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.
Atomic Structure
Atoms have a tiny positive nucleus orbited by negative electrons. The nucleus contains almost all the atom's mass.
The Nucleus
Made of protons (positive) and neutrons (neutral). We describe nuclei using proton and nucleon numbers.
Isotopes
Atoms of the same element with different numbers of neutrons. Same protons, different mass.
Fission & Fusion
Splitting large nuclei or joining small ones releases enormous amounts of energy from the nucleus.
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.
Think of it like this:
If the nucleus were the size of a marble sitting in the middle of a football stadium, the electrons would be tiny specks flying around the upper seats. The atom is 99.9% empty space—mostly vacuum between the nucleus and the electrons.
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.
Relative Mass of Electrons
Electrons have a relative mass of 1/2000 compared to a proton. We say their mass is "negligible"—so small we often ignore it completely when calculating atomic mass.
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.
Why This Mattered: This experiment was the first evidence that atoms have a nuclear structure. It overturned the Plum Pudding Model and gave us the nuclear model we use today.
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.
Ion
- Positive ions:
loses
- Negative ions:
gains