The Nuclear Model of the Atom

What You'll Learn

This chapter explains how atoms are structured, what the nucleus contains, and the massive energy released when nuclei split or fuse. By the end, you'll understand why atoms hold together, how we describe them mathematically, and why nuclear reactions are so powerful.

The Atom: A Miniature Solar System

Atomic Structure: The Big Picture

An atom is incredibly simple in concept: it has a small, dense, positively charged nucleus in the centre, surrounded by negatively charged electrons in orbits around it. That's it. But the scale is mind-blowing.

Just How Small?

Atoms have a radius of about 1 × 10⁻¹⁰ m (0.0000000001 m). To put that in perspective: you could fit roughly 100 million atoms side by side across the width of your thumbnail. Yet the nucleus is even tinier—over 10,000 times smaller than the atom itself. And here's the wild part: despite being so small, the nucleus contains almost all of the atom's mass. The electrons barely weigh anything.

Rutherford's Alpha Scattering Experiment: The Proof

Before 1909, scientists thought atoms were like raisin puddings—a uniform ball of positive charge with electrons scattered throughout (the "Plum Pudding Model"). Then Ernest Rutherford's team fired alpha particles (helium nuclei) at thin gold foil and watched what happened.

The Results Were Shocking:

• Most alpha particles passed straight through — This meant atoms are mostly empty space. If the positive charge were spread throughout, particles would be deflected more.
• Some were deflected (bent) sharply — They came close to something very small and very positively charged that repelled them.
• A tiny few bounced straight back — They hit the dense centre head-on. This proved the nucleus is real and extremely small.

Ions: Atoms with an Electrical Charge

In a neutral atom, the number of protons (positive) exactly equals the number of electrons (negative), so the overall charge is zero.

But atoms can gain or lose electrons to become more stable. When they do, they become ions—electrically charged particles.

• Positive ions: Formed when an atom loses electrons. More protons than electrons now. (Examples: Na⁺, Ca²⁺)
• Negative ions: Formed when an atom gains electrons. More electrons than protons now. (Examples: Cl⁻, O²⁻)

The Nucleus: Protons, Neutrons & Numbers

What's Inside the Nucleus?

The nucleus contains exactly two types of particles:

• Protons: Positively charged particles. Each proton has a relative charge of +1.
• Neutrons: Neutral particles (no charge). Relative charge = 0. They have roughly the same mass as protons.

Because the nucleus is made of protons and neutrons, the nucleus has an overall positive charge. This is what attracts and holds the negative electrons in their orbits.

Describing the Nucleus: Proton Number (Z) and Nucleon Number (A)

Every nucleus can be completely described by just two numbers. These are so important they're part of the standard notation.

Finding the Number of Neutrons

If you know the proton number (Z) and nucleon number (A), you can always find neutrons:

Example: Carbon-12 has nucleon number 12 and proton number 6. So it has 12 − 6 = 6 neutrons.

Nuclide Notation: Reading and Writing Nucleus "Codes"

Physicists write nucleus symbols in a special way called nuclide notation, which looks like this:

    A   (Nucleon Number / Mass Number - top)
   ---X
    Z   (Proton Number / Atomic Number - bottom)

    Example: ¹²C or C-12
             (Carbon-12 or Carbon 12)
            

How to read it:

• X = Chemical symbol of the element (C = carbon)
• A (top number) = Nucleon number (total particles in nucleus)
• Z (bottom number) = Proton number (identifies the element)

Isotopes: The Same Element, Different Weight

Here's something that confuses many students: not all atoms of the same element are identical. They can have different numbers of neutrons.

Real Example: Hydrogen has three isotopes:

• Protium (¹H): 1 proton, 0 neutrons. This is regular hydrogen.
• Deuterium (²H): 1 proton, 1 neutron. About 1 in every 5,000 hydrogen atoms is deuterium.
• Tritium (³H): 1 proton, 2 neutrons. Extremely rare—about 1 in a billion billion hydrogen atoms. It's radioactive.

All three are hydrogen (they all have 1 proton, so Z = 1), but their nucleon numbers differ (A = 1, 2, 3). This affects their stability and mass, but not their basic chemical properties.

Protons, Neutrons & Electrons: Properties & Charges

Relative Charge: Comparing Particles on the Charge Scale

The three subatomic particles have different charges. Instead of using Coulombs (which are tiny numbers), physicists use relative charge—a simple ratio that compares each particle to the fundamental unit of charge.

Here are the relative charges you must memorize:

Why these numbers?

• A proton has charge +1.6 × 10⁻¹⁹ C, so its relative charge is (+1.6 × 10⁻¹⁹) ÷ (1.6 × 10⁻¹⁹) = +1.
• An electron has charge −1.6 × 10⁻¹⁹ C, so its relative charge is (−1.6 × 10⁻¹⁹) ÷ (1.6 × 10⁻¹⁹) = −1.
• A neutron has no charge, so relative charge = 0 ÷ (1.6 × 10⁻¹⁹) = 0.

Nuclear Charge (Extended Tier)

The nuclear charge is simply the overall charge of the nucleus—how many protons it contains.

Example: Carbon has proton number 6, so its nuclear charge is 6 × (+1) = +6.

Relative Mass: Why Electrons Don't Count

Protons and neutrons have almost identical mass, which we define as 1 unit each. Electrons are much, much lighter.

This is why we ignore electron mass when calculating atomic mass. The mass of an atom is essentially the mass of its nucleus—protons + neutrons.

Example: Oxygen-16 has nucleon number 16, so its relative mass is approximately 16.

Fission & Fusion: Energy from the Nucleus

Why Nuclear Reactions Release Enormous Energy

When a nucleus splits (fission) or joins (fusion), a tiny amount of its mass is converted into energy. Thanks to Einstein's E = mc², even a tiny mass converts to a huge amount of energy because c (the speed of light) is enormous.

Nuclear Fission: Splitting the Nucleus

Fission is when a large, unstable nucleus breaks apart into two smaller nuclei (called daughter nuclei), releasing 2 or 3 neutrons and massive energy.

How it works:

1. A neutron smashes into a large nucleus (like uranium-235).
2. The nucleus becomes unstable and splits into two smaller nuclei.
3. 2–3 extra neutrons are released with tremendous kinetic energy.
4. Gamma rays are emitted (high-energy radiation).
5. Energy is released as the products fly apart at extreme speed.

Fission Process:

    Neutron →    [Large Nucleus]  →  [Daughter Nucleus 1]
                                      [Daughter Nucleus 2]
                                      + Neutrons (2–3)
                                      + Gamma rays
                                      + Lots of kinetic energy
            

Fission Equations: Reading Nuclear Reactions

Fission reactions are written like chemical equations, but with nuclei instead of atoms. Here's a real example:

What's happening here:

• 235 U (uranium-235) + 1 neutron are the reactants (inputs).
• 92 Kr (krypton-92) + 141 Ba (barium-141) + 3 neutrons + energy are the products (outputs).
• Notice: The top numbers add up on both sides (235 + 1 = 92 + 141 + 3). Conservation of nucleon number.
• The bottom numbers also add up (92 + 0 = 36 + 56 + 0). Conservation of proton number.

Why Large Nuclei Undergo Fission

Large nuclei like uranium and plutonium are naturally unstable. Too many protons repel each other, and there aren't enough neutrons to hold them together. They eventually break apart—either spontaneously (radioactively) or triggered by a neutron collision.

Chain Reactions: In nuclear power plants and bombs, the neutrons released by one fission trigger fission in neighbouring nuclei, creating a chain reaction that releases enormous energy in a controlled (or uncontrolled) manner.

Nuclear Fusion: Joining Nuclei

Fusion is the opposite of fission. Two light nuclei join together to form a heavier nucleus, releasing energy and often a neutron.

Where it happens: Stars like the Sun are powered by fusion. Hydrogen nuclei fuse at extreme temperatures and pressures to form helium, releasing the energy that makes stars shine for billions of years.

Why fusion is hard on Earth: Fusion requires extreme temperature (millions of degrees) and pressure to overcome the repulsion between positively charged nuclei. We can achieve it for an instant in hydrogen bombs, but sustained fusion in power plants is still an unsolved engineering challenge.

Fusion Equations: Reading Nuclear Reactions

Here's the main fusion reaction that powers the Sun:

What's happening:

• Deuterium (²H, heavy hydrogen with 1 proton + 1 neutron) + regular hydrogen (¹H, 1 proton) are the reactants.
• Helium-3 (³He, 2 protons + 1 neutron) + energy are the products.
• Top numbers: 2 + 1 = 3 ✓ (nucleon number conserved)
• Bottom numbers: 1 + 1 = 2 ✓ (proton number conserved)

Mass-Energy Conversion in Nuclear Reactions

In both fission and fusion, the products weigh slightly less than the reactants. This "missing mass" has been converted to energy via E = mc².

Although the mass difference is tiny (fractions of a percent), because c² is so enormous (9 × 10¹⁶ m²/s²), even minuscule mass gives tremendous energy.

What to Memorise: Key Terms & Formulas

Concepts Checklist: Can You Tick Them All?

Work through this checklist as you revise. You should be able to tick everything before your exam.

Exam Tips: Common Mistakes & What Examiners Look For

You've Got This!

You now understand the nuclear model of the atom—how atoms are structured, how we describe them mathematically, what isotopes are, how ions form, and why nuclear reactions release enormous amounts of energy. These ideas are fundamental to physics and chemistry.

Your revision strategy:

1. Work through this guide section by section.
2. Try the practice questions—don't peek at the answers first.
3. Go back and tick the concepts checklist. If you can't tick it confidently, reread that section.
4. Create flashcards for the "What to Memorise" section—formulae, key terms, and particle properties.
5. Practice balancing nuclear equations until it becomes automatic.
6. Review the exam tips regularly so you avoid the common pitfalls.

Final thought: This material isn't just for exams. The nuclear model explains real phenomena—radioactivity, nuclear power, how stars work, and nuclear medicine. Understanding atoms at the nuclear level is understanding the foundation of matter itself.

You're ready. Now go ace this! 🚀