History of The Atom

Question 1

What is an Atom?

Answer 1

Smallest unit of matter

Question 2

Who was Aristotle?

Answer 2

Believed that there were 4 ‘elements’ that make up everything:

Earth
Air
Fire
Water

He was in 400 B.C

Question 3

Who was Democritus?

Answer 3

Theorized that if you cut something in half, over and over, eventually it will be “uncuttable”
Named the “uncuttable” particle the atom

(From the Greek word “atomos”, meaning indivisible)

He was approx 440 B.C

Question 4

Who was John Dalton?

Answer 4

Dalton proposed a modern atomic model based on experimental evidence rather than logic and reasoning. He suggested that:

1. All matter is made up of neutral atoms.
2. Atoms of the same element are identical, meaning they have the same mass and behave in the same way.
3. Atoms are small, solid spheres.

This model is known as the ‘Billiard Ball Model’.

He was in year 1803

Question 5

Who was J.J Thompson?

Answer 5

J.J. Thomson tested Dalton’s Model through an experiment in which he heated metal in a vacuum tube. Tiny particles left the metal and traveled to the opposite end of the tube. When he placed a charged rod near these particles, he discovered they were negatively charged.

Based on this evidence, Thomson proposed that:

1. Atoms contain smaller parts with a negative charge called electrons.
2. The electron has a very small mass compared to the entire atom.
3. Electrons are scattered throughout a large positive blob of material.

This led to the development of the Chocolate Chip Cookie Model.

He was in year 1904

Question 6

Who was Ernest Rutherford?

Answer 6

Rutherford Experiment 1
Rutherford and his team, while experimenting with radioactive elements, discovered a new particle which they called the proton. They found that the proton was positively charged (+1).

Rutherford Experiment 2: Gold Foil Experiment
In their Gold Foil Experiment, Rutherford and his team shot positive particles at a sheet of gold atoms. They expected the particles to pass straight through because of the large spaces between atoms.

Gold Foil Experiment Results
Most of the particles did pass through as expected, but some were deflected to the side. This unexpected result led Rutherford to rethink the structure of the atom.

Rutherford’s Model
From the gold-foil experiment, Rutherford concluded in 1911 that:

1. The positive charge of the atom and most of its mass are concentrated in a very tiny space called the nucleus.
2. Most of the atom is empty space between the electrons and the nucleus.

This led to the development of the Cherry Pit Model of the atom.

He was in year 1911

Question 7

Who was James Chadwick?

Answer 7

James Chadwick was weighing atoms and became confused because the mass of the atoms didn’t align with what was expected from only protons and electrons. To investigate further, he shot protons at some beryllium.

He noticed some mystery particles traveling away.

Chadwick wondered:

• If the mystery particle was positively charged, it would be a proton.
• If the mystery particle was negatively charged, it would be an electron.

However, the particles he observed were neither positively nor negatively charged. This led him to conclude that they were neutrally charged particles, which he later identified as neutrons.

He was in the 1900s

Question 8

Who was Niels Bohr?

Answer 8

Bohr-Rutherford Model

While working under Rutherford, Bohr published his model of the atom.

He proposed that:
Electrons are moving very quickly
Electrons circle the nucleus in energy shells called orbits!

This is our current Model and is called the:
Bohr-Rutherford Model