A little bit of Chemical History

Greek Philosophers• suggested matter was made of 4

elements - Earth, Air, Fire, Water. Eg: bone was made of ¼ earth, ¼ water and ½ fire

• Aristotle added a fifth element “ether” for the gods – found only in the heavens

• He believed chemical change is a change in form eg: iron and rust were different forms of the same stuff.

Atoms• Proposed by Democritus who

imagined cutting gold into ever smaller pieces until you could cut a particle no more. He called this “ not cuttable” or “ a tome”

• Substances had different properties due to atoms being different – water – smooth atoms, fire- spiky, earth – rough.

• Ahead of time and not listened to until years later

Alchemists• Arabs found Aristotle’s writing and

became excited about being able to turn matter from one form to another – named alchemy

• Alchemists devoted lives to searching for the right purification techniques and developed most of our lab glassware and techniques .

• Also extracted compounds with medicinal properties – we buy these today from ‘chemists.

Elements• Boyle (1600’s) believed Democritus’

ideas • Experimented with gold and silver and

said that whatever he did he couldn’t change them into earth air or water.

• Proposed “ element is a substance that can’t be broken down into anything simpler”

• British chemists discovered carbon dioxide, hydrogen and oxygen and demonstrated that water can be made by burning two parts H2 in one part O2

• 1700’s Antoine Lavoisier investigated oxygen and recognised as an element.

• 1797 Joseph Proust invented the law of definite proportions - “a given compound must always contain the same proportions of elements”

• Proust also distinguished compounds of elements and mixtures

Atoms and Molecules• John Dalton (1766 – 1844) – elements made of

small indivisible “atoms”• Atoms of an element all the same and different

to atoms of other elements• Atoms cannot be changed into other types of

atoms• Compounds made when atoms combine in

whole number ratios• Main difference between elements is mass of

atoms

Electric particles• Batteries constructed in 1800’s from zinc + copperplates with saline soakedcardboard in between – the first stored electricalenergy

• Faraday suggested electricity was linked to the elements produced in electrolysis and that electricity was “particulate” in nature

• Physicists discovered they could produce a stream of negatively charged particles called cathode rays that would travel between negative and positive electrodes in a vacuum. These particles always had the same properties

Screen coated with compoundsthat glow when electrons strike

Beam of electrons

Plum pudding model• Thomson investigated the charge/mass ratio on cathode

ray particles and found that the charge was the same size as hydrogen but the mass was 1836 times smaller

• Named the particle electron• Proposed a model like a plum pudding with negative

electrons scattered throughout positive stuff

Negative electrons

Positive atom stuff

New Zealand’s moment of chemistry fame

• Ernest Rutherford, around 1895, performed an experiment with gold foil where he bombarded it with tiny positive “alpha” particles

• Most particles went through the gold foil undisturbed most of atom is empty space

• A small number came straight back the way they went in centre of atom very dense and small

• Some deflected at large angles strong positive charge in small area instead of weak positive stuff

Alpha beam

Lead block

Gold foil

Rutherford’s model of an atom consisted of a highly charged positive nucleus containing nearly all of the mass of the atom in a small space with negative electrons in a large space around the outside. The nucleus is only 1/10000 of the size of the atom!

• Meanwhile the Periodic table was being organised by Mendeleev who put all information about elements on cards and tried to make an order. He started putting them in order of atomic mass but also lined up elements with similar properties.

• He discovered gaps where he thought elements should be and predicted they would be discovered - he was right!

Other findings• Henry Mosely – discovered the

positive charge on an atom is linked to atomic number.

• Neutrons discovered in 1920 – 1930• 1935 – Thomson discovered isotopes

when he discovered two kinds of neon atoms with different masses.

• Once the neutron was discovered isotopes were explained by having different numbers of neutrons for the same number of protons

• YouTube - The Atom Song