Welcome to Atomic Structure!
In this chapter, we are going to dive into the tiny building blocks that make up everything in the universe: atoms. Don't worry if this seems tricky at first—atoms are so small we can't see them with our eyes, but by the end of these notes, you'll understand how they are built, how they change, and why some of them are "radioactive." Knowing about atoms helps us understand everything from how stars shine to how doctors treat cancer.
1. The Structure of an Atom
Every atom is like a tiny solar system. It has a center and things orbiting around it.
The Size and Scale
- Atoms are incredibly small. They have a radius of about \(1 \times 10^{-10}\) metres (that’s 0.0000000001 metres!).
- The nucleus is the "sun" at the center. It is even smaller—less than 1/10,000 of the radius of the whole atom.
- Analogy: If an atom were the size of a huge football stadium, the nucleus would be like a small marble sitting on the center spot!
What's Inside?
The atom is made of three subatomic particles:
- Protons: Found in the nucleus. They have a positive charge (+1) and a mass of 1.
- Neutrons: Found in the nucleus. They have no charge (0) and a mass of 1.
- Electrons: These orbit the nucleus in energy levels (shells). They have a negative charge (-1) and almost no mass.
Quick Review: In a normal atom, the number of electrons is always equal to the number of protons. Because the positive and negative charges are equal, the atom has no overall electrical charge.
Energy Levels
Electrons live in shells. If an atom absorbs electromagnetic radiation, an electron can jump to a higher energy level (further from the nucleus). If the atom emits radiation, the electron drops back down to a lower level (closer to the nucleus).
Key Takeaway: Almost all the mass of an atom is concentrated in the tiny, positive nucleus at the center.
2. Mass Number, Atomic Number, and Isotopes
To identify atoms, we use two special numbers often seen on the Periodic Table.
- Atomic Number: The number of protons. This tells you which element it is (e.g., every Carbon atom has 6 protons).
- Mass Number: The total number of protons + neutrons.
Example: For Sodium (\({}^{23}_{11}\text{Na}\)):
The Atomic Number is 11 (so it has 11 protons).
The Mass Number is 23 (so it has 23 protons + neutrons).
To find the neutrons: \(23 - 11 = 12\) neutrons.
What are Isotopes?
Isotopes are atoms of the same element (same number of protons) but they have a different number of neutrons. They have the same chemical properties but different masses.
What are Ions?
If an atom loses one or more of its outer electrons, it becomes a positive ion (because it now has more positive protons than negative electrons).
Key Takeaway: Protons define the element; neutrons define the isotope; electrons define the charge.
3. How the Model of the Atom Changed
Science is always moving! As we got better equipment, our "picture" of the atom changed.
- Ancient times: People thought atoms were tiny spheres that couldn't be divided.
- The Plum Pudding Model: After discovering electrons, scientists thought the atom was a ball of positive charge with negative electrons stuck in it (like blueberries in a muffin).
- The Alpha Particle Experiment: Scientists fired tiny positive particles at thin gold foil. Most went through, but some bounced back! This proved that the mass must be concentrated in a tiny nucleus at the center.
- The Nuclear Model (Bohr): Niels Bohr suggested electrons orbit at specific distances (energy levels).
- The Neutron (Chadwick): About 20 years after the nucleus was accepted, James Chadwick proved that neutrons existed in the nucleus.
Did you know? It took over 100 years of experiments to get to the model of the atom we use in class today!
4. Nuclear Radiation
Some atomic nuclei are unstable. To become stable, they give out radiation. This is a random process called radioactive decay.
Types of Radiation
- Alpha (\(\alpha\)): Two neutrons and two protons (a helium nucleus). It is big, heavy, and very ionising but can be stopped by a piece of paper.
- Beta (\(\beta\)): A high-speed electron ejected from the nucleus. It happens when a neutron turns into a proton. It can be stopped by a thin sheet of aluminum.
- Gamma (\(\gamma\)): An electromagnetic wave. It is very penetrating and needs thick lead or concrete to stop it.
- Neutron (n): A neutron can also be emitted from the nucleus.
Quick Comparison:
Most Ionising: Alpha (damages cells easily if inside the body).
Most Penetrating: Gamma (can travel through almost anything).
Key Takeaway: Radiation is just an unstable nucleus trying to "relax" and become stable.
5. Half-Life
Radioactive decay is random, so we can't predict when one single atom will decay. Instead, we use half-life.
Half-life is the time it takes for the number of nuclei in a sample to halve, or the time it takes for the count-rate (activity) to fall to half its initial level.
Activity is measured in Becquerels (Bq).
Example: If a sample has an activity of 800 Bq and a half-life of 2 hours:
After 2 hours: 400 Bq
After 4 hours: 200 Bq
After 6 hours: 100 Bq
Common Mistake: Students often think "half-life" means the time until the radiation is gone. It doesn't! It only means the time it takes to half. It never quite reaches zero.
Key Takeaway: A short half-life means the source is very radioactive at the start but becomes safe quickly. A long half-life means it stays radioactive for a long time.
6. Radioactive Contamination and Irradiation
It is important to know the difference between being near radiation and having radiation on you.
- Irradiation: Exposing an object to nuclear radiation. The object does not become radioactive. (Like sitting near a fire—you get warm, but you don't become the fire).
- Contamination: The unwanted presence of radioactive atoms on or inside an object. This is more dangerous because the atoms will keep decaying inside you.
Safety and Peer Review
Because radiation can be hazardous (causing mutations or cancer), scientists must publish their findings. These are peer-reviewed, meaning other scientists check the work to make sure it is accurate and safe.
Key Takeaway: To stay safe, we use lead aprons (protection from irradiation) and gloves/masks (protection from contamination).
Final Summary Review
- Atoms: Tiny, positive nucleus (P+N), negative electrons in shells.
- Models: Plum Pudding (old) \(\rightarrow\) Nuclear Model (new).
- Isotopes: Same protons, different neutrons.
- Radiation: Alpha (big/strong), Beta (fast electron), Gamma (wave).
- Half-life: Time taken for activity to halve.