Journey Inside the Atom · Lesson 4 of 14
Rutherford's Model of an Atom
“A tiny nucleus explains the scattering evidence but creates a new problem of atomic stability”
• Derive Rutherford's main conclusions from scattering. • Describe the planetary model. • Compare the sizes of the atom and nucleus. • Explain the stability problem. • Describe the proton and electrical neutrality.
The gold foil experiment did more than reject Thomson's model. It provided clues for a new picture of the atom. Rutherford asked what arrangement could allow most particles to travel freely while occasionally causing a very strong repulsion.
Rutherford's Model of an Atom
The Nucleus and Empty Space
The extremely small dense central region of an atom that contains positive charge and most of the atomic mass.
Rutherford concluded that most of an atom is empty space. Positive charge and most of the mass are concentrated in a tiny central nucleus. Electrons were pictured as moving around this nucleus, somewhat like planets around the Sun, which is why the model is often called the planetary model.
Connecting Evidence With Conclusions
| Observation | Conclusion |
|---|---|
| Most alpha particles passed through | Most of the atom is empty space. |
| Some alpha particles were deflected | Positive charge is present in the atom. |
| A few were strongly deflected | Positive charge and most mass are concentrated in a tiny nucleus. |
The Scale of the Nucleus
The approximate diameter of an atom is about 10⁻¹⁰ m, while the diameter of a nucleus is about 10⁻¹⁵ m. In diameter, the atom is therefore about one hundred thousand times larger than its nucleus. If an atom were enlarged to the size of a cricket ground, the nucleus would be comparable to a tiny pepper grain near the centre.
Problem
Estimate how many atoms of diameter 10⁻¹⁰ m would span paper of thickness 0.1 mm.
- 1.Convert 0.1 mm to metres: 0.1 mm = 10⁻⁴ m.
- 2.Divide the sheet thickness by one atomic diameter.
- 3.10⁻⁴ ÷ 10⁻¹⁰ = 10⁶.
- 4.The estimate is about one million atoms across the thickness.
Limitations of Rutherford's Model
An electron moving in a circular path continually changes direction and is therefore accelerating. In the classical picture used here, an accelerating charged electron should lose energy. Its orbit would shrink, the electron would spiral inward and it would eventually fall into the positive nucleus. If this happened, atoms would collapse, yet ordinary matter is stable.
Rutherford's model explains the scattering experiment but does not explain atomic stability. A model can be a major improvement and still remain incomplete.
Discovery of the Proton
A positively charged subatomic particle in the nucleus with relative charge +1 and much greater mass than an electron.
The positive charge of the nucleus is associated with protons. Since the electron has relative charge −1 and the proton +1, a neutral atom must contain equal numbers of protons and electrons.
Quiz
Which description best matches Nucleus?
Which description best matches Proton?
Which term matches this description: The extremely small dense central region of an atom that contains positive charge and most of the atomic mass.
Which term matches this description: A positively charged subatomic particle in the nucleus with relative charge +1 and much greater mass than an electron.
Which statement is a key takeaway from this lesson?
Practice Problems
- Use the scattering observations to explain Rutherford's three conclusions.
- Why can a nucleus contain most of the mass while taking up little space?
- Explain the stability problem in Rutherford's model.
- A neutral atom contains seventeen protons. How many electrons does it have?
- Why was Rutherford's model better than Thomson's model even though it was incomplete?
Key Takeaways
• Rutherford proposed that an atom contains a very small, dense, positively charged nucleus. • Most of the atom's volume is empty space, with electrons present outside the nucleus. • The nucleus contains most of the mass of the atom. • Rutherford's model explained scattering results but could not fully explain why orbiting electrons remain stable.