History of the Atomic Model Seeing the Invisible

Can matter be divided into smaller and smaller pieces forever? A Big Debate Can matter be divided into smaller and smaller pieces forever? What do you think? YES! NO! Democritus Aristotle

Atomic Models: Greek Democritus (460-370 B.C.)  Matter can not be divided forever Smallest piece = “atom” (Gk “atomos” = “not to be cut”) He had no way of knowing what atoms looked like!

The word “atom” comes from a Greek word that means “unable to be cut” … and you kept on cutting the leftover piece in half… Imagine you had a piece of gold that you then cut in half… …and then you cut one of these smaller pieces in half… Gold Go ld

The word “atom” comes from a Greek word that means “unable to be cut” Eventually you would have 1 piece of gold left. If you cut it in half, you wouldn’t have gold any more – you’d have something else. This tiny, tiny single piece of gold is called an atom of gold. An atom is the smallest particle of an element that acts like the element. …and kept going… …and kept going… …and kept going… An atom of gold

He hypothesized that atoms were: Democritus He hypothesized that atoms were: Small & Hard Diff in shape & size Infinite Always moving Capable of joining But no one believed him!

Time Goes By… 1600s-1700s: Key experiments occur which support Democritus’s ideas. Robert Boyle (1627-1691) Antoine Lavoisier (1743-1794)

Dalton (1766-1844) English chemist and school teacher Did many experiments, studying gasses and proposed atomic theory

Dalton’s Atomic Theory 1. All matter is made up of atoms Atoms cannot be divided into smaller particles All atoms of the same element are exactly alike 4. Different elements are made of different kinds of atoms

Dalton’s Atom Model Solid Sphere 10

Atomic Models: J.J. Thompson Passed electricity through an uncharged gas The gas gave off rays to show it was NEGATIVELY charged How? Negative charges must come from inside the atom! => Electrons! Click here for more on the Discovery of the electron

But wait… How can an atom be NEUTRAL if it is full of negatively charged particles (electrons?)

Atomic Models: J.J. Thompson The atoms are neutral… How? + charges must be present to balance - charges + & - lumped in a cluster he said looked like “plum pudding”

Thompson’s Model

Atomic Models: Rutherford Passed + Charged Particles through gold foil Most passed right through Atom is mostly empty space Some bounced off at odd angles Nucleus must be + Calculate size of nucleus

Rutherford’s Gold Foil Experiment start experiment DO NOT OPEN RADIOACTIVE MATERIAL INSIDE + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + LOOK INSIDE THE GOLD

Rutherford’s Work What's Different This Time??? start experiment DO NOT OPEN RADIOACTIVE MATERIAL INSIDE + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + LOOK INSIDE THE GOLD

Rutherford’s Model

So the atom is made up of positive and negative particles. Okay… So the atom is made up of positive and negative particles. Where are the electrons found in the atom?

Atomic Models: Bohr Electrons are found in specific energy levels Like planets around the sun

Bohr’s Model

Atomic Models: Wave Electrons move so fast that it is impossible to determine their location Move in all directions around the nucleus

Electron Cloud Model Current Atomic Model- Electrons travel in a region surrounding the nucleus called the electron cloud