A Brief History of the Science of Chemistry,

and the Discovery of the Atom and its Subatomic Particles

A. Greeks – first to try to explain chemical reactions

1. By 400 B.C. there were four fundamental substances: earth, air, fire, and water. All matter was composed of various mixtures of these four.

2. Democritus – contemporary of Socrates – influenced by his teacher, Leucippus, who introduced him to the idea that matter was not continuous – best known for his atomic theory: the world is made of void and tiny particles called atoms

3. Plato and Aristotle were philosophers of the same general period and more influential than Democritus and rejected his idea of matter being composed of atoms : matter was continuous and made of only one thing called hyle

B. The next 2000 years were dominated by Alchemists who were trying to find a way to turn cheap metal into gold – responsible for the discovery of many metals

C. Boyle, Newton, Priestly

D. Lavoisier – did combustion experiments carefully measuring the reactants and products and found that the total starting mass was equal to the total ending mass: the law of conservtion of mass – wrote the first modern chemistry textbook in 1789

After this time, chemistry became based on measurements obtained in experiments.

E. Proust – experiments show that given compounds always have the same element mass ratio: the law of constant composition

F. John Dalton – alchemist – English school teacher – hampered by being poor, not articulate, a poor illustrator, and not a skilled experimentalist (perhaps because he was color-blind) – gave first detailed description of color blindness – he revolutionized chemistry – only recreation was lawn bowling

1. in 1808 formulated the first modern atomic theory

a. all matter composed of small particles called atoms

b. atoms are the smallest part of matter and can't be created or destroyed or broken apart (subdivided)

c. atoms of an element are identical; atoms of different elements are different

d. atoms unite in definite ratios to form compounds

e. atoms combine, separate or are rearranged in chemical reactions

2. the law of multiple proportions – not based on experiments – combining masses of one element with another are in the ratio of small whole numbers

3. first to try to make a table of the relative masses of the elements known at the time – not always correct: noted water had a 1:8 mass ratio so H=1 and O=8 and the formula for water must be OH

G. Gay-Lussac , Amadeo Avogadro

H. Berzelius – made measurements of relative masses, with poor lab setup measured the mass of the then known 50 elements fairly accurately – developed the bases for the modern symbol system for elements and how to write formulas

I. Ben Franklin – positive and negative charge

J. Henri Becquerel – some compounds of uranium produced high energy radiation and were detected on photographic plate exposed to it - radioactivity

K. Marie and Pierre Curie

L. Crooke – Crookes tube

M. Joseph John Thomson – English physicist at Cambridge – late 1890's and early 1900's

1. conductivity of gases at low pressures using cathode ray tube and found that cathode rays were bent by both magnetic and electric fields – produced by different kinds of electrodes and the relative ratio was always the same no matter what gas was used => must be a basic particle – negatively charged – called cathode rays electrons

2. in 1897 measured mass to charge ratio of electron

3. modified cathode ray tube and found rays going in opposite direction to electrons, called canal rays

4. canal rays were particles that were equal but opposite in charge to electrons – hydrogen gas used and the mass of these particles was determined to be 1836 times greater than the mass of an electron

5. since an atom is neutral there must be equal positive and negative charges => a neutral atom contains equal numbers of protons and electrons

6. proposed that the atom was a diffuse cloud of positive charges with negative electrons scattered throughout it, called the Plum Pudding

8. in 1912 he noted that there were two different neon atoms that behaved chemically alike but had different masses

N. Aston – student of Thomson, worked with neon atoms of different masses, called isotopes

O. Robert Milikan – American

1. oil drop experiment – oil drops were dropped between two plates in an electric field and the voltage on the plates was varied => was able to suspend the oil drop between the plates – by knowing the mass of the drop and the voltage where the voltage equalled the force of gravity (the force causing the oil drop to fall) so the drop doesn't move, he was able to calculate the electric charge

2. the charge was always some integer multiple of some small number => must be the electric charge on a single electron

3. combined with Thomson's work, the mass of an electron and a proton could be calculated

P. Ernst Rutherford – New Zealand physicist– student of Thomson at Cambridge

1. worked with Becquerel's radioactivity, named positive rays alpha rays (a), negative rays beta rays (b) and neutral rays gamma rays (g)

2. named canal ray particles protons

3. Noble Prize in chemistry in 1908 for his investigation into the structure of the atom (resented award in chemistry, he was a physicist and looked down his nose at chemists)

4. 1911-1912 tried to prove Thomson's Plum Pudding model of the atom with his gold foil experiment

a. shot “bullets” made from helium atom stripped of electrons (a particles) at thin gold foil, expected most to go through and maybe some deflected

b. results startled him – most went through and some were deflected at acute angles but some were deflected straight back 180º at the source

c. this could only be caused by a small, dense, positive center within the gold foil

d. using different foils gave the same results

e. center has 99.9% of the mass of the atom

f. center has diameter only 0.01% of the atom

6. nuclear atom – the atom is a vast empty space with a positively charged, dense, central core surrounded by a large negatively charged cloud, the core charge equals the cloud charge

7. in 1920 he predicted the existence of a third basic particle

Q. Moseley – 1913

1. used X-rays and found that the X-ray wavelength varied depending on the element used and correlated with the number of positive charges – the number of positive charges is equal to the number of protons; atomic number = number of protons

R. Irene and Fredric Joilot-Curie

S. Chadwick – 1932 found a high energy, no charge particle with a mass about equal to the mass of a proton (1839 times electron mass), called the neutron

T. Bohr – founder of modern physics, electrons must be in orbits of a specific energy – they can absorb energy to move to higher level creating an excited state and they can give off energy as light and move back to the original ground state; Planetary Model of the atom - electrons orbit the nucleus (same as Rutherfords) in specific paths (determined by their energy) like the planets orbit the sun

U. Modern Model of the Atom - electrons are not confined to a specific orbit but can be found anywhere in the electron cloud (same nucleus as Rutherfords)

subatomic particle	location	relative mass	relative charge	made of:
proton	nucleus	1 amu	1	3 quarks, UUD
neutron	nucleus	1 amu	0	3 quarks, DDU
electron	electron cloud	0 amu	-1	–––––

Nuclear Symbols:

X = element symbol

a = atomic number = #protons

b = atomic mass = #protons + # neutrons

c = charge

zero: #protons = #electrons

positive ion: #protons > #electrons, electrons are lost, called cation

negative ion: #protons < #electrons, electrons are gained, called anion

atomic number = #protons = a

atomic mass = #protons + # neutrons = b

#protons = atomic number

#neutrons = b - a

#electrons = a – c

Hyphen notation: mass number written after the name of the element; e.g. carbon-14

Average Atomic Mass: the weighted average of the atomic masses of the naturally occurring isotopes of an element

%₁*mass₁ + %₂*mass₂ + %₃*mass₃ + ..... = average atomic mass