Atomic Theory

Let’s Take a Trip Through Time!

Democritus vs. Aristotle 400BC 350BC

Democritus 460 – 370 B.C. Matter composed of Atoms basic elements from which all matter is made atoms move in a void (empty space) atoms have different shapes (round, pointy, oily, have hooks, etc.) Ideas rejected by leading philosophers because void = no existence

First Concept of an Atom Atoms are solid, homogeneous, indivisible particles

Aristotle 350 BC Matter can be subdivided into smaller and smaller particles forever Matter composed of combination of 4 elements: air, earth, water and fire No such thing as empty space – had the “ether”

Aristotle won that debate! For 2000 years most people believed that the 4 elements were: air, earth, water, and fire There was no such thing as empty space!

Phlogiston Theory Around 1700’s: wood  calx (ash) + phlogiston (↑ air) iron  calx (rust) + phlogiston (↑ air)

Antoine LaVoisier Demonstrated importance of measurements in chemistry Disproved the phlogiston theory First to state Law of Conservation of Mass

LaVoisier was the father of modern chemistry, but he lost his head in 1794!

J. Proust – 1797 Law of Definite Proportions In pure compound: elements combine in definite proportions to each other H2O CO2

John Dalton 1766-1844

Dalton’s Model Billard Ball model

Dalton’s Atomic Theory Matter is composed of very small particles called atoms All atoms of a given element are identical Atoms of different elements are different Atoms are indivisible and cannot be created or destroyed Different atoms combine in whole number ratios to form compounds In a reaction, atoms are rearranged

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

In a reaction, atoms are rearranged + +  +

1904 Thomson’s Model Plum Pudding model (aka: chocolate chip cookie model)

J.J. Thomson 1856-1940 Discovered electron 1897 Cathode Ray Experiment Discovered isotopes 1913

Cathode Ray Tube Experiment Thought cathode rays were streams of particles smaller than atoms magnetic field will deflect beam of charged particles

Thomson’s discovery meant that the atom WAS divisible!

JJ Thomson’s Ideas matter is neutral!

Plum Pudding Model Negative electrons in a soup of positive charges

positive charge evenly spread out while the negative charge is in bits – like chocolate chips in cookies source

Proton – Discovered by 1920 Thomson & Goldstein – 1907 - discovered heavy particle with positive charge Rutherford – 1918 - shot alpha particles at nitrogen gas and got hydrogen - hydrogen had to come from nitrogen - suggested hydrogen nucleus was a particle & named it the proton

Rutherford’s Model 1910 Nuclear model

Ernest Rutherford 1871-1937 atom’s mass is mostly in the center of atom Nucleus nucleus has a positive charge

Rutherford’s Experiment - 1911

Compared to electron, alpha particle is massive & fast Rutherford’s exp’t: animation Compared to electron, alpha particle is massive & fast If Thomson’s model correct, alpha particle wouldn’t be affected - electrons are tiny - positive charge uniformly spread

What did Rutherford conclude from the particles that bounced back? What did Rutherford conclude from the particles that went straight through? mostly empty space What did Rutherford conclude from the particles that bounced back? hit something really heavy & positively charged source

Results of Rutherford’s Expt Results DO NOT match Thomson’s model

Rutherford Model NUCLEAR Model

Rutherford proposed nuclear atom : dense central core (+ charged) surrounded by mostly empty space where electrons are found Rutherford did not speculate on how the electrons were arranged around the nucleus

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 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 An angstrom = 1 X 10-10 m 6 BILLION Cu atoms in a line = less than 1 meter! Can we “see” an individual atom?

James Chadwick 1891-1974 Worked with Rutherford Discovered Neutron 1932 Nobel Prize in Physics 1935

Bohr’s Model 1913 Planetary model

Niels Bohr 1885-1962 His Model: Nucleus surrounded by orbiting electrons at different energy levels Electrons have definite orbits Worked on the Manhattan Project (US atomic bomb)

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

Bohr Model for Nitrogen

Quantum Mechanical model Modern Model 1926 Quantum Mechanical model

Ernst Schrödinger 1887-1961 Quantum Mechanical 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

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

Which one of you will develop a better theory and win the Nobel Prize? Next Atomic Theory ? Which one of you will develop a better theory and win the Nobel Prize?