Introduction to the Atomic Model Advanced Chemistry Ms. Grobsky

Exploring the Black Box Imagine you’ve received a present that you are not allowed to open. You REALLY want to know what’s inside! What might you do to figure out the contents?

Exploring the Black Box In your group, try to figure out the contents and internal structure your present WITHOUT opening it Note: Do NOT shake the box too violently as the contents may fall out! As a group, be sure to answer the questions on the hand-out!

So What is a Black Box? A black box refers to any system that has inner workings that cannot be directly observed because it is not possible or feasible Can you think of any other black box examples?

The Black Box and Science An atom is an example of a black box

The Atomic Model and the Nature of Science When scientists wanted to determine the structure of an atom, they were not able to actually see inside of the atom They had to rely on empirical evidence, indirect evidence, observation, and inference when developing their ideas-just as you did when determining the contents of your present!

The Atomic Model and the Nature of Science Scientists then developed a model to express their new ideas The understanding of the structure of the atom has changed many times over the course of centuries As new evidence emerged, the ideas of the atomic model changed Lecturing about the scientists of the atomic model isn’t quite as fun and interesting as understanding: HOW was the atomic model developed? WHY has the atomic model changed over the years?

The “What” of the Atomic Model A Brief History

Democritus This is the Greek philosopher Democritus who began the search for a description of matter more than 2400 years ago He asked: Could matter be divided into smaller and smaller pieces forever, or was there a limit to the number of times a piece of matter could be divided? 400 BC

Atomos His theory: Matter could not be divided into smaller and smaller pieces forever, eventually the smallest possible piece would be obtained This piece would be indivisible He named the smallest piece of matter “atomos,” meaning “not to be cut”

Atomos  To Democritus, atoms were small, hard particles that were all made of the same material but were different shapes and sizes  Atoms were infinite in number, always moving and capable of joining together

Democritus vs. The Philosophers The eminent philosophers of the time, Aristotle and Plato, had a more respected, (and ultimately wrong) theory Aristotle and Plato favored the earth, fire, air and water approach to the nature of matter Their ideas held sway because of their eminence as philosophers The atomos idea was buried for approximately 2000 years

Then Along Came Dalton’s Model In the early 1800s, the English chemist John Dalton performed a number of experiments that eventually led to the acceptance of the idea of atoms

Dalton’s Atomic Model All elements are composed of atoms Atoms are invisible and indestructible particles

Dalton’s Atomic Model Continued Atoms of the same element are exactly alike H H

Dalton’s Atomic Model Continued Atoms of different elements are different O H

Dalton’s Atomic Model Continued Compounds are formed by the joining of atoms of two or more elements H O

Dalton’s Atomic Model This theory became one of the foundations of modern chemistry BUT it needed work

Thomson’s Plum Pudding Model In 1897, the work of an English scientist J. J. Thompson provided the first hint that atoms were made up of even smaller particles

Thomson Model Thomson studied the passage of an electric current through a gas As the current passed through the gas, it gave off rays of negatively charged particles

Thomson Model This surprised Thomson, because the atoms of the gas were uncharged Where had the negative charges come from? Where did they come from?

Thomson concluded that the negative charges came from within the atom A particle smaller than an atom had to exist The atom was divisible! Thomson called the negatively charged “corpuscles,” today known as electrons Since the gas was known to be neutral, having no charge, he reasoned that there must be positively charged particles in the atom But he could never find them

And... negative (-) Thomson thought the positive (+) and negative (-) particles were s p r e a d o u t in atoms like: p p u l d u d m i s n (plums in puddin’)

And Then There was Rutherford… In 1908, the English physicist Ernest Rutherford was hard at work on an experiment that seemed to have little to do with unraveling the mysteries of the atomic structure You will explore the methods performed in Ernest Rutherford’s experiments and help “develop” a model for an unseen object!