Historical Developments

in Chemistry

SC.912.P.8.3 – 4 Major Models of the Atom

SC.912.N.3.3 – Scientific Law vs. Scientific Theory

Democratus (400 BC) –

-If you cut a substance in

half forever you end up

with an uncuttable particle

= atom

Atoms vs. Elements• Element: pure substance that cannot be

separated into simpler substances

• Made up of only 1 kind of atom

• Combine to form compounds

• Atom: the smallest particle into which

an element can be divided and still be

the same substance

• Atoms make up elements

John Dalton (1766-1844) –

1. All matter is made of atoms;

atoms are indivisible and

indestructible.

2. All atoms of a given element are

identical in mass and properties.

3. Atoms join with other atoms to

make new substances.

*Disclaimer*

Modern Atomic Theory elaborates more,

but Dalton’s theory became the theoretical

foundation in chemistry

(e.g. atoms can be destroyed in nuclear

reactions but not by chemical reactions,

different kinds of atoms = isotopes)

“Atoms hard; dense (like billiard balls)”

J.J. Thomson (1856-1940)--Discovered the electron

-Realized the accepted model of an

atom did not account for negatively

or positively charged particles

-”Plum Pudding” model; explained

some of the atom’s electrical

properties, but failed to recognize the

atom’s positive charges as particles.

Thomson’s Cathode Ray Tube Experiment

YouTube URL: https://youtu.be/O9GoyscbazkOr search YouTube for “Cathode Ray Tube”

Thomson’s Cathode Ray Tube Experiment

YouTube URL: https://youtu.be/Rb6MguN0Uj4Or search YouTube for “Discovery of the Electron: Cathode Ray Tube Experiment”

Ernest Rutherford (1871-

1937)-- Gold Foil experiment showed the

mass of an atom was concentrated

in a nucleus

- Nuclear Model of the Atom - Did not yet theorize about neutrons

(not discovered until 1932 by James Chadwick)

Fun-fact

Ernest Rutherford was Thomson’s student! ....who

disproved Thomson’s Plum Pudding model.

Note:All you need to understand about alpha particles at this

point is that they are positively charged particles

Rutherford’s Gold Foil Experiment

YouTube URL: https://youtu.be/XBqHkraf8iEOr search YouTube for “Rutherford Gold Foil Experiment – Backstage Science”

Niels Bohr (1885-1962)-- Electron Shell Model

(or the Quantum Model)

- Suggested that:

- electrons in an atom revolve around the

nucleus only in certain discrete,

separate orbits, or “shells”

Erwin Schroedinger (1887-1961)-- Mathematical model (wave functions) for

the distribution of electrons in an atom

– “Electron Cloud Model”

- Probability of where electrons are

- 3-dimensional model better

describes 3-dimensional space

- Bohr’s model was only 1-

dimensional

Werner Heisenberg (1901-1976) - Uncertainty principle – the position and

velocity cannot both be measured at the same

time

- “Since you could never with great

certainty measure more than one

property of a particle, you could only

work with probability & mathematical

formulations”

n

“Law” vs. “Theory”

Scientific LAW• “A set of observed regularities expressed in a

concise verbal or mathematical statement”

• “A statement that a particular phenomena always occurs if certain conditions are present”

• Describes what nature does under certain conditions (predicts what will happen)

• Resists change over time

Scientific THEORY

• “An explanation for an observation (or series of

observations) that is substantiated by a

considerable body of evidence”

• “A body of knowledge and explanatory

concepts that seek to increase our

understanding (explain) a major phenomenon

in nature”

• Explains how nature works

Sooo… what about Models? • Constructed to help explain complex concepts

LAW or THEORY?

Sir Isaac Newton’s _____ of Gravity

• LAW

• Gravity is described as-is, not explained in terms of how it probably came about

• “Ms. O., what’s gravity?”

• “Ms. O, why is there gravity?”

Practice

The law of gravity describes a relationship between two masses. The more

massive an object is or the closer it is to another body, the more the gravitational

attraction between the two objects.

However, the law is limited because it cannot explain which of the following?

A) what gives the larger object more mass

B) why we experience gravity on earth and not in space

C) why there is a gravitational attraction in the first place

D) why does the smaller object also attract the larger object

Practice

The law of gravity describes a relationship between two masses. The more

massive an object is or the closer it is to another body, the more the gravitational

attraction between the two objects.

However, the law is limited because it cannot explain which of the following?

A) what gives the larger object more mass

B) why we experience gravity on earth and not in space

C) why there is a gravitational attraction in the first place

D) why does the smaller object also attract the larger object

Who created this model?

• The Plum Pudding modelWhat is this model called?

• Thomson

Who disproved it? • Rutherford

Which experiment

disproved it? • Gold Foil Experiment

What is he most known for

discovering? • The electron

Whose theory does the other

model represent? • Rutherford’s NUCLEAR

Model

What is the main difference

between the two models?• Bohr’s model requires discrete electron orbits

Which model represents

Bohr’s atomic theory?

Whose model laid the foundation for

modern Atomic Theory? • Dalton

What did his theory say? (Hint: 3 things)1. All matter is made up of atoms, which are indivisible

2. All atoms of a given element are the same mass and properties

3. Atoms join with other atoms to make new substances

Summary

• Dalton – laid the theoretical foundation for Atomic Theory

• Thomson – discovered the electron using Cathode Ray experiment; developed Plum Pudding Model

• Rutherford – disproved Plum Pudding Model; replaced it with Nuclear Model after conducting Gold Foil Experiment which provided evidence for an dense, positively-charged atomic nucleus

• Bohr – Replaced Rutherford’s model with the electron shell model (or Quantum theory); key feature = discrete electron orbits with specific energy levels that prevent negatively-charged

electrons from collapsing into the positively-charged nucleus

• Shroedinger – electron cloud model; said that the position and location of electrons in an atom is better described with probabilities (a three-dimensional model rather than Bohr’s one-dimensional model)

• Scientific Theory – well-supported explanations of phenomena (WHY x happens)

• Scientific Law – well-supported descriptions of phenomena (WHAT happens when x occurs)

**Theories do not become laws, and laws do not become theories**

Anatomy of the Atom

SC.912.P.8.4 – Structure & Composition of Atoms

Construct/Draw an atomic model

Label using the word bank, and

Please include the electrical charge• “Word Bank”

• PROTON - electrical charge?

• NEUTRON - electrical charge?

• ELECTRON - electrical charge?

• NUCLEUS - electrical charge?

• ELECTRON CLOUD –

electrical charge?

PROTONS

• CHARGE of proton?

• (+) Positive

• LOCATION of proton?

• Nucleus

• SUPPORT THESE ANSWERS USING WHAT YOU KNOW ABOUT

THE SCIENTISTS WHO HELPED DEVELOP ATOMIC THEORY

• E.g. Particular experimental observation? An inference?

ELECTRONS

• CHARGE of electron?

• (-) negative

• LOCATION of electron?

• Electron cloud

• SUPPORT THESE ANSWERS USING WHAT YOU KNOW ABOUT

THE SCIENTISTS WHO HELPED DEVELOP ATOMIC THEORY

• E.g. Particular experimental observation? An inference?

NEUTRONS

• CHARGE of neutron?

• 0 - Neutral (no charge)

• LOCATION of neutron?

• nucleus

• We did not discuss neutrons so this one is given to you

NUCLEUS

• Electrical charge of nucleus?

• (+) positive

• Location of nucleus?

• In the center of the atom

• SUPPORT THESE ANSWERS USING WHAT YOU KNOW ABOUT

THE SCIENTISTS WHO HELPED DEVELOP ATOMIC THEORY.

E.g. Particular experimental observation? An inference?

ELECTRON CLOUD

• Electrical charge of electron cloud?

• (-) negative

• Location of electron cloud?

• Everywhere that isn’t the nucleus

• SUPPORT THESE ANSWERS USING WHAT YOU KNOW ABOUT

THE SCIENTISTS WHO HELPED DEVELOP ATOMIC THEORY.

E.g. Particular experimental observation? An inference?

Forces of Attraction/Repulsion> electrostatic forces: affects objects that have charge

more mass in an atom = larger electron cloud = larger

magnitude of distortions in the electron distribution

=> Dipoles!

DIPOLE: a charge separation

Atomic Parts & What to Know

• Subatomic Particles – Proton, Neutron, Electron

• Location – Nucleus or Electron Cloud?

• Charge – Positive, Negative, or Neutral?

• Forces of attraction and repulsion between subatomic particles

• E.g. electrical charge (opposite charges attract, like charges repel)

• RELATIVE SIZE of proton, neutron, and electron

• Which is the SMALLEST?

Another form of note-taking/organizing information into a GRAPHIC ORGANIZER.Your Task: Draw/Make a graphic organizer that summarizes the Historical

Developments of Atomic Theory

Please include the following information:

- Name of scientists (in chronological order)

-Their discovery or idea

-Their experiment (if applicable)

-The name of their model and a sketch of it

-What makes each theory/idea different from the last? (This part does not apply to

Democratus)

•ACTIVITY/HOMEWORK (it’s only homework if you do not finish in class)

Another form of note-taking/organizing information into a GRAPHIC

ORGANIZER.Your Task: Draw/Make a graphic organizer that summarizes the atomic

structure

Please include the following information:

- the 3 subatomic particles (charge, location, relative size)

- the 2 different atomic structures (charge, location)

- label a picture of an atom that includes all of the above

•ACTIVITY/HOMEWORK (it’s only homework if you do not finish in class)