The Development of Atomic Theory

Objectives: Students will be able to:

• Make inferences based on observations in order to make conclusions about atoms.

• Research a specific atomic model in order to make a presentation to another group.

Warm Up:What do dinosaurs and atoms

have in common?

What do Dinosaurs and Atoms have in Common?

No one has seen an atom or a dinosaur directly. We know of their existence only by indirect evidence. Our theories of both dinosaurs and atoms has changed over time based on this indirect evidence.

What Do You Know About Atoms?

Using a white board you may:–Draw your representation of an atom.

–If you are not sure, draw what you think an atom might look like.

–Describe what you know about atoms.

Exploration Activity

• In your team, make and record observations about the contents of two sealed bags. DO NOT open bags. Use your senses of touch, smell, and hearing to help you make your observations.

• You will make a prediction of what is in the bags.

• Be prepared to share your findings and explain your reasoning behind your predictions.

You have 5 minutes to fill in chart and answer 1st question.

Activity (continued)

• After hearing the presentations, answer the questions on the back of the hand-out individually.

• What did you learn from the activity to explain how scientists study atoms.

Scale of the Universe

• Now that we know atoms are small, how small are they?

Do Theories in Science Stay the Same?

• Ideas and theories in Science change as new information is gathered.

Our theory about the atom has changed over time as new studies are done. Even though no one has ever seen an atom up close, we are still able to make new discoveries – just like we have made new discoveries about dinosaurs.

This fossil evidence shows us that some dinosaurs may evolved into birds.

Democritus was an ancient Greek philosopher who lived from 460 - 370 B.C.

What did Democritus conclude about cutting matter in half? There was a limit to how far you could divide matter. You would eventually end up with a piece of matter that could not be cut.

He thought matter is like motion. It cannot be divided in half forever. The tortoise and hare would never finish the race if you could keep dividing the distance to the finish line in half forever.

Who was Democritus?

What does the Greek word atomos mean?

• The Greek word “atomos” means not able to be divided

or “indivisible.”

• Aristotle was a very famous Greek philosopher who believed that matter could be divided into smaller and smaller pieces forever. He held a very strong influence on popular belief and his views on this were accepted for two thousand years.

Why was Democritus’s ideas not accepted?

What is an Atom?• An atom is the smallest

particle that an element can be divided and still be that element.

• For example the smallest particle of carbon is a single atom of carbon. If you divide it is no longer carbon anymore.

PIECES OF CARBON

CARBON ATOM

John Dalton 1776-1844• Two thousand years later

a British chemist and schoolteacher brings back Democritus’s idea of the atom

• He performed many experiments to study how elements join together to form new substances

• He found that they combine in specific ratios (remember the electrolysis of water) and he supposed it was because the elements are made of atoms.

What 3 new ideas did John Dalton propose about the atom?

• All substances are made up of atoms which are small particles that cannot be created, divided, or destroyed.

• Atoms of the same element are exactly alike and atoms of different elements are different.

• Atoms join with other atoms to form different substances

When magnesium burns, it combines with oxygen. In magnesium oxide, the ratio of the mass of magnesium to the mass of oxygen is always about 3 : 2. Magnesium dioxide has a fixed composition.

Dalton’s Atomic Theory

Dalton made these wooden spheres as a model to represent the atoms of different elements. A tiny, solid sphere with a different mass represents each type of atom.

Dalton’s Atomic Theory

A theory must explain the data from many experiments. Because Dalton’s atomic theory met that goal, the theory became widely accepted.

Over time, scientists found that not all of Dalton’s ideas about atoms were completely correct. They revised the theory to take into account new discoveries.

Dalton’s Atomic Theory

J.J. Thomson 1856-1940• What particle did

Thomson discover?

• J.J. Thomson discovered that atoms are made of smaller negatively-charged particles called electrons.

• Thomson’s discovery was the result of doing experiments with “cathode ray tubes”

Thomson’s Cathode Ray Experiment

Stream of electrons is attracted to positively charged plate here.

"What are these particles?are they atoms, ormolecules, or matter in a stillfiner state of subdivision?“ quote by Thomson

When the current was turned on, the disks became charged, and a glowing beam appeared in the tube.

• Thomson hypothesized that the beam was a stream of charged particles that interacted with the air in the tube and caused the air to glow.

• Thomson observed that the beam was repelled by the negatively charged plate and attracted by the positively charged plate.

Thomson’s Model of the Atom

Evidence for Subatomic Particles

Thomson concluded that the particles in the beam had a negative charge because they were attracted to the positive plate. He hypothesized that the particles came from inside atoms because

• no matter what metal Thomson used for the disk, the particles produced were identical.

• the particles had about 1/2000 the mass of a hydrogen atom, the lightest atom.

Thomson’s Model of the Atom

Thomson’s Model

Thomson revised Dalton’s model to account for these subatomic particles.

• The atom has neither a positive nor a negative charge, but there must always be some positive charge in the atom.

• The atom is filled with a positively charged mass of matter that has negative charges evenly scattered throughout it.

Thomson’s Model of the Atom

The Plum Pudding Model• Thomson did not know how the

electrons in an atom were arranged. He believed they were mixed throughout an atom.

• He proposed that the atom was a sphere of positively charged material. Spread throughout the atom were the negatively charged electrons similar to plums in a pudding or chocolate chips in ice cream.

Ernest Rutherford(1871 - 1937)

• Awarded the Nobel Prize in Chemistry for his discovery of alpha particles, positively charged particles emitted from radioactive elements

• Was a student of J.J. Thomson but disagreed with the “Plum Pudding Model”

• Devised an experiment to investigate the structure of positive and negative charges in the atom.

An Interactive Model of Rutherford’s Gold Foil

Experiment

http://micro.magnet.fsu.edu/electromag/java/rutherford

Click here

4.1 Studying Atoms

The Gold Foil Experiment

Rutherford’s Atomic Theory

Beam of alpha particles

Source of alpha particles

Slit

Deflected particle

Undeflected particle

Screen

Gold atoms

Alpha particles

Alpha particles

Nucleus

4.1 Studying Atoms

Discovery of the Nucleus

The alpha particles whose paths were deflected must have come close to another charged object. The closer they came, the greater the deflection.

However, many alpha particles passed through the gold without being deflected. These particles did not pass close to a charged object.

Rutherford’s Atomic Theory

What did most of the particles shot at the gold foil do?

• Most of the particles traveled straight through the gold foil

What was the surprising behavior of a few of the particles?

• A few of the particles were deflected and some even bounced back

Rutherford’s Revised Atomic Theory (1911)

Result: Most of the positively charged particles went straight through the gold foil.

Atomic Theory: Most of the matter of the atom is found in a very small part of the atom. This is called the nucleus of the atom. It is very tiny and extremely dense.

Result: Some of the positively charged particles were deflected or even bounced back.Atomic Theory: Like charges repel so the nucleus must have a positive

charge. If electrons have a negative charge they could not be in a positively charged nucleus. Electrons must surround the nucleus at a distance.

Result: The diameter of the nucleus is 100,000 times smaller than the diameter of the entire gold atom.

Atomic Theory: Atoms are mostly empty space with a tiny, massive nucleus at the center .

Thomson’s model did not explain all of the evidence from Rutherford's experiment. Rutherford proposed a new model.

• The positive charge of an atom is not evenly spread throughout the atom.

• Positive charge is concentrated in a very small, central area.

• The nucleus of the atom is a dense, positively charged mass located in the center of the atom.

Rutherford’s Atomic Theory

The Houston Astrodome occupies more than nine acres and seats 60,000 people. If the stadium were a model for an atom, a marble could represent its nucleus. The total volume of an atom is about a trillion (1012) times the volume of its nucleus.

Rutherford’s Atomic Theory

The Bohr Model of the AtomWhat did Bohr learn about electron movement?

• Bohr proposed that electrons move in paths at certain distances around the nucleus.

• Electrons can jump from a path on one level to a path on another level.

• Click here http://www.colorado.edu/physics/2000/quantumzone/bohr.html

The Modern Theory of the Atom

• Electrons travel in regions called “electron clouds”

• You cannot predict exactly where an electron will be found

http://www.fearofphysics.com/Atom/atom3.html

Energy Levels

The energy that an electron has is based on its location around the nucleus. (Electrons that are closer to the nucleus have less energy than those that are farther away from the nucleus)

How can bookshelves help you understand the movement of electrons?

•Each shelf represents an energy level

•Each book represents an electron

•You can move a book to a higher or lower shelf with the correct amount of energy.

•A book cannot be between shelves

(An electron can move by gaining or losing energy but can never be between energy levels)

How small are atoms?

THERE ARE 2 X 1022 ATOMS IN A PENNY. If all the atoms in a penny were blown up to the size of a grain of sand they would cover the entire state of California

What can a scanning tunneling electron

microscope show us?

• These images do not show an actual picture of an atom. They show a color-enhanced image of the surface of a material at the atomic level.

Image of hydrogen atom

The Universe and Atoms start here

• A Star Turned Inside Out

Large Hadron Collider

• LHC

• CERN – Interactive Game

Formative Assessment

• Kahoot on Atomic Structure 4-1

Closure

• Do theories in science stay the same? Explain.

• What do you now know about current atomic theory and how it developed?

Homework

• p. 105 Writing to Persuade Imagine you live in ancient Greece. Assume all you know about matter is what you can observe with your five senses. You have heard the views of both Democritus and Aristotle about matter. Write a paragraph supporting one of their views.