🔬 Core Physics (9223): Study Notes - Atomic Structure
Hello future physicists! Welcome to the exciting world of Nuclear Physics. Before we can understand why some atoms break apart (radioactivity) or how nuclear power works, we need to know what atoms are made of.
This chapter, "Atomic Structure," is the essential foundation. Don't worry if this seems tricky at first; we'll break down the tiniest parts of matter into simple, manageable steps!
1. The Nuclear Model of the Atom
We now use the Nuclear Model to describe the atom. Imagine an atom is like a tiny solar system:
- A very dense, tiny centre called the Nucleus.
- Mostly empty space surrounding the nucleus.
- Tiny particles orbiting the nucleus in shells.
The crucial thing to remember (especially for Nuclear Physics) is how small the nucleus is. If an atom were the size of a football stadium, the nucleus would be like a tiny marble in the centre! This tiny marble holds almost all the atom’s mass.
Key Takeaway: The atom is mostly empty space, with a tiny, dense, central nucleus.
2. Subatomic Particles: The Building Blocks
Every atom is built from three fundamental particles. These particles determine the atom's identity and its electrical behaviour.
2.1. The Particles and Their Locations
It is vital to know where these particles are found and what their charge and mass are (relative to each other).
- Protons (p): Found in the nucleus. They have a positive charge.
- Neutrons (n): Found in the nucleus. They have zero charge (neutral).
- Electrons (e): Orbit the nucleus in shells. They have a negative charge.
2.2. Charge and Mass Comparison
In physics, we use "relative mass" and "relative charge" to simplify calculations.
| Particle | Relative Mass | Relative Charge |
|---|---|---|
| Proton | 1 | +1 |
| Neutron | 1 | 0 (Neutral) |
| Electron | Very small (approx. 1/1836) | -1 |
💡 Memory Aid:
- Proton = Positive.
- Neutron = Neutral.
- The heavy particles (Protons and Neutrons) are found in the nucleus. Electrons are so light, they hardly contribute to the mass of the atom.
Did you know? The force holding the protons (who naturally repel each other because they are all positive) and neutrons together in the nucleus is called the strong nuclear force. This is the source of energy we use in nuclear reactors!
3. Defining the Atom: Atomic and Mass Numbers
Every element (like Hydrogen, Oxygen, Uranium) has a unique signature based on how many protons it has. We use two important numbers to describe an atom: the Atomic Number (Z) and the Mass Number (A).
3.1. Atomic Number (Z) – The Identity Card
The Atomic Number (symbol Z) is the most important number for identifying an element.
- \(Z\) = The number of Protons in the nucleus.
- If the atom is neutral (no overall charge), then the number of Protons must equal the number of Electrons (\(P = E\)).
Analogy: The Atomic Number is like your unique ID number or social security number. Change this number, and you change the entire element!
3.2. Mass Number (A) – The Weight
The Mass Number (symbol A) represents the total mass of the nucleus. Since protons and neutrons are the heavy particles (mass = 1), we just count them up.
- \(A\) = The total number of particles in the nucleus (Protons + Neutrons).
- \(A = P + N\)
⚠️ Quick Review Box:
Protons = Z (Atomic Number)
Nucleus Mass = A (Mass Number)
4. Calculating Protons, Neutrons, and Electrons
You will often see atoms represented using this standard notation:
\(^{\text{Mass Number (A)}}_{\text{Atomic Number (Z)}} \text{X}\)
Where X is the chemical symbol for the element. Let's use Uranium-235 as an example, written as \(^{235}_{92}\text{U}\).
Step-by-Step Calculation (Uranium-235)
We need to find P, N, and E for neutral \(^{235}_{92}\text{U}\).
1. Find the Protons (P):
- Look at the Atomic Number (\(Z\)), which is the bottom number.
- \(Z = 92\). Therefore, Protons = 92.
2. Find the Electrons (E):
- Since the atom is neutral, the number of negative charges (Electrons) must equal the number of positive charges (Protons).
- Electrons = Protons = 92.
3. Find the Neutrons (N):
- Neutrons = Mass Number (\(A\)) - Atomic Number (\(Z\)).
- \(N = 235 - 92\)
- Neutrons = 143.
Common Mistake to Avoid: Students sometimes forget that the Mass Number (A) includes both protons and neutrons. Always subtract Z from A to find the neutrons.
5. Isotopes: Same Element, Different Weights
This concept is absolutely essential for understanding Nuclear Physics, as many radioactive elements exist as isotopes.
5.1. Definition of Isotope
Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons.
- Same element = Same Atomic Number (\(Z\)).
- Different weight = Different Mass Number (\(A\)).
Since isotopes have the same number of protons and electrons, they react identically in chemical reactions. Their differences only matter in the nucleus—which is why they are so important in Nuclear Physics!
5.2. Example: Carbon Isotopes
Carbon has an atomic number of 6 (\(Z=6\)). This means all carbon atoms have 6 protons.
- Carbon-12 (\(^{12}_6\text{C}\)): 6 Protons, 6 Neutrons (12 - 6 = 6). This is stable.
- Carbon-14 (\(^{14}_6\text{C}\)): 6 Protons, 8 Neutrons (14 - 6 = 8). This isotope is unstable and is used for carbon dating (it is naturally radioactive!).
Key Takeaway: If you change the number of protons, you change the element. If you change the number of neutrons, you create an isotope of that element. Unstable isotopes are the source of radioactivity, which we will study next.
✅ Chapter Summary: Atomic Structure
Here are the absolute must-know facts for this chapter:
- The atom consists of a tiny, dense, positive nucleus and orbiting electrons.
- The nucleus contains Protons (P, +1 charge, mass 1) and Neutrons (N, 0 charge, mass 1).
- The Atomic Number (Z) is the number of protons and determines the element.
- The Mass Number (A) is the total number of protons + neutrons.
- For a neutral atom, P = E.
- Neutrons = A - Z.
- Isotopes are atoms of the same element (same P) but different numbers of neutrons (different A).
Keep practising those calculations! You've successfully mastered the building blocks needed for our journey into nuclear reactions. Well done!