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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
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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 + copper plates with saline soaked cardboard in between – the first stored electrical energy • 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 Beam of electrons Screen coated with compounds that glow when electrons strike
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 nucleusis 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
