The History of the Atom

History of the Atom Timeline Modern Quantum Cloud Model post 1930 Thomson 1897 Rutherford 1912 Bohr 1913 Democritus 460 BC and Dalton 1803 AD

Marble Model Plum Pudding Model The Nuclear Model The Planetary Model

Black Boxes Black Boxes are anything that you cannot see inside Like a cell phone… Or a locked box… Or an atom…

THE GREEK ATOM Democritus (around 460-370 BC.), "a-tomos" and signifies "indivisible". All matter is composed of indivisible atoms, which are too small to be seen. There is a void, which is empty space between atoms. Atoms are completely solid. Atoms are homogeneous, with no internal structure. Atoms are different in their size, their shape and their weight.

Early Greek Philosophers The ancient Greeks (~450 BC) believed there only four elements: Earth Air Water Fire

Aristotle The idea of the atom was strongly opposed by Aristotle and others. All substances were combinations of elements and elemental qualities. Fire, water, earth, and air. The atom receded into the background.

Democritus’ idea of the atom was largely ignored until an English schoolteacher did some experiments over 2000 years later, he was… John Dalton (1766-1804) Leading to his atomic theory…

Dalton’s Atomic Theory (1803) All matter is made up of small indivisible atoms. All atoms of one element are alike, but they are different from atoms of other elements In compounds, atoms join in whole number ratios. (Law Definite Proportions- Proust) Atoms are neither created nor destroyed in any chemical reaction. (Law of Conservation of Mass- Lavoisier) In chemical reactions, atoms are combined, separated, or rearranged.

Dalton's Atomic Theory All matter is made up of small indivisible atoms. Figure 2.1 John Dalton (1766-1844)

Dalton's Atomic Theory All atoms of one element are alike, but they are different from atoms of other elements Figure 2.1 John Dalton (1766-1844)

Dalton's Atomic Theory 3. In compounds, atoms join in whole number ratios. (Law Definite Proportions- Proust)

Dalton's Atomic Theory Atoms are neither created nor destroyed in any chemical reaction. (Law of Conservation of Mass- Lavoisier) In chemical reactions, atoms are combined, separated, or rearranged. Figure 2.1 John Dalton (1766-1844)

John Dalton’s Atomic Theory Unit 2 - Lecture 1: Structure of the Atom John Dalton’s Atomic Theory Almost right. A good start. very small Structure of the atom after Dalton (ca. 1810)

Thomson’s Cathode Ray Tube Experiments and the Discovery of the Electron

JJ Thomson (1856-1940) Thomson used the cathode ray tube to help establish the identity of the electron 1897. Click here to listen to him talk about it Gives off electrons Vacuum Magnet - redirected the electrons- nobody knows why the magnetic field bends

Unit 2 - Lecture 1: Structure of the Atom J.J. Thomson (1897): Cathode Rays Atoms subjected to high voltages give off cathode rays.

But, what was that green light? Was it a light? Was it a particle? To test this, he brought a magnet close to the cathode ray tube to see what would happen.

Unit 2 - Lecture 1: Structure of the Atom J.J. Thomson: Cathode Rays Cathode rays can be deflected by a magnetic field. Cathode rays are negatively charged particles (electrons). Electrons are in atoms.

So, the magnet caused the cathode ray to move. What does that tell us? Would a magnet affect a light from a flashlight? (you could try this at home) Probably not. Therefore,! the cathode ray must be a particle

Thomson also noticed… That the cathode ray was coming out of the negative end (cathode) of the tube and going toward the positive end (anode) Therefore, because opposites attract, he concluded that the cathode ray must be negative He called these particles… Negative end Positive end

ELECTRONS! The discovery would alter Dalton’s model of the atom because now there is something inside it But, in addition to the negatively charged electrons, there must be something giving it a positive charge because the overall charge of the atom is neutral (not negative) Let’s look at Thomson’s model of the atom…

Original Model of the Atom Plum Pudding Model These newly discovered electrons must have been balanced by some sort of positive charge. Thomson proposed a 'plum pudding' model, with positive and negative charge filling a sphere. Even Thomson's student Rutherford, who later proved model incorrect, believed it at the time.

Thomson’s Plum Pudding Model of the Atom He believed the atom was made of positively charged stuff with negatively charged particles scattered throughout Why the “plum pudding” model? What is “plum pudding”?

An American analogy to his atom would be… Plum Pudding is an English dish sort of like bread pudding with raisins in it. An American analogy to his atom would be… …Is like… But we’ll still refer to it as the “Plum Pudding” model in class

Unit 2 - Lecture 1: Structure of the Atom J.J. Thomson – The Electron “Plum pudding” model: Negative electrons are embedded in a positively charged mass. Electrons (-) Unlike electrical charges attract, and that is what holds the atom together. Positively charged mass Structure of the atom after Thomson (ca. 1900)

Rutherford and the Nucleus

Ernest Rutherford New Zealand physicist (1871-1937) In 1909, Rutherford performed the Gold Foil Experiment. In it, he shot alpha particles (very small, very dense, very fast particles) at a thin layer of gold foil.

Radioactivity Three types of radiation were discovered by Ernest Rutherford:  particles (positive, charge 2+, mass 7400 times of e-)  particles (negative, charge 1-)  rays (high energy light) Figure 2.8

He expected all of the alpha particles to go straight through It would be like if you were shooting bullets at a cake…all of the bullets (or alpha particles) would go straight through the cake (or gold foil atoms)

Unit 2 - Lecture 1: Structure of the Atom Ernest Rutherford (1909) Scattering experiment: firing alpha particles at a gold foil

Rutherford’s Gold Foil Experiment Expected- particles to go straight through the plum pudding with its diffuse positive charge Actual- some particles bounced back! 1 in 8000,

Most of the alpha particles went straight through but a very tiny amount were deflected at odd angles? That could only happen if there was something very tiny in the atom that was dense enough to deflect the alpha particles. Like this…

Rutherford’s Reaction The amazed Rutherford commented that it was "as if you fired a 15-inch naval shell at a piece of tissue paper and the shell came right back and hit you."

The Nuclear Atom Thomson’s model could not be correct. Some alpha particles bounce off the gold foil. This means the mass of the atom must be concentrated in the center and is positively charged! Thomson’s model could not be correct. Figure 2.11

DISCOVERY OF THE NUCLEUS Rutherford proved that an atom was mostly empty space with a very small, very dense, positively charged nucleus in it. Rutherford Video

Unit 2 - Lecture 1: Structure of the Atom Ernest Rutherford The Nucleus and the Proton The mass is not spread evenly throughout the atom, but is concentrated in the center, the nucleus. The positively charged particles in the nucleus are protons. Electrons (-) are now outside the nucleus. Structure of the atom after Rutherford Planetary Model (published 1911)

DISCOVERY OF NUCLEONS Rutherford understood that the nucleus is itself composed of nucleons. These nucleons are of two types: positively charged, it's a proton. neutrally charged, it's a neutron The neutron was effectively discovered in 1932 by Chadwick.

Unit 2 - Lecture 1: Structure of the Atom James Chadwick – The Neutron In the nucleus with the protons are particles of similar mass but no electrical charge called neutrons. Electrons (-) are now outside the nucleus in quantized energy states called orbitals. (From Niels Bohr and quantum mechanics) The positively charged particles in the nucleus are protons. + n n Structure of the atom after Chadwick (1932)

BOHR ATOM In order to take account of atomic stability, Danish physicist, Niels Bohr created a new model of the atom in 1913..

BOHR ATOM The orbits of the electrons can't be just anywhere but are "quantified”; only certain particular orbits are permitted for the electron. It's not until one jumps from one orbit to another that it can emit (or absorb) light.

Modern Atomic Model Also known as the quantum atomic model. Describes the atom as mostly empty space with a tiny massive nucleus with protons and neutrons at the center. Surrounding the nucleus is a cloudlike region of electrons moving too fast to describe their location in space and time.

Unit 2 - Lecture 1: Structure of the Atom proton (+) neutron electrons - responsible for the volume and size of the atom, negatively charged 10-10 m nucleus - responsible for the mass of the atom, positively charged 10-14 m

What is the relative size of the atom….? An atomic model the size of Busch Stadium would contain a pea sized nucleus containing 95.95% of the atoms mass. The pea at the pitcher’s mound would be the nucleus, and a firefly flying around outside would be an electron! The atom is mostly….. Empty space! Nothing!

Subatomic Particles Protons and electrons are the only particles that have a charge. Protons and neutrons have essentially the same mass. The mass of an electron is so small we ignore it. Table 2.1

History of the Atom Timeline Modern Quantum Cloud Model post 1930 Thomson 1897 Rutherford 1912 Bohr 1913 Democritus 460 BC and Dalton 1803 AD

Marble Model Plum Pudding Model The Nuclear Model The Planetary Model