How the model of the atom changed, from solid spheres to the nuclear model, and the particles inside an atom.
What you will learn
- Describe how the model of the atom developed
- Explain what alpha scattering showed
- State the relative mass and charge of protons, neutrons and electrons
A model that kept changing
- Early on: atoms are tiny spheres that cannot be divided.
- The electron is discovered, so the plum pudding model: a ball of positive charge with electrons scattered through it.
- Alpha scattering: positive alpha particles fired at thin gold foil. Most go straight through, a few are deflected, a very few bounce back. So most of the atom is empty space, with the mass and positive charge in a tiny nucleus.
- Bohr: electrons orbit the nucleus at fixed distances (shells).
- Later: the nucleus contains protons, and then neutrons (Chadwick).
Why the neutron had to exist: atoms were heavier than their protons alone could explain. Helium has 2 protons but a mass of 4, so the nucleus had to hold another particle with mass but no charge. With no charge it was hard to detect, so it was found later, by Chadwick in 1932.
The rest of this lesson and its 10 practice questions are locked.
In this lesson
- A model that kept changing
- Inside an atom
- Try it: Find the protons
10 practice questions, marked as you go; about 20 minutes in all.
Key terms
Plum pudding, Nucleus, Protons, Neutrons, Significant figures
In this unit: Atomic structure and the periodic table
Atoms, formulae and equations, separating mixtures, the model of the atom, isotopes, electrons in shells and the groups of the periodic table.