Physics (0625) | The nuclear model of the atom

Understanding the Nuclear Model of the Atom (IGCSE Physics 0625)

Hello future Physicists! This chapter is where we dive into the microscopic world to understand how matter—everything around us—is built. We're talking about atoms, the tiny building blocks that hold the universe together. Don't worry if it seems abstract; we will use analogies and simple steps to break down the nuclear model. Let's get started!

1. The Basic Structure of the Atom (Core Content)

For a long time, people thought atoms were solid, uniform balls. But modern physics shows us a much more exciting structure—mostly empty space!

Key Components of the Atom

An atom has two main regions:

• The Nucleus: This is the tiny, dense centre of the atom. It carries a positive electrical charge.
• Electrons: These are tiny particles carrying a negative electrical charge. They orbit the nucleus in shells or energy levels.

Analogy: Think of the atom like a mini Solar System. The nucleus is the heavy, central Sun, and the electrons are the light planets orbiting far away.

Neutral Atoms and Ions

In a neutral atom, the total positive charge in the nucleus must perfectly balance the total negative charge carried by the orbiting electrons.

If the number of protons (positive) equals the number of electrons (negative), the atom has zero overall charge.

An atom can become charged (an ion) by gaining or losing electrons:

• Positive Ion: Formed when an atom loses one or more electrons (it now has more protons than electrons).
• Negative Ion: Formed when an atom gains one or more electrons (it now has more electrons than protons).

2. Discovering the Nucleus: The Rutherford Scattering Experiment (Supplement)

Don't worry if this sounds complex—it's just a famous experiment that proved our current model! Before 1911, the "Plum Pudding" model suggested positive charge was spread out like pudding, with electrons scattered inside like plums. Ernest Rutherford proved this wrong using alpha particles.

The Experiment Setup

Rutherford fired beams of positively charged alpha (\(\alpha\)) particles at a sheet of extremely thin gold foil. They monitored where the particles ended up using a fluorescent screen.

Observations and Conclusions (The Evidence)

Rutherford expected the alpha particles to pass straight through with only minor deflection, based on the old model. Instead, they observed three main things:

1. Most particles passed straight through or were only slightly deflected.
   Conclusion: The atom is mostly empty space. The nucleus is very small compared to the size of the whole atom.
2. A tiny number (about 1 in 8000) bounced back, sometimes directly backwards.
   Conclusion: There must be a tiny, extremely dense central mass in the atom, containing almost all of the atom's mass.
3. The particles that deflected widely were repelled.
   Conclusion: Since alpha particles are positive, the central mass (the nucleus) must also be positively charged, causing electrostatic repulsion.

This experiment provided the key evidence supporting the current nuclear model: a tiny, massive, positively charged nucleus surrounded by vast empty space where electrons orbit.

3. The Nucleus: Protons and Neutrons (Core and Supplement)

Now let's zoom in on the nucleus. It is made up of two types of particles, collectively called nucleons.

Composition and Relative Charges (Core)

Defining the Numbers (Core and Supplement)

To describe any atom, we use two key numbers:

1. Proton Number (\(Z\)) or Atomic Number (Core/Supplement)
   

   \(Z\) is the number of protons in the nucleus.

   Why is this important? \(Z\) determines the element. Every atom with the same proton number is the same element (e.g., all atoms with \(Z=6\) are Carbon).

   Supplement Link: The proton number (\(Z\)) is directly related to the relative charge of the nucleus (it defines the magnitude of the positive charge).

2. Nucleon Number (\(A\)) or Mass Number (Core/Supplement)
   

   \(A\) is the total number of nucleons (protons + neutrons) in the nucleus.

   Supplement Link: The nucleon number (\(A\)) is directly related to the relative mass of the atom (since electrons have negligible mass).

Calculating Neutrons (Core)

Since the Nucleon Number (\(A\)) is the total of protons and neutrons, you can always find the number of neutrons (\(N\)) using this simple formula:

Number of Neutrons (\(N\)) = Nucleon Number (\(A\)) - Proton Number (\(Z\))
\(N = A - Z\)

Nuclide Notation (Core)

We use a special notation to quickly summarize the composition of any nucleus:

\[ \text{}_{\text{Z}}^{\text{A}}\text{X} \]

Where:

• \(\text{X}\) is the chemical symbol for the element (e.g., H for Hydrogen, C for Carbon).
• \(\text{A}\) is the Nucleon Number (Mass Number) at the top.
• \(\text{Z}\) is the Proton Number (Atomic Number) at the bottom.

Example: An atom of Carbon-14 is written as \(\text{}_{6}^{14}\text{C}\).

This means: Protons (\(Z\)) = 6. Nucleons (\(A\)) = 14. Neutrons = \(14 - 6 = 8\).

4. Isotopes (Core)

If the proton number (\(Z\)) defines the element, what happens if the number of neutrons changes?

What is an Isotope?

Isotopes are atoms of the same element that have the same number of protons (\(Z\)) but different numbers of neutrons (\(N\)).

Since the number of neutrons is different, their nucleon numbers (\(A\)) are also different.

Analogy: Imagine a car model (the element, defined by the engine/Z). Isotopes are the same car model but with different weight tires (the neutrons/A).

Example: Hydrogen has three main isotopes:

• Protium (\(\text{}_{1}^{1}\text{H}\)): 1 proton, 0 neutrons.
• Deuterium (\(\text{}_{1}^{2}\text{H}\)): 1 proton, 1 neutron.
• Tritium (\(\text{}_{1}^{3}\text{H}\)): 1 proton, 2 neutrons.

They are all Hydrogen because they all have \(Z=1\), but they have different masses. Note that elements may have more than one isotope.

5. Nuclear Processes: Fission and Fusion (Supplement)

In nuclear physics, we study changes to the nucleus itself. The two key processes release huge amounts of energy.

A. Nuclear Fission (Splitting)

Fission is the process where a large, unstable nucleus (like Uranium-235) is split into two smaller, more stable nuclei.

• It is often triggered by firing a neutron at the large nucleus.
• It releases energy and usually releases more neutrons, leading to a chain reaction (this is the process used in nuclear power stations).

Qualitative Description of Mass/Energy Changes: During fission, the total mass of the products (the smaller nuclei and released particles) is slightly less than the original mass of the large nucleus. This 'missing' mass is converted directly into a massive amount of energy (as described by Einstein's famous equation, \(E=mc^2\)).

\(Large\ Nucleus \longrightarrow Two\ Smaller\ Nuclei + Neutrons + Energy\)

B. Nuclear Fusion (Joining)

Fusion is the process where two small, light nuclei (like Hydrogen isotopes) join together to form a single, larger, more stable nucleus.

• This process requires extremely high temperatures and pressures (millions of degrees Celsius) to overcome the electrostatic repulsion between the positive nuclei.
• This is the energy source that powers the Sun and other stars!

Qualitative Description of Mass/Energy Changes: Similarly to fission, during fusion, the total mass of the final nucleus is slightly less than the total mass of the two starting nuclei. This 'missing' mass is converted into colossal amounts of energy.

\(Two\ Small\ Nuclei \longrightarrow One\ Larger\ Nucleus + Energy\)

Did you know? Scientists around the world are researching ways to control nuclear fusion on Earth, as it offers a potentially clean and nearly limitless source of energy!