o We use models to help us understand things that are too:

BIGsmall

Fasts l o w

or complicated to see in “real life”

Why Use Models?

Scientists have used the results of experiments to help them create a model of the atom.

The atomic model has undergone many changes, and continues to develop and change…

from THIS to THIS

Model of the Atom

Ancient Greek philosopher (460 – 370 BC)

Suggested that matter cannot be cut into smaller pieces forever

Gave a name to the smallest particle of matter:

the ATOM

Democtritus

ατομοσ

Dalton was an English schoolteacher

Began teaching mathematics and chemistry at the age of 12

Revived the idea of Democritus’ “smallest piece” of matter

John Dalton (1766 – 1844)

“All matter is made of tiny particles called “atoms”

Atoms are indivisible and indestructible Atoms of the same element are identical Atoms of different elements differ in some

fundamental way Atoms combine in simple whole number

ratios to form compounds

Dalton’s Model of the Atom

These wooden balls, were the first models made to represent atoms - used by John Dalton to demonstrate atomic theory.

Joseph Thomson (1856-1940)

English physicist

Worked with Cathode-Ray Tubes (CRTs)

Credited with the discovery of the electron

Call me “JJ”

Mysterious particles emanated from the cathode end

These particles were deflected by magnetic and electric fields

They were very small and negatively charged

Cathode Ray Tubes

These particles were called “electrons” and were assumed to be a part of all matter

The “Plum Pudding” model:

Thomson’s Model of the Atom

New Zealand-born (1871 – 1937), student of JJ Thomson

Famous “Gold Foil Experiment” Discovered that atom has a small, positive

NUCLEUS with negative electrons moving around it - mostly empty space.

Earnest Rutherford

Gold Foil Experiment

“It was quite the most incredible event that ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you!”

Gold Foil Experiment

Thomson Vs Rutherford Model

Used Max Planck’s quantum theory to show that…

Electrons exist in different “energy levels”

Neils Bohr (1885 – 1962)

Electron “clouds” and “orbitals”

Schrodinger and Pauli

A fellow researcher with Rutherford

Discovered years later that the nucleus contained not one, but TWO types of particles

This second particle was called the “neutron” because it had no electrical charge

James Chadwick…mmmm, neutrons…

Small, dense nucleus made up of…. positive protons, neutral neutrons, with negative electrons, in “orbit” around

nucleus

Current Atomic Model

Democritus – ATOMOS

Dalton – atoms as “solid balls”

Thomson – “plum pudding” model, with negative electrons

Rutherford – small positive nucleus, atom is mostly empty space

Chadwick – Nucleus contains protons and neutrons

Summary