Lesson 1-1

Democritus (460–370 BC) believed that matter is made of small, solid objects called atomos, from which the English word atom is derived.

Early Ideas About Matter

Lesson 1-1

Early Ideas About Matter (cont.)

• Aristotle (384–322 BC) did not believe that empty space exists, but instead believed that all matter is made of fire, water, air, and earth.

• Because Aristotle was so influential, his ideas were accepted and Democritus’s ideas about atoms were not studied again for more than 2,000 years.

Lesson 1-2

John Dalton combined data from his own scientific research with data from the research of other scientists to propose a new atomic theory.

Dalton’s Atomic Model

Lesson 1-3

An atom is the smallest piece of an element that still represents that element.

The Atom

What is a copper atom?

Lesson 1-3

• Atoms of different elements are different sizes, but all are very, very small.

• You cannot see atoms with just your eyes or even with most microscopes.

The Atom (cont.)

How would you describe the size of an atom?

Lesson 1-3

• The 1981 invention of a high-powered microscope, called a scanning tunneling microscope (STM), enabled scientists to see individual atoms for the first time.

• Scientists have learned that atoms are not the smallest particles of matter.

The Atom (cont.)

Lesson 1-4

Following his experiments with cathode ray tubes, scientist J.J. Thomson concluded that cathode rays were made of small, negatively charged particles which he called electrons.

Lesson 1-4

An electron is a particle with one negative charge (1–).

Thomson—Discovering Electrons

electron

from Greek electron, means ―amber,‖ the physical force so called because it first was generated by rubbing amber. Amber is a fossilized substance produced by trees.

Lesson 1-4

• Because atoms are neutral, or not electrically charged, Thomson proposed that atoms also must contain a positive charge that balances the negatively charged electrons.

• Thomson’s proposed atom was a sphere with a positive charge evenly spread throughout and negatively charged electrons within it.

Thomson—Discovering Electrons (cont.)

Lesson 1-4

Thomson’s model of the atom contained a sphere of positive charge with negatively charged electrons within it.

Lesson 1-5

Scientist Ernest Rutherford set up experiments to test Thomson’s atomic model and to learn more about what atoms contain.

Rutherford—Discovering

the Nucleus

Lesson 1-5

Rutherford expected the positive alpha particles to travel straight through the foil without changing direction.

Lesson 1-5

Some alpha particles traveled in a straight path, as expected. But some changed direction, and some bounced straight back.

Lesson 1-5

Rutherford—Discovering

the Nucleus (cont.)

Given the results of the gold foil experiment, how do you think an actual atom differs from Thomson’s model?

Lesson 1-5

• Rutherford concluded that most of an atom’s mass and positive charge is concentrated in a small area in the center of the atom called the nucleus.

• Additional research showed that the positive charge in the nucleus was made of positively charged particles called protons.

Rutherford—Discovering

the Nucleus (cont.)

Lesson 1-5

• A proton is an atomic particle that has one positive charge (1+).

• Negatively charged electrons move in the empty space surrounding the nucleus.

Rutherford—Discovering

the Nucleus (cont.)

Lesson 1-5

Rutherford’s model contains a small, dense, positive nucleus. Tiny, negatively charged electrons travel in empty space around the nucleus.

Lesson 1-6

• James Chadwick discovered that, in addition to protons, the nucleus also contained neutrons.

• A neutron is a neutral particle that exists in the nucleus of an atom.

Discovering Neutrons

Lesson 1-6

• Niels Bohr proposed that electrons move in circular orbits, or energy levels, around the nucleus.

• Electrons closer to the nucleus have less energy than electrons farther away from the nucleus.

Bohr’s Atomic Model

Lesson 1-6

• More research showed that, although electrons have specific amounts of energy, energy levels are not arranged in circular orbits.

• When an electron moves from a higher energy level to a lower energy level, energy is released—sometimes as visible light.

Bohr’s Atomic Model (cont.)

Lesson 1-6

In Bohr’s model of the atom, electrons move in circular orbits around the atom.

Lesson 1-6

Bohr’s Atomic Model (cont.)

How did Bohr’s model of the atom differ from Rutherford’s?

Lesson 1-6

• In the modern atomic model, electrons form an electron cloud.

• An electron cloud is an area around an atomic nucleus where an electron is most likely to be.

The Modern Atomic Model

Lesson 1-6

In this atom, electrons are more likely to be found closer to the nucleus than farther away.

Lesson 1-6

The Modern Atomic Model (cont.)

How has the model of the atom changed over time?

Lesson 1-6

• Protons and neutrons are made of smaller particles called quarks.

• Scientists theorize that there are six types of quarks: up, down, charm, strange, top, and bottom.

• Protons are made of two up quarks and one down quark.

Quarks

Lesson 1-6

• Neutrons are made of two down quarks and one up quark.

• The current atomic model might change with the invention of new technology that aids the discovery of new information.

Quarks (cont.)

Lesson 1 – LR1

A. atom

B. electron

C. nucleus

D. proton

Which term describes a particle with one negative charge?

Lesson 1 – LR2

Whose model of the atom contained a sphere of positive charge with negatively charged electrons within it?

A. Dalton

B. Democritus

C. Rutherford

D. Thomson

Lesson 1 – LR3

A. electron cloud

B. neutron

C. nucleus

D. proton

Which term refers to an area around an atomic nucleus where an electron is most likely to be?