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 car clothing model model airplane

Let’s Take a Trip Through Time!

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

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

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

Atoms are small… new ringsold 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… Atoms are indivisible… EVIDENCE: Earth has been around long time. Even though mountains get washed away, rocks continue to exist. New plants grow where old plants die. 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 ® fastenerVelcro hooks and loopsGreek idea of solid atom

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

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 gray box? If not, you may be color blind. (More males are color blind than females)

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, and other properties; atoms of different elements are different in size, mass, and other properties (found out not exactly correct)

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

So Dalton’s atoms are kind of like billiard balls Atoms combine in whole-number ratios

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

#5: In chemical reactions, atoms are combined, separated or rearranged 2H 2 O + 2Na  2NaOH + H 2 2H 2 O + 2Na  2NaOH + H 2

In a reaction, atoms are rearranged +  + +

Dalton’s Chemical Symbols We use different symbols today

Matter is electrical! Michael Faraday (English chemist, born 1791) In 1834 learned that matter would interact with electricity Realized that electricity had to be made up of particles that could be counted, but did not know what they were (were electrons)

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

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

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

electric or electric or magnetic fields magnetic fields deflect beam of deflect beam of charged particles charged particles

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

More About Charge… Opposite charges attract, alike charges repel (push away.

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

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

RULE: Matter is NEUTRAL so… for every one electron [(-) charge] there must also be one positive charge to equal the zero charge (“neutral”) of matter

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

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

Ernest Rutherford Discovered the proton in 1920 Won Nobel Prize in chemistry in 1908 Discovered that most of mass of atom is in nucleus

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

To test plum pudding model of atom, Rutherford sent tiny radioactive particles called  (alpha) particles, like bullets, towards thin sheet of gold foil.

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

But even though most  particles went straight through, a few were deflected, like this so the particles must have hit something: really heavy really heavy (+) charged (+) charged

The gold foil experiment scattering results meant the Plum pudding model of the atom was INCORRECT, so Rutherford had to think of new model that fit what he saw so… proposed Nuclear Model

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

Rutherford did not speculate how electrons arranged around nucleus Rutherford proposed Nuclear atom

Problem with Rutherford’s Model To prevent (-) electrons from being attracted to (+) nucleus, electrons have to orbit nucleus like Earth orbits sun But… electrons need energy to orbit nucleus, and there’s no source of energy for this so every Rutherford 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 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! 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 Most atoms are 1-2 angstroms across An angstrom = 1 X m An angstrom = 1 X m 6 Billion Cu atoms in a line = less than 1 meter! 6 Billion Cu atoms in a line = less than 1 meter! Can we “see” an atom? Can we “see” an atom?

James Chadwick Worked with Rutherford Worked with Rutherford Discovered Neutron 1932 Discovered Neutron 1932 Nobel Prize in Physics 1935 Nobel Prize in Physics 1935

Niels Bohr Created quantized atom model, 1915 Worked on Manhattan Project Won Nobel Prize in 1922 In Bohr’s model, electrons can only move between energy levels in the atom, emitting energy when they jump from higher to lower levels, absorbing energy when they jump from lower to higher levels.

Bohr addressed the issue of electrons in the atom Nucleus surrounded by electrons orbiting at different energy levels Nucleus surrounded by electrons orbiting at different energy levels Electrons have definite orbits Electrons have definite orbits

Quantum Mechanical Theory Electron in a Hydrogen atom

Bohr’s new atomic model had quantized energy levels, meaning the electrons could only move by jumping between levels (numbered n = 1, n = 2, n = 3, etc.)

electrons absorb energy when they jump out, and emit (send out) energy when they jump in

Bohr’s Planetary Model Electrons travel only in specific orbits Electrons travel only in specific orbits Each orbit has definite energy Each orbit has definite energy Inner orbit (n=1): least energy Inner orbit (n=1): least energy Outer orbit (n=7): most energy Outer orbit (n=7): most energy Atoms emit radiation when e - jumps from outer orbit to inner orbit Atoms emit radiation when e - jumps from outer orbit to inner orbit Outermost orbits determine atom’s chemical properties Outermost orbits determine atom’s chemical properties

Erwin Schrödinger Wave Model Austrian scientist ( ) won the Nobel Prize in physics in 1933 calculated wave model of hydrogen atom in 1926 also called cloud model, quantum mechanical model, modern model of the atom atom model we still use today

In Schrödinger’s wave model of atom, electron behaves as energy wave as well as a matter particle (Light also behaves as particle and wave) Einstein had predicted that energy and matter were related in his equation E = mc 2 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) Electron’s energy is quantized (has only certain values) Electrons in probability zones called “orbitals”, not orbits Electrons in probability zones called “orbitals”, not orbits - location cannot be pinpointed - location cannot be pinpointed Electrons are particles & waves at same time Electrons are particles & waves at same time Electrons move around nucleus at speed of light Electrons move around nucleus at speed of light

Orbitals

A Missing Particle – The Neutron James Chadwick discovered the last major piece of the atom in 1932 discovered the neutron, which has no charge (“neutral”) won the Nobel Prize in physics in 1935 neutron weighs about as much as a proton while electron is much smaller ( )

The Complete Modern Atom An atom is: mostly empty space nucleus has most of mass of an atom nucleus contains protons & neutrons electrons are in energy levels around nucleus electrons jump between levels, emitting and 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 develop a better theory and win the Nobel Prize?