1.3 Atomic Theory

1-3A Behaviour of Gases Demo

Early Greek Philosophers (5th century BC)

• Debated what the universe was made up of

• Democritus theorizes that everything made of tiny particles he calls atomos

Aristotle (4th century BC)• Lived 2500 years ago• Most respected Greek

philosopher• Creator of the “4

elements” theory– Believed that

everything in the world was made up of earth, air, fire & water

The Alchemists• Aristotles theory persisted for

over 2000 years!• During the middle ages, everyone

was attempting alchemy. • These attempts were all

unsuccesful, but resulted in an enormous amount of accidental scientific discoveries!

• These discoveries had many questioning Aristotles 4 elements theory

John Dalton (1766 – 1844)

• Video of Dalton’s candle/water/jar experiment• Since the flame ‘used up’ something in the air,

obviously air could be broken down further, so was not a building block of matter!

• Credited with disproving Aristotle and developing a new way of explaining matter

Dalton’s Atomic Theory

1. All matter is made of small particles called atoms.2. Atoms cannot be created, destroyed, or divided

into smaller particles.3. All atoms of the same element are identical in

mass and size, but they are different in mass and size from the atoms of other elements.

4. Compounds are created when atoms of different elements link together in definite proportions.

• Dalton’s theory shows why the alchemists were never successful.

• Metals are made of specific atoms, and atoms cannot be turned into other atoms!!

J.J. Thomson (1856-1940)

• Studied electric currents in gas discharge tubes

• Discovered (in 1897) that currents are streams of negatively charged particles he called electrons.

• He thought that atoms must be composed of smaller particles – a mind boggling idea at the time!

•British physicist

Thomson’s “Raisin Bun” Model of the Atom

•Atom is a positively charged ball studded with negatively charged electrons

• Thomson’s raisin bun model didn’t last long, as it was quickly followed by a new model proposed by a scientist from New Zealand…

Ernest Rutherford

Ernest Rutherford (1871 – 1937)

•Expected alpha particles to pass straight through

•But every once in a while, a particle was reflected back!

• Rutherford had discovered the nucleus!– The tiny, dense, positively charged centre of the

atom.• 10 years later he determined that the nucleus

was actually made up of 2 types of particles:– A proton – positively charged particle– A neutron – particle with no charge

Niels Bohr (1885 – 1962)

• A Danish physicist who worked for Rutherford

• He studied the parts of the atom surrounding the nucleus

•Bohr was particularly interested in the electrons that existed in this space

• He studied light given off by gases that glowed when an electric current was passed through them

• He discovered that electron surround the nucleus in specific energy ‘levels’ or ‘shells’

Bohr’s Model: Electrons have different amounts of energy

•Basically, electrons orbit the nucleus on specific paths•If energy is supplied, they can jump to higher energy level paths•When the energy supply stops, the electrons ‘fall’ back to their original path

Inside the Atom…

• An atom is the smallest particle of an element that retains the properties of the element

• Atoms are made up of 3 types of subatomic particles:– Proton (positively charged, found in the nucleus)– Neutron (no charge, found in the nucleus)– Electron (negatively charged, found outside of the nucleus)

The Subatomic Particles

•The nucleus contains protons and neutrons• It is extremely tiny but incredibly dense• Takes up 0.01% of the space in an atom

but accounts for 99% of the mass!•Electrons have almost no mass but take up a huge amount of space (99.99% of the atom)