: These are atoms of the same element (same number of protons) but with a different number of neutrons. This means they have the same atomic number but a different mass number.
Symbol Format: We write isotopes like this: \( ^{13}_{6}C \).
The top number (13) is the Mass Number.
The bottom number (6) is the Atomic Number.
Quick Review Box
Protons = Positive (Both start with P!)
Neutrons = Neutral (Both start with Neu!)
Electrons = Negative
In a normal atom, the number of protons equals the number of electrons, so the overall charge is neutral.
Takeaway: The nucleus is small, dense, and positive. Isotopes are just versions of elements with "extra" or "fewer" neutrons.
2. How the Atomic Model Changed
Our ideas about atoms haven't always been the same. Science changes when new evidence is found!
- Plum Pudding Model: J.J. Thomson thought the atom was a ball of positive charge with negative electrons stuck in it like fruit in a pudding.
- Rutherford’s Alpha Scattering: Ernest Rutherford fired alpha particles at thin gold foil. Most went through, but some bounced back! This proved the atom is mostly empty space with a tiny, dense, positive nucleus in the middle.
- Bohr Model: Niels Bohr suggested that electrons orbit the nucleus at specific distances (shells).
Did you know? Electrons can move between these orbits! If they absorb electromagnetic radiation, they move to a higher shell (further from the nucleus). When they move back down, they emit radiation.
3. Types of Radiation
Some nuclei are "unstable" (they have too much energy or the wrong balance of particles). To become stable, they spit out radiation. This is a random process—we can't predict exactly when one nucleus will decay!
The Four Main Types:
1. Alpha (\(\alpha\)): Two protons and two neutrons (the same as a Helium nucleus). It is big, heavy, and very ionising but can't travel far. It is stopped by a sheet of paper.
2. Beta Minus (\(\beta^-\)): A fast-moving electron emitted from the nucleus. It happens when a neutron turns into a proton. It is stopped by a few mm of aluminium.
3. Beta Plus (\(\beta^+\)): A fast-moving positron (the antimatter version of an electron). It happens when a proton turns into a neutron.
4. Gamma (\(\gamma\)): An electromagnetic wave. It has no mass and no charge. It is very penetrating and is only stopped by thick lead or concrete.
Ionising Power: This is the ability to knock electrons off atoms to create ions. Alpha is the strongest at this; Gamma is the weakest.
Common Mistake to Avoid
Don't confuse penetrating with ionising!
Alpha is the "strongest" hitter (most ionising) but the "weakest" at traveling through things.
Gamma is the "weakest" hitter but the "best" at traveling through things.
Takeaway: Radiation is emitted to make a nucleus stable. Each type has different strengths and can be stopped by different materials.
4. Radioactive Decay Equations
We use equations to show what happens during decay. The rule is: The total mass and charge must be the same on both sides!
Beta Minus Decay: A neutron turns into a proton and an electron.
\( ^{14}_{6}C \rightarrow ^{14}_{7}N + ^{0}_{-1}e \)
Notice the mass (14) stays the same, but the atomic number increases by 1 because we gained a proton!
Beta Plus Decay: A proton turns into a neutron and a positron.
\( ^{13}_{7}N \rightarrow ^{13}_{6}C + ^{0}_{+1}e \)
The mass stays the same, but the atomic number decreases by 1.
Gamma Decay: The nucleus just "rearranges" itself to lose energy. The mass and atomic numbers do not change.
5. Half-Life and Activity
Activity: The rate at which a source decays. It is measured in Becquerels (Bq). 1 Bq = 1 decay per second.
Half-life: The time it takes for half of the undecayed nuclei in a sample to decay. It is also the time it takes for the activity to drop to half of its original value.
Analogy: Imagine you have 1000 popcorn kernels. The "half-life" is the time it takes for 500 of them to pop. After another half-life, you'd have 250 left, then 125, and so on.
Calculating Net Decline:
If an isotope has a half-life of 2 hours, how much is left after 4 hours?
Step 1: 4 hours is two half-lives.
Step 2: After the 1st half-life, 1/2 is left.
Step 3: After the 2nd half-life, 1/2 of 1/2 is left = 1/4.
Takeaway: You can't predict when one atom will decay, but half-life allows us to predict what a large group of atoms will do over time.
6. Background Radiation and Safety
Background Radiation: Low-level radiation that is around us all the time.
Origins: Earth (rocks like granite, radon gas) and Space (cosmic rays from the Sun).
Detecting Radiation:
- Photographic Film: Becomes darker as it absorbs more radiation.
- Geiger-Müller (GM) Tube: Clicks every time it detects radiation.
Contamination vs. Irradiation:
Irradiation: Being exposed to radiation from a nearby source. It stops the moment you move away. (Like standing near a fire to get warm).
Contamination: Getting radioactive atoms on or in your body. These atoms will keep decaying until they are removed. (Like getting hot coal stuck in your pocket!).
Safety Precautions: Use lead-lined aprons, keep your distance, use robotic arms for handling, and limit time spent near sources.
7. Fission and Fusion
These are nuclear reactions that release massive amounts of energy.
Nuclear Fission (Splitting):
This happens in nuclear power stations. A large nucleus (like Uranium-235) absorbs a neutron and splits into two smaller "daughter nuclei," releasing energy and more neutrons.
Chain Reaction: The neutrons released can hit other Uranium atoms, causing them to split too. We control this using:
- Control Rods: Made of boron, they absorb extra neutrons to slow the reaction down.
- Moderators: Slow down neutrons so they are easier for the Uranium to "catch."
Nuclear Fusion (Joining):
The creation of a larger nucleus from two smaller nuclei. This is the energy source for stars (like the Sun). It releases much more energy than fission!
The Problem: It is very hard to do on Earth. Protons are positively charged, so they repel each other (electrostatic repulsion). To force them together, you need extremely high temperatures and pressures.
Quick Review Box
Fission = Splitting (think 'fissure' or a crack).
Fusion = Joining (think 'fusing' things together).
Takeaway: Fission is used in power plants today. Fusion is the goal for the future because it is cleaner and more powerful, but the conditions needed are incredibly difficult to maintain.