5.1 Development of the Model of the Atom

Question 1

John Dalton

Answer 1

In 1804 John Dalton agreed with Democritus that matter was made of tiny spheres (“atoms”) that couldn’t be broken up, but he reckoned that each element was made up of a different type of “atom”.

Question 2

J.J. Thompson

Answer 2

J.J. Thompson discovered particles called electrons that could be removed from atoms. So Dalton’s theory wasn’t quite right. This led scientists to believe that atoms were spheres of positive matter with tiny negative electrons stuck in them like currants in a cake.

Question 3

Rutherford

Answer 3

• The Rutherford-Geiger-Marsden alpha particle scattering experiment fired a beam of alpha particles at thin gold foil. It was expected that the particles would pass straight through the gold sheet, or only be slightly deflected. But although most of the particles did go straight throw the sheet, some were deflected more than expected, and a few were deflected back the way they had come - something the original model couldn’t explain.
• These results provided evidence that the gold atom was mostly empty space - most of the alpha particles weren’t deflected. It also provided evidence that the atom had a central, tiny nucleus containing most of the atom’s mass (as most alpha particles passed straight through, but some were deflected by large angles) which was positively charged.

Question 4

Niehs Bohr

Answer 4

• Rutherford, Geiger and Marsden’s nuclear model described the atom as a positively charged nucleus orbited by a cloud of negative electrons.
• Scientists realised that electrons in a ‘cloud’ around the nucleus of an atom like this would be attracted to the nucleus, causing the atom to collapse. Niels Bohr got around this by adapting the initial model to show that electrons orbiting the nucleus can only do so at certain distances.
• This means that electrons can move within (or sometimes leave) the atom.

Question 5

When Democritus first conceived of atom theory, around 500 BC, how did he describe atoms? (3)

Answer 5

• The smallest possible unit of matter
• Small spheres
• Separated from each by empty space

Question 6

How Rutherford developed the nuclear model

Answer 6

• In Rutherford’s experiments, alpha particles were fired at a thin sheet of gold foil.
• Most particles passed through, but some were deflected off course.
• This caused him to hypothesise that there was a dense region of positive charge at the centre of the atom that repelled the alpha particles.
• As a result he developed the nuclear model of the atom, in which there was a central positive nucleus, surrounded by negative electrons.

Question 7

In the 20th century, Chadwick discovered neutral particles in the atomic nucleus. What are these particles called?

Answer 7

Neutrons