History of the Atom Bellringer: K: Write what you know about atoms W: write what you want to know about atoms Objective: SWBAT explain how the modern model of the atom has evolved

Table Set Up Date Person Idea/Discovery Keyword(s)

(greek for indivisible) HISTORY OF THE ATOM Democritus develops the idea of atoms 460 BC He pounded up materials in his pestle and mortar until he had reduced them to smaller and smaller particles which he called: ATOMA (greek for indivisible)

HISTORY OF THE ATOM ATOMS John Dalton 1808 Suggested that all matter was made up of tiny spheres that were able to bounce around with perfect elasticity (they can bounce forever and never slow down) and called them ATOMS

HISTORY OF THE ATOM 1808 John Dalton All elements are composed (made up) of atoms. It is impossible to divide or destroy an atom. All atoms of the same elements are alike. (One atom of oxygen is the same as another atom of oxygen.)

HISTORY OF THE ATOM 1808 John Dalton Atoms of different elements are different. (An atom of oxygen is different from an atom of hydrogen.) Atoms of different elements combine to form a compound.

HISTORY OF THE ATOM ELECTRON Joseph John Thompson 1898 found that atoms could sometimes eject a far smaller negative particle which he called an ELECTRON

HISTORY OF THE ATOM PLUM PUDDING MODEL 1904 Thompson develops the idea that an atom was made up of electrons scattered unevenly within an elastic sphere surrounded by a soup of positive charge to balance the electron's charge like plums surrounded by pudding. PLUM PUDDING MODEL

HISTORY OF THE ATOM Ernest Rutherford 1910 Supervised Geiger and Marsden carrying out his famous experiment. They fired Helium nuclei at a piece of gold foil which was only a few atoms thick. they found that although most of them passed through. About 1 in 10,000 hit

HISTORY OF THE ATOM gold foil helium nuclei helium nuclei They found that while most of the helium nuclei passed through the foil, a small number were deflected and, to their surprise, some helium nuclei bounced straight back.

HISTORY OF THE ATOM Rutherford’s new evidence allowed him to propose a more detailed model with a central nucleus. He suggested that the positive charge was all in a central nucleus. This held the electrons in place by electrical attraction. He also suggested that atoms were mostly empty space. However, this was not the end of the story.

HISTORY OF THE ATOM 1913 Niels Bohr Studied under Rutherford at the Victoria University in Manchester. Bohr refined Rutherford's idea by adding that the electrons were in orbits. Rather like planets orbiting the sun, with each orbit only able to contain a set number of electrons.

Bohr’s Atom electrons in orbits nucleus

History of the Atom 1932 James Chadwick neutron Discovered a neutrally charged particle located in the nucleus of an atom. This new particle has the same mass as a proton. He called this particle a neutron

History of the atom 1962 Erwin Schrödinger Quantum Mechanical Model “Electron Cloud” Refers to a region outside the nucleus where the electron is most likely to be found Because we cannot know exactly where an electron is at any given time, we use the model to predict the places where it probably is. (This idea is based on quantum mechanics and Heisenberg’s uncertainty principle)*

Bohr Model vs. Electron Cloud

HELIUM ATOM Shell proton N + - + N - neutron electron

ATOMIC STRUCTURE Particle Charge proton + charge neutron No charge electron - charge

Atomic Timeline Activity

Activity Create a timeline of the evolution of the atomic model. Include all major events from Democritus to the present. Each event needs: The year The person responsible A short description of the new idea An illustration of what people thought the atom looked like at that time.

Expectations Working quietly- silent or whispering to your neighbor (level 1) Raise your hand for help Working productively and independently You can get up for more supplies, but not to socialize You will need to turn this in at the end of class, so you need to work effectively without wasting time.