Atomic Structure - The Nuclear Model

An atom

The smallest particle of an element that still has the properties of that element

John Dalton

All matter made of small particles called atoms

Atoms are indivisible

Atoms cannot be created nor destroyed

Atoms of the same element have the same mass

William Crookes

Discovered cathode rays
→ Streams of negatively charged particles called electrons

When small objects (Metallic Maltese Cross) were placed in the path of the rays, they would cast a shadow at the end of the tube. Showing that cathode rays travel in straight lines

George Stoney

Suggested the name electron for the smallest unit of electric charge

JJ Thomson

Showed that cathode rays (electrons) are negatively charged particles

Measured the charge to mass ration - e/m of electron

Proposed a ‘Plum Pudding’ model
→ An atom is a positively charged sphere with negatively charged electrons embedded in it

Robert Millikan

Oil Drop Experiment
Measured the size of the charge of electron, therefore the mass of electron could now be calculated

Ernst Rutherford (1909)

Gold Foil Experiment
→ Bombarded gold foil with alpha particles (He 2+)
→ Used a zinc sulphide to detect the alpha particles

Observations and Conclusions of the Gold Foil Experiment

1) Most passed straight through → Atom is mainly empty space

2) Some were deflected through at large angles → A-particles passed close to a very positive core and were repelled

3) A very small number bounced straight back → A-particles must have hit a very small, very dense core

Niels Bohr

Electrons orbited the nucleus

Orbits had characteristics energy → Energy levels

Rutherford (1920)

Bombarded lighter elements such as nitrogen and oxygen with a-particles

→ Small positive particles were given off (Protons)

Assumptions of Rutherford’s Nuclear Model

→ Atoms contain a small dense core called the nucleus. Most of the mass of the atom is located here.

→ The atom is mostly empty space.

→ Electrons are scattered around in space surrounding the nucleus. This space is called the electron cloud. He proposed they were moving but didn’t state exactly the motion of an electron.

Limitation of Rutherford’s Nuclear Model

→ Since like charges repel, why does the nucleus not fly apart due to the repulsion between the protons in the nucleus?

→ Since opposite charges attract, the model couldn’t explain why electrons don’t spiral into the nucleus. We know that this doesn’t happen in real life as most atoms are stable

James Chadwick

Bombarded beryllium with a-particles

→ Small neutral particles were given off (Neutrons)

Limitation of the updated nuclear model of the atom

→ Original limitations still stand

→ The fact that the nuclear model cannot explain how electrons stay in motion around the nucleus without crashing into it is a major limitation of the nuclear model

Proton

Mass - 1
Charge - +1
Location - Nucleus

Nuetron

Mass - 1
Charge - Nuetral
Location - Nucleus

Electron

Mass - N/A
Charge - -1
Location - Orbiting the nucleus