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~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 ofEarth, wind, fire and water
Dalton’s Atom
The Atom – Circa 1800
indivisible
1st 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 10-
19C
cathode ray tube gave offX-rays
Roentgen 1870’s
Becquerel Discovers
radioactivity(spontaneous 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 Experiment3 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 Numbernumber of protons plus neutrons(not on periodic table)
Atomic Massthe weighted average of the massnumbers for all the isotopes of the atom(on periodic table – bottom)
Symbols
Name-mass number
charge
XMass 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
3
1
1
2
1
1
1
Ions
Atoms that gain or lose electrons and become charged
Charge= protons- electrons
Practice with ions
1. Write the symbol for calcium+2-41
How many protons, electron, neutrons
2. Write the symbol for oxygen-2-18.
How many protons, electron, neutrons?
Atomic Mass
Make table:
MassX Abundance =
Atomic Mass= __________
Atomic Mass
Make table:
MassX Abundance = ________
Cu-63 69.1% =
Cu-65 30.9% = +_________
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-63 69.1% Copper-65 30.9%
• Uranium-234 .001% Uranium-235 .71% Uranium-238 99.23%