~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%