Atomic History and Theories

Atom Definition: the smallest particle of any element that retains the properties of that element.

Practice Quiz What is the smallest particle of magnesium that retains the properties of magnesium?

Electron Microscopes With the use of tunneling electron microscopes we can sort of “see” atoms Image of copper atoms Image of silicon atoms

These electron microscopes were invented in the mid-1900’s These electron microscopes were invented in the mid-1900’s. How did scientists figure out atomic structure without being able to look at any atoms?

The History of Atomic Theory

But first….THEORY What is a theory in science? A theory is a well-tested explanation of what happens in nature. In layman’s terms, if something is said to be “just a theory,” it usually means that it is a mere guess, or is unproven. But in scientific terms, a theory implies that something has been well-tested and verified many times by multiple groups of scientists.

But first….Law A scientific law is a statement of something that seems to be true in the natural world, but doesn’t explain why. Example: The Law of Gravity. Newton could use this law to predict the behavior of a dropped object, but he couldn't explain why it happened.

Remember! A LAW describes what happens in nature, but doesn’t explain why. A THEORY is an explanation of what happens in nature based on repeated observations or experiments.

The History of Atomic Theory

Democritus Greek philosopher, not scientist Proposed fourth century B.C.E. (Before Common Era) first to suggest that matter is made of tiny particles called atoms “atomos” “atomos” = Greek for “indivisible” ideas were rejected by Aristotle, who was very influential, and therefore forgotten for two thousand years.

Democritus’ atomic model: atoms were small, hard particles that were different shapes and sizes

John Dalton First to propose an ATOMIC THEORY based on science: 1. Each element is composed of small particles called atoms. 2. All atoms of a given element are identical*; the atoms of different elements are different. 3. Atoms are not created or destroyed in chemical reactions. 4. Compounds are formed when atoms of more than one element combine.

Dalton’s atomic model: a tiny sphere that is indivisible

All matter is composed of extremely small particles called atoms Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, & other properties ELEMENT 1 ELEMENT 2 ELEMENT 3 ELEMENT 4 Atomic Theory

Atomic Theory Atoms cannot be created or destroyed Atoms of different elements combine to form compounds In chemical rxns, atoms are combined, separated, or rearranged + + Atomic Theory

J.J. Thomson First to suggest that there were particles smaller than the atom** Discovered the first subatmic particle…….. the ELECTRON! Developed the “plum pudding model.”

Thomson’s “plum-pudding” model: the atom is composed of electrons surrounded by a cloud of positive charge to balance the electron's negative charge

Remember! Thompson vs. Dalton Thompsons model proved Daltons Atomic Theory was wrong…. Atoms are divisible into smaller subatomic particles

Ernest Rutherford gold foil experiment discovered the NUCLEUS, and realized that it was very dense and positively charged suggested that most of the atom is empty space

Gold Foil Experiment: Rutherford shot tiny positively-charged alpha particles through a thin sheet of gold foil What Rutherford expected: What really happened:

Gold Foil Experiment: Some of the alpha particles bounced back. What Rutherford expected: What really happened:

Such huge deflections could mean only one thing… some of the alpha particles had run into massive concentrations of positive charge and, since like charges repel, they had been hurled straight back by them.

Rutherford’s atomic model: All of the positive charge is crammed inside a tiny, massive nucleus about ten thousand times smaller than the atom as a whole. The atom is mostly empty space.

Animation of Rutherford’s experiment http://www.mhhe.com/physsci/chemistr y/essentialchemistry/flash/ruther14.swf

Rutherford’s Conclusion Eight years after the Gold Foil Experiment Nucleus contains PROTONS, positively charged particles

Scientists were looking for missing mass Scientists were looking for missing mass. The protons and electrons in the atom didn’t weigh enough to account for the mass of the atom

James Chadwick Rutherford’s co-worker proved the existence of neutrons, neutral particle in the atom’s nucleus.

Chadwick’s model of the atom Protons and neutrons are in the nucleus, which only takes up one millionth of a billionth of the volume of the atom. The electrons are in the electron cloud, which constitutes almost all of the volume of the atom.

In fact, if an atom were expanded to the size of a cathedral, the nucleus would be about the size of a housefly!

Neils Bohr Proposed that electrons are made up of different shells Quantum Model: atoms have different energy states Lowest allowable energy state = Ground state Electron moves to a higher energy level = Excited state Must add energy, ex. Heat Electrons move to a higher energy orbit, but fall to a lower orbit when it loses energy Releases light Correctly predicted the emissions spectrum for H

Subatomic Particles to Know Location Mass Charge Proton Nucleus 1 amu +1 Neutron Electron Cloud Practically zero -1

Foldable Outside Democritus Dalton Thompson Rutherford Chadwick Bohr Inside Include a description of each scientists contribution to the atom. Draw a picture of each model. Example on the board