HISTORY of the ATOM

• Early Greeks, including Aristotle, believe that all matter composed of 4 elements: earth, water, air and fire.

• These elements were eternal and unchanging (like a stucco wall).

• Different substances were made up of the elements in different proportions,

• But Not All subscribed to this idea

• Democritus – reasoned that you could divided matter into smaller and smaller portions until to could not further subdivide it

• These particles were both invisible and structureless (could not be further subdivided)

• Called these particles ATOMS• Matter was made up of these

discrete units or atoms (like a brick wall versus the stucco wall)

•Failed to win the debate because–Aristotle led the opposition–No experimental data to support the idea

• The Scientific Revolution: to name a few

• Copernicus – Astronomer; heliocentric theory of the Solar System – 1574

• Galileo – Astronomer and Physicist developed the telescope and affects of gravity – 1632

• Johannes Kepler- Astronomer with his three laws of planetary motion – 1634

• Rene Descartes- Mathematician – 1635 Cartesian physics

• Isaac Newton – Philosopher & Mathematician – Laws of Motion – 1687

•Chemistry awakens in the Eighteenth Century

•Antoine Lavoisier – French chemist: “Father of Modern Chemistry” – circa 1780

• Conducted Experiments with precise measurements & records

• Weight of Products = Weight of Reactants (starting chemicals)

• Law of Conservation of Matter

• Joseph Proust – French, who around 1799 showed that the proportion by mass of the elements in a given compound is always the same

• 10 grams of cupric sulfide is composed of 6.6 g of copper & 3.4 g of sulfur

• 100 grams of cupric sulfide is composed of 66 g of Cu and 34 g of S

• Relative percentages is 66% Cu 34% S

• Called Law of Definite Proportion

•John Dalton- •Developed the Atomic Theory in 1803 by proposing:

•All elements are composed of atoms which are indivisible and indestructible particles

•All atoms of the same element are exactly alike; in particular they all have the same mass

•Atoms of different elements are different; in particular they have different masses

•Compounds are formed by the joining of atoms of two or more elements; the number of respective atoms is a ratio of small whole numbers

•Atoms of the same elements can combine in two or more different ratios to form two or more different compounds

•Are all his tenants correct?•We’ll save that for later in this unit

The Dalton Atom – Billiard Ball Model

• Atoms were solid neutral spheres• Each reactant was an different

type of atom that are rearranged into the new products by a definite ratio by mass

•Dalton’s Theory supported the experimental data of his time and the following theories:

•Law of Conservation of Matter•Law of Definite Proportions•Law of Multiple Proportions

Law of Multiple Proportions

•Atoms of different elements always combine in whole number ratios

•Sulfur & Oxygen combine•2.0g S reacts with 2.0g O•2.0g S reacts with 3.0g O

• Since the mass of sulfur is constant we know their ratio is 1:1

• Now we reduce the oxygen masses to their lowest ratio by dividing through by 2.0 g (the smallest of the two values)

• 2/2 = 1 and 3/2 = 1.5• But we do not have a whole number

ratio• Multiply both by 2 and we have a 2:3

ratio• 1 atom S:2 atom O; 1 atom S:3 atom O

• Let’s try another example: Carbon and Oxygen

• Carbon monoxide - there is 1.33 g O per 1.00 g of C

• Carbon dioxide – there is 2.66 g O per 1.00 g of C

• Ratio is 1:2; said another way, there is exactly two times the oxygen per gram in CO2 as in CO

• Today we think atoms instead of grams

Modern Atomic Theory

• In the 1870’s William Crookes conducted studies using Discharge Tubes

• He discovered that a magnet could deflect the path of the cathode ray

• Concluded that the cathode rays carry an electrical charge

• JJ Thomson (1897) expanded this work to include an electrical field with similar results

• Regardless of the metal used in the cathode tube, result always the same

• Cathode rays deflected towards the positive electrode

• Concluded particles have a negative charge; called them corpuscles

• Known today as electrons (1891)

•PLUM PUDDING MODEL•Thomson proposed a model,

sometimes called the "plum pudding" or "raisin cake" model, in which thousands of tiny, negatively charged corpuscles (given the name electrons by Stoney in 1891) swarm inside a sphere of positively charged matter.

• Lord Rutherford performed his gold foil experiment in 1909

• Bombarded the gold foil with alpha particles (rays from radioactive decay)

•Nearly all the particles passed straight through hitting the zinc sulfide backstop as if there was no gold foil present

•Concluded that most of the atom is empty space

•He also noticed that a few were deflected

•The amount of deflection varied A few particles were deflected at large angles of 90o or more

•A small portion of the atom was solid which had to be positively charge since electrons are negatively charged and an atom’s overall neutral charge

Rutherford AtomEmpty Space or Nuclear

Model

•Conclusion- each atom contains a small dense positively charged central portion or nucleus with electrons in mostly empty space around it