CHEMISTRY Unit 1.3/1.6 (AQA - history of the atom)

What did scientists believe about atoms before the discovery of the electron?

Tiny spheres that could not be divided.

What atomic model was proposed following the discovery of the electron?

The plum pudding model

What is the plum pudding model of the atom?

A ball of positive charge with negative electrons embedded in it.

What experimental results led to the nuclear model of the atom?

The alpha particle scattering experiment

What were the two main conclusions from the alpha particle scattering experiment?

The mass of an atom is concentrated at the centre (nucleus) and the nucleus is charged.

How did Niels Bohr adapt the nuclear model?

electrons orbit the nucleus at specific distances.

Why did Niels Bohr’s nuclear model gain acceptance?

theoretical calculations agreed with experimental observations.

What particles were discovered that made up the positive charge of the nucleus?

Protons (positive particles, each having the same amount of positive charge).

Whose experimental work provided evidence for the existence of neutrons within the nucleus?

James Chadwick (about 20 years after the nucleus became an accepted scientific idea).

What is the main difference between the plum pudding model and the nuclear model?

Plum pudding model: A ball of positive charge with electrons embedded throughout it, and no central nucleus.

• Nuclear model: A tiny, charged, dense positive nucleus at the centre containing most of the mass, with electrons orbiting around it.

Why do scientific models get changed or replaced over time?

Because new experimental evidence is discovered that the old model cannot explain.