Chapter 4: Glow in the Dark. Introduction List as many things as you can think of that “glow” What do you have to do to make these “glowing” things “glow”?

Chapter 4: Glow in the Dark

Introduction

List as many things as you can think of that “glow”

What do you have to do to make these “glowing” things “glow”?

This chapter will introduce the chemistry needed to understand how glowing things work

Section 4.1: Development of Atomic Theory

Objective: Describe the development of modern

atomic theory

Ancient GreeceVarious philosophers considered the many

mysteries of life.Aristotle concluded that matter was composed of

4 elements - earth, air, fire, and water – and that it could be divided endlessly into ever smaller pieces.

Democritus was the first person to propose the idea that matter was not infinitely divisible. He believed that matter was composed of atomos or atoms; atoms were solid & indivisible.

John Dalton

• Aristotle's idea went unchallenged for 2000 yrs.

• John Dalton (1766-1844), an English schoolteacher and chemist, revised Democritus’ ideas based on careful & accurate scientific research that he conducted himself.

Dalton’s Atomic Theory (1803)All matter is made of tiny particles “atoms”Atoms cannot be created, divided, destroyed or

changed into other types of atomsAtoms of the same element have identical

propertiesAtoms of different elements have different propertiesAtoms of different elements combine in whole-

number ratios to form compoundsChemical changes join, separate or rearrange

atoms in compounds

(p. 124 in text)

J. J. Thomson

• Because of Dalton’s atomic theory, most scientists in the 1800s believed that the atom was like a tiny solid ball that could not be broken up into parts.

• In 1897, a British physicist, J.J. Thomson, discovered that this solid-ball model was not accurate.

Thomson’s Cathode Ray Tube

• Thomson’s experiments used a vacuum tube (a tube that has had all the gases pumped out of it) called the cathode ray tube.

Cathode Ray Tubes

Metal plate (cathode) releases stream

Cathode ray

When connected to a battery, electrodes at the end of the tube seemed to generate a “cathoderay.” The cathode ray looked like a ray of light traveling through the tube, from the cathode plate to the anode plate.

Metal plate (anode) to which stream travels

Cathode Ray Tubes & Charge

-

+

Negatively charged plate

Positively charged plate

Ray is deflected away from negative plate and towards positive plate

Next Thomson put charged plates outside the tube.He found that the rays bent towards a positivelycharged plate and away from a negative one.

Thomson’s conclusions

Since like charges repel, Thomson’s knew that there was something negatively charged in the cathode ray.

Since there were no particles in the tube, these negative particles had to come from the atoms of the metal plates.

Since all types of metal produced the same result, the negative charge had to be in all types of atoms.

In 1897, Thomson announced that the rays were electrons and they had a negative charge

Theories change

Thomson’s evidence showed Dalton’s idea of solid, uniform atoms was incorrect.

In addition, since atoms themselves are not negatively charged but neutral, scientists believed there had to be other particles in the atom, especially positively charged ones.

The Plum Pudding Model

Eugene Goldstein conducted experiments to find the positive parts (protons) and determined they had the opposite charge as the electron but were 1837 times heavier!

Thomson developed a model of the atom called the “plum pudding” model.

Also Called The Cookie Dough Model

The “chips” are the negative electrons.

The “dough” is the positive portion

The “chips” are stationary and don’t move within the “dough”

Gold Foil Experiment

In 1911, a team of scientists led by Ernest Rutherford in England carried out the first of several important experiments that revealed an arrangement far different from the plum pudding model of the atom.

This team included Geiger and Marsden.


Geiger and Marsden (under Rutherford’s direction) bombarded very thin gold foil with radioactive particles (alpha particles “”)

They were to observe the direction the particles took as a result of passing through the foil.



It was believed that if the plum pudding model was correct, the alpha particles would pass straight through the gold atoms.

Instead, researchers found that some alpha particles were deflected at very wide angles.

Observations & Conclusions

Most of the alpha particles passed straight through the foil with no deflectionThese particles did not run into anything; they

traveled through empty space.Some particles had slight deflections

These particles ran into something much smaller than themselves.

A few particles were deflected at wide angles – some came straight back!These particles ran into something very dense

Rutherford’s Nuclear ModelTo explain the results, Rutherford proposed a new model of the atom – the nuclear model.

In this model, atoms are nearly all empty spaceThere is a small area of the atom that contains most of the mass. This area caused the wide deflections seen.

This area is called the nucleus. Protons are found here.

Electrons (the smaller particles), the cause of the small deflections, are found in the space outside the nucleus.

The Neutron

The protons (+) and electrons (-) could explain the charges of the various parts of the atom that were observed.They could not explain the total mass of atoms.Neutrons were proposed in 1920’s but not confirmed until 1932 by James Chadwick.

Neutrons had mass similar to protons and no charge. They were located in the nucleus.

Revisions to the Nuclear Model

In 1913, Neils Bohr (who was working for Rutherford) believed Rutherford’s model needed improvement.

Bohr’s Atomic Model

Bohr performed experiments with hydrogen atoms & light.He determined that electrons are in levels according to how much energy they have (energy levels) and that only certain energy amounts were allowed.

Bohr’s Atomic ModelBohr proposed that, within energy

levels, electrons are found in specific circular paths, or orbits, around the nucleus.

Bohr’s model came to be known as the planetary model.

The Bohr Model

The orbit closest to the nucleus contains the lowest energy electrons.

The first level can hold 2 electrons, then the next two levels can each hold 8 and then levels farther out can hold 18.

Pictures of the Bohr Models

Oxygen

Use of the Bohr Model now

We no longer believe electrons are in circular orbits.

However, this is still a convenient way to show energy levels on 2-dimensional paper.

Modern Atomic Theory

Bohr’s research lead the way for the study of quantum mechanics (the study of tiny particles) in the 1920’s.

Quantum mechanics uses calculus equations to show how the electrons act as both particles and waves.

These equations show the most probable location of electrons in the atom (known as atomic orbitals).

Documents

Chapter 4: Glow in the Dark. Introduction List as many things as you can think of that “glow” What do you have to do to make these “glowing” things “glow”?