The History of the Atom
What is matter made of? This is a difficult question, because we can’t see the smallest parts of matter
Empedocle A Greek around 400 BC He divided matter into four elements, that he also called "roots": These roots were Earth Air Water Fire
Democritus (another Greek) Named the smallest piece of matter the atom which means ‘indivisible’ He and his followers thought that all atoms were made of the same material but were different shapes and sizes His ideas were forgotten for 2100 years
But these were just ideas In the 1800’s a man named Dalton was investigating the properties of gases He noticed that gases combine as if they were made of individual particles His theory of atomic structure is called the Billiard Ball Model
The Billiard Ball Model Atoms are indivisible and indestructible Atoms of different elements are different Chemical compounds are formed by combining atoms
Remember, Dalton had never seen an atom!
Thomson In 1897 he came up with a different model He did experiments that showed there were negatively charged particles in atoms He called these particles electrons
Raisin Bun Model Thomson thought the atom was made of positively charged material with negatively charged particles embedded within it Sometimes this is called the ‘Plum Pudding Model’
Thomson had never seen an atom! Thomson was doing experiments where he passed an electric current through gases He discovered that negative particles were being emitted from the gas Therefore atoms must contain charged particles Thomson’s model was based on his experimental evidence His model explains the evidence that he collected.
Rutherford In 1908 this English scientist was shooting alpha particles at very thin pieces of gold foil Alpha particles are heavy particles with a positive charge The gold foil was very thin
What Happened? He expected that the alpha particles would pass through the gold foil without a problem A screen behind the gold foil was used to detect the alpha particles after they passed through the foil
This is his experiment
His results were surprising! Instead of passing through the gold foil, some of the particles bounced straight back in the opposite direction Most of the particles passed right through What did this mean?
Can you imagine? Rutherford said it was like shooting a cannonball at a piece of tissue paper and having it bounce back
Rutherford explained his results by coming up with a new model of the atom The atom must be mostly made up of empty space That is why most of the alpha particles passed through the gold foil
What was Rutherford’s explanation? There must be a very dense, positively charged region in the centre of the atom That is what caused the alpha particles to change direction Rutherford called this the nucleus
Atoms are electrically neutral Therefore there must be negative particles in an atom Rutherford thought that they orbited around the nucleus He called them electrons
Rutherford’s Model is similar to our picture of the Solar System Sometimes it is called the Planetary Model
Think of a marble in a baseball stadium That is the size of the nucleus compared to the size of an atom!
In 1913 Rutherford’s model was revised by Neils Bohr Bohr noticed that only certain energies of light would interact with the electrons of atoms This meant that electrons could only have certain energies
Bohr’s Model His picture of the atom had electrons in specific orbits around the nucleus These orbits had certain energies
Bohr’s Model of the atoms is very useful even today Later experiments have shown that the atom is really very complicated The motion of particles can only be described with complicated math Quantum Mechanics uses math to predict the movement of electrons Bohr’s Model is still used for simple descriptions of atoms
The End!!!