~450BC

Who was the first to propose that matter was made of tiny indivisible particles ?

Democritus’s atom matter was made up of tiny invisible indivisible particles in constant motion surrounded by an empty space (void) solid (no void inside) eternal (indestructible) Differ from one another in shape, size, arrangement

Aristotle Disapproved of Democritus’ idea of atoms Believed that all matter was comprised of Earth, wind, fire and water

Dalton’s Atom The Atom – Circa 1800 indivisible

1 st Modern Atomic Theory 1. Matter consists of tiny particles called atoms. 2. Atoms of one element are alike and are different from atoms of another element. 3. Atoms are indestructible and unchangeable (law conservation of matter). Lavoisier 4. Atoms combine in simple whole number ratios to form compounds (law of definite proportion).Lavoisier

Lavoisier (early 1700’s) Father of modern chemistry Law of conservation of matter – matter not created not destroyed

Faraday 1839 Electrolysis experiments  Structure of atoms related to electricity  Charge is somehow involved in binding elements together to form compounds

Crooke’s tube/Cathode Ray tube 2 electrodes  Cathode: negatively charged electrode  Anode: positively charged electrode Rays from cathode to anode

Deflection of a charged particle 1. mass 2. velocity 3.charge of the particle 4.Strength of magnet 5.amount of charge on the plates

JJ Thomson Experiments with cathode ray tube: Conclusion:  Cathode ray was a stream of negatively charged particles called electrons  Later protons were found going in an opposite direction

Plum Pudding Model Sphere of positive charge with electrons embedded in it.

Millikan’s Oil Drop Experiment 1909 Determined charge of electron using Thomson’s charge to mass ratio Charge = 1.6 X C

cathode ray tube gave off X-rays Roentgen1870’s

Becquerel Discovers radioactivity(spontane ous emission of particles or rays) when photographic film becomes exposed in the presence of a sample of uranium Refers to Pierre and Marie Curie

Marie and Pierre Curie (1900’s) Discovered Radium (Ra) and Polonium (Po). Marie won 2 Nobel prizes Marie died of cancer and Pierre was killed by a horse drawn milk truck Daughter Irene continued their work

Rutherford 1909 Lead Box Experiment 3 types of radiation from radioactive substances: α, β, γ Gold foil experiment  Positively charged alpha particles (He nucleus) were shot at a thin piece of gold foil  Expected α particles to go right through

Gold Foil Experiment

Rutherford’s Conclusion -Atom mostly empty space -Positive charge and mass concentrated in tiny dense -nucleus

The neutron Chadwick – 1932 Neutral radiation by nuclear bombardment nuclear reaction No charge but same mass as proton

Neutral Carbon-12 atom

Subatomic particles Proton  Inside nucleus  +1 1 amu Neutron  Inside nucleus  0  1amu Electron  Outside nucleus  -1  O amu(1/2000amu)

Don’t get confused Atomic number number of protons (on periodic table- top) Mass Number number of protons plus neutrons (not on periodic table) Atomic Mass the weighted average of the mass numbers for all the isotopes of the atom (on periodic table – bottom)

Symbols Name-mass number charge X Mass number Atomic number

Atoms Protons=electrons Atoms have no charge

ISOTOPES Atoms with the same number of protons but a different number of neutrons.

Examples Hydrogen-3 H Hydrogen-2 H Hydrogen-1 H

Ions Atoms that gain or lose electrons and become charged Charge= protons- electrons

Practice with ions 1.Write the symbol for calcium How many protons, electron, neutrons 2.Write the symbol for oxygen How many protons, electron, neutrons?

Atomic Mass Make table: MassXAbundance= Atomic Mass= __________

Atomic Mass Make table: MassXAbundance= ________ Cu %= Cu %=+_________ Atomic Mass= __________

Atomic mass Element X has 2 isotopes X-10 and X-11. If the atomic mass is 10.8, what is the abundance of each?

Pennies Pre 1982 – mostly copper penny cost more than a penny to make Except in 1943 pennies were zinc coated steel – “steel pennies” Post 1982 – 97% zinc coated with copper since zinc is less expensive than copper

What image did the early Greeks have of atoms?

Further developments Moseley: found elements contain a unique positive charge  Number of protons in nucleus is what determines the identity of the element  Periodic table arranged according to atomic number Chadwick (1930’s)  Discovers neutron

Practice 1. Write the symbol for sodium-24 How many protons, electron and neutrons? 2.Write the symbol for Tin-123 How many protons, electron and neutrons?

Proust 1799 Law of constant composition (law of definite proportions) Observed that a compound always contained the same elements in the same proportion by mass example: water is always 88.9% oxygen and 11.1% hydrogen by mass

Calculate the atomic masses Copper % Copper % Uranium % Uranium % Uranium %