Atomic Theory

Past and Present: pieces of a puzzle

The First Atomic Hypothesis

• Democritus (460 – 370 BC): – Greek philosopher

– Speculated that matter is composed of atoms which move through empty space

– Atoms are solid, homogeneous, indestructible and indivisible

– Different types of atoms have different sizes and shapes.

– “Atomos” = indivisible

The Wrong Direction

• Aristotle (384 – 322 BC):

– Aristotle rejected Democritus’ ideas and said empty space could not exist. Instead, he believed that matter is only made of earth, fire, air and water.

– Very influential philosopher of the time whose ideas were accepted.

– His denial of the existence of atoms went unchallenged for nearly 2000 years!!

The Early Atomic Hypotheses

• Boyle (1627-1691): – In 1650, without proof, Boyle

stated that matter was composed of atoms. He discovered a gas law.

• Newton (1642-1727): – In 1670, without proof,

stated that matter was composed of atoms. He discovered calculus and g____.

Law of Conservation of Mass

• Lavoisier (1760): – Founded modern chemistry – Discovered 33 elements and

organized them into 4 categories (Gases, Metals, Nonmetals, Earths-chalk,clay,magnesia)

– Discovered combustion – Discovered the Law of

Conservation of Mass • Matter cannot be created nor

destroyed. • Mass of reactants equals the mass

of products.

Law of Conservation of Mass Mass is conserved during chemical reactions!

Law of Conservation of Mass

• In a chemical reaction atoms separate, combine or rearrange, but, atoms are not created, destroyed or divided in the process.

• The number of atoms of each type is the same before or after the reaction. (see previous slide)

• The mass of the reactants is the same as the mass of the products.

John Dalton’s Atomic Theory

• Dalton (1766-1844): • John Dalton, a practicing Quaker

left school at age 11 and returned at age 12 as a teacher in England

• In 1803 after studying the works of Democritus and Boyle and others, and performing numerous experiments, he proposed his atomic theory

• It was the first atomic theory based on scientific evidence.

John Dalton’s Atomic Theory

1. Matter is composed of extremely small particles called atoms

2. Atoms are indivisible and indestructible

3. Atoms of a given element are identical in size, mass and chemical properties

4. Atoms of a specific element are different from those of another element

5. Different atoms combine in simple whole-number ratios to form compounds

6. In a chemical reaction atoms are separated, combined or rearranged

Law of Multiple Proportions: when 2 elements combine to

form different compounds in small whole number ratios

(predicted by Dalton’s theory)

(NH4)2Cr2O7 (orange) Cr2O3 (green) CrO3 (red)

2.167 g : 1.000 g 1.083 g : 1.000 g 2.167 g : 1.083 g 2 : 1

Dmitri Mendeleev

• 1834-1907 He organized the 56 elements that were known in 1869 according to similarities and masses and noticed the periodicity (repeating pattern) in their properties. He was puzzled by inconsistencies like Tellurium and Iodine (he thought Iodine should be heavier) and attributed it to inaccuracies in atomic mass measurements of the time. Because of this periodicity, he could predict where unknown elements (not yet discovered) would appear on the periodic table.

• Scientists still thought the atom was the smallest particle of matter for several decades until……….

Discovery of Electrons

• J.J. Thomson (1856-1940): – In 1897, Thomson (English physicist)

conducted the Cathode Ray Tube Experiment (CRT). (led to the invention of TV).

• Thomson discovered that cathode rays were streams of negatively charged particles.

• He later determined the charge to mass ratio of an electron and found the mass to be much smaller than a H atom (lightest known atom).

• Thomson realizes Dalton is incorrect and there are particles smaller than an atom.

• He proposes the Plum Pudding Model of the atom (a uniform, positively charged sphere with e-)

Cathode Ray Tube

Henry Gwyn Jeffreys Moseley 1887-1915

• British scientist

• discovered that atoms contain a unique positive charge in the nucleus which exactly identifies the element

• He named this positive charge the atomic #

• Henry Moseley

Electron’s Charge & Mass is Determined

Millikan (American physicist:1868-1953)

*In the early 1910s, using the Oil Drop Apparatus, Millikan discovered the charge of an electron.

“so good was Millikan’s experimental setup and technique that the charge he measured almost 100 years ago is within 1% of the currently accepted value” (Glencoe)

* Knowing the electron’s charge and using Thompsons’ charge/mass ratio, Millikan calculated the electron’s mass at 9.1x10^-28g.

Millikan Oil Drop Experiment

The Gold Foil Experiment The nucleus is discovered.

• The question “what accounts for the mass of an atom if the electron mass is so small?” was still unanswered.

• Rutherford (1871-1937): – In 1911, Rutherford shot alpha particles

from Thorium at a thin layer of gold foil to test Thompson’s model of the atom. • Most went straight through • Some deflected a little • A few were deflected a lot (cannon ball off TP)

– Conclusion: • Atoms are mostly empty space • Atoms have a very small dense nucleus that is

positively charged. (<--This dot (if a nucleus) would weigh as much as 70 automobiles!)

The Gold Foil Experiment

Neutron Discovered

• Chadwick ( a coworker of Rutherford) – In 1932 he discovered the nucleus contains

another particle called the neutron by bombarding Li, Be, and B with alpha particles.

42α ---------------> Li Target ------------> 10n

– The neutron has no charge and has a mass

1840 times heavier than that of an electron • Neutron Mass = 1.675 x10-24 g • Neutron Charge = 0

+

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Niels Bohr’s Atom • Danish physicist working in Rutherford’s laboratory in 1913 proposed a quantum model of the H atom. • Electrons orbit the nucleus in orbits, like a solar system; multiple energy levels

Planetary Model

Quantum Mechanical Model – like a shot gun blast

• Erwin Schrödinger 1887-1961 • Born in Vienna, Austria • University of Berlin professor • Nobel Prize 1933 • Based on waves of light and probability.

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Modern Atomic Structure

• Electron Cloud:

– Electrons located outside the nucleus

– Volume of atom is mostly empty space • Most atoms’ radii = 1 x10-10 m = 1Ǻ

• Nucleus:

– Protons and neutrons are located in the nucleus.

– Most of the mass of the atom is in the nucleus. • Nucleus size = 1 x10-15 m

Atomic Models

Atomic Size – So how big is it?

• A cubic cm of air (size of a sugar cube) has 4.5 x 10^19 atoms! • 45,000,000,000,000,000,000 atoms

• How many atoms would it

take to fill a universe?

http://www.youtube.com/watch?v=0fKBhvDjuy0&feature=player_embedded

IBM’s Scanning Tunneling Electron Microscope

Quarks, Leptons, Hadrons………..

• No model can accurately represent every known aspect of the atom!

• Youtube video: Exploring the Atomic Composition of Steel

http://www.youtube.com/watch?v=fz5gzQJq-0s

• Ted Talk (size of an atom)

http://ed.ted.com/lessons/just-how-small-is-an-atom