A History of Atomic Theory Atomic Models

What is a model ? MODEL: detailed, 3-D representation of an object, (typically on smaller scale than original) model airplane model car clothing model

Let’s Take a Trip Through Time!

It Started with the Greeks DEMOCRITUS • Lived in Greece 2500 years ago (460-370 BC) • “Father of modern science”

Demacritus said: “All matter is made of atoms that are tiny, indestructible and indivisible.”

Democritus Where did Democritus get his ideas for Greek banknote with Democritus and drawing of atom Where did Democritus get his ideas for small, indestructible, indivisible atoms?

Atoms are small… new rings old ring EVIDENCE: Old gold rings wear away slowly, getting thinner and thinner, but you never see gold atoms on your finger, so gold atoms must be very small!

Atoms are indestructible… EVIDENCE: Earth has been around long time Mountains wash away but rocks continue to exist New plants grow where old plants die Atoms are indivisible… EVIDENCE: None This was HYPOTHESIS made by Democritus to explain nature as he saw it.

What did Democritus think atoms looked like? Appearance of atoms assumed based on behavior: Liquids pour Solids are hard and rigid

Liquids pour. Why? Liquids require a container If liquid atoms were like little balls they would roll out when you tip the container Greek idea of liquid atom

Solids are rigid. Why? cocklebur plant cockleburs stick on clothing and each other little hooks on cockleburs Velcro® fastener Velcro hooks and loops Greek idea of solid atom

Aristotle had other ideas Greek banknote and coin picturing Aristotle ARISTOTLE: Famous Greek philosopher, born 384 BC. student of Plato (another famous philosopher) teacher of Alexander the Great (who later conquered the world)

Aristotle’s idea of matter Aristotle did not believe Democritus’s idea of atoms was correct Aristotle believed all matter made from four elements: Earth Air Fire Water

Aristotle’s idea of matter Aristotle was more famous than Democritus, so people believed him, even though he was wrong! Democritus’s idea of atomos (atoms) was lost for nearly 2000 years until John Dalton brought it back in 1803

Atom idea lost for ~2000 years

John Dalton, New Atom,1803 Born in England, 1766 Studied chemistry, physics, and color blindness Brought back Democritus’s idea of an indivisible atom

Color blindness Can you see a number in this box? If not, you may be color blind. (More males are color blind than females)

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Dalton’s Theory of Atoms Five parts to Dalton’s modern atomic theory: # 1: Elements are made of extremely small particles called atoms

# 2: Atoms of given element are identical in size, mass, & other properties; Atoms of different elements are different in size, mass, 7 other properties (later found not to be exactly correct)

#3: Atoms cannot be subdivided, created, or destroyed (later proved wrong)

So Dalton’s atoms are considered to be like billiard balls

#4: Atoms of different elements combine in simple whole-number ratios to form chemical compounds (H2O 2:1 ratio H:O)

#5: In chemical reactions, atoms are combined, separated and/or rearranged

Ex. of a chemical reaction: + +  + 2H2O + 2Na  2NaOH + H2

Dalton’s Chemical Symbols We use different symbols today

Matter is electrical! Michael Faraday (English chemist, born 1791) 1834: learned matter interacts with electricity Realized electricity had to be made up of particles that could be counted, but didn’t know what they were (electrons)

J.J. Thomson English physicist (1856-1940) won Nobel Prize in 1906 1897: discovered ELECTRON - studied cathode rays using Crooke’s tube - showed atoms were divisible

Crooke’s tube | cross-shaped anode cathode | – cross-shaped shadow Cathode rays (stream of electrons) move from metal cathode (on left) to cross-shaped anode (on right) & cast cross-shaped shadow on glass (on right) So … Electrons are particles with negative charge

Cathode Ray Tube Experiment thought cathode rays were streams of particles smaller than atoms

electric & magnetic fields deflect beam of charged particles

What do we mean by charge? ELECTRICAL CHARGE: property of matter: has either more or fewer electrons than protons ELECTRONS are negatively charged (-1) PROTONS are positively charged (+1) MATTER IS NEUTRAL (no charge): (+) charges equal (-) charges so cancel out each other batteries have (+) and (-) ends

More About Charge… Opposite charges attract, Alike charges repel

JJ Thomson’s discovery of (-) charged electrons proved that atoms were divisible!

Thomson’s “Plum pudding” atom – electron ENGLISH PLUM PUDDING raisin

RULE: Matter is NEUTRAL (no charge) so… for every one electron [(-) charge] there must also be one (+) charge

Dalton’s Billard Ball Model we move from Dalton’s Billard Ball Model to Thomson’s “Plum pudding” model

Thomson’s Plum Pudding Model: (+) charge evenly spread out while (-) charge is in bits – like raisins in plum pudding (or chocolate chips in cookies)

Ernest Rutherford 1871-1937 1920: discovered proton 1908: won Nobel Prize (Chemistry) Discovered that most of mass of atom is in nucleus

Rutherford’s Experiment – 1911 Rutherford’s exp’t: animation

Rutherford tested plum pudding model of atom: sent tiny radioactive alpha (a) particles, like bullets, towards thin sheet of gold foil

Rutherford gold foil experiment expected a particles to pass straight through, like this… most a particles went straight through or didn’t bump into anything showing most of atom is empty space

even though most a particles went straight through, a few were deflected, meaning a particles must have hit something: really heavy (+) charged

gold foil experiment scattering results showed Plum pudding model of atom was INCORRECT, so Rutherford devised new model that fit his results… proposed Nuclear Model

The NUCLEAR atom model Rutherford concluded: all atoms have dense, positive (+) center (contains most of mass of atom) atom is mostly empty space (except very tiny (–) electrons) electrons (–) / The NUCLEAR atom model nucleus / [protons (+)]

Rutherford model did not speculate how electrons arranged around center (name nucleus came later)

Problem with Rutherford’s Model To prevent (-) electrons from being attracted to (+) nucleus, electrons must orbit nucleus like Earth orbits sun But… e- need energy to orbit nucleus; Rutherford never atom would “die” yet Real atoms do not die

So how big is the nucleus compared to the entire atom? If atom as big as football stadium, nucleus smaller than flea on 50-yard line! If atom big as period at end of sentence in textbook, it would have mass of 70 cars!

So how big is an atom? Most atoms are 1-2 angstroms across 1 Å = 1 X 10-10 m 6 Billion Cu atoms in a line = less than 1 meter! Can we “see” an atom?

James Chadwick 1891-1974 worked with Rutherford 1932: discovered neutron 1935: Nobel Prize (Physics)

Niels Bohr 1885 - 1962 Created quantized atom model, 1915 Worked on Manhattan Project Won Nobel Prize in 1922 Bohr’s model: e- only move from one energy level to another in atom; emit energy when jump from higher to lower levels: absorb energy when jump from lower to higher levels.

Bohr addressed issue of atom’s electrons nucleus surrounded by e- orbiting at different energy levels e- have definite orbits

Quantum Mechanical Theory Electron in a Hydrogen atom

Bohr’s new atomic model had quantized energy levels e- only move by jumping levels (numbered n = 1, n = 2, n = 3, etc.)

electrons: absorb energy when they jump away from nucleus emit energy when they jump towards nucleus

Bohr’s Planetary Model electrons travel only in specific orbits each orbit has definite energy inner most orbit (n=1): least energy outer most orbit (n=7): most energy atoms emit radiation when e- jumps from outer orbit to inner orbit outermost orbits determine atom’s chemical properties

Erwin Schrödinger Wave Model Austrian scientist (1887-1961) 1933: won Nobel Prize (Physics calculated wave model of hydrogen atom (1926) atomic model called: cloud model quantum mechanical model modern model atom model

If we could see an electron it might look like this “cloud” Schrödinger’s wave model of atom: e- behaves as energy wave AND as matter particle (light also behaves as particle and wave) Einstein had predicted that energy and matter were related in his equation E = mc2 If we could see an electron it might look like this “cloud”

Modern/Wave/Cloud Model electron’s energy is quantized (has only certain values) electrons in probability zones called “orbitals”, not orbits - location cannot be pinpointed electrons are particles & waves at same time electrons move around nucleus at speed of light

Orbitals

A Missing Particle – The Neutron 1932: James Chadwick discovered last major piece of atom discovered neutron (“neutral”, no charge) 1935: won Nobel Prize (Physics) neutron weighs about as much as proton while electron is much smaller (1891-1974)

The Complete Modern Atom An atom is: mostly empty space nucleus: most of atom’s mass nucleus contains protons & neutrons electrons in energy levels around nucleus electrons jump between levels, emitting & absorbing energy as jump

The development of atomic theory represents the work of many scientists over many years

Next Atomic Theory ? Which one of you will perhaps develop a better theory for the atomic model and win a Nobel Prize in the future?