1. Atoms
and
Molecules

2. Hmmmm… Imagine cutting a gold bar in half. Then cut in half again.
…and again.
EVERYBODY WRITES: Could you keep cutting that bar in half forever? Why or why not?
How small could you cut up that gold and it still be gold?

3. Atoms, Molecules The Early History of Chemistry Before 16th Century
Greeks were the first to attempt to explain why chemical changes occur.
Alchemy: Attempts to change cheap metals into gold. They invented the idea of atoms, that matter is not continuous. They discovered several elements and learned to prepare mineral acids.

4. The Early History of Chemistry
16th Century
German develop the systematic metallurgy (extraction of metal from ores)
Swiss develop the medicinal application of minerals
17th Century
Robert Boyle: First chemist to perform quantitative experiments

5. Law of Conservation of Mass (Lavoisier)
16 X
8 Y +
8 X2Y
Law of Conservation of Mass: Mass is neither created nor destroyed in chemical reactions.

6. Dalton’s Atomic Theory
John Dalton published his atomic theory in 1803
Elements are made up of tiny particles called atoms.
Each element is characterized by the mass of its atoms. Atoms of the same element have the same mass, but atoms of different elements have different masses.
The chemical combination of elements to make different chemical compounds occurs when atoms join in small whole-number ratios.
Chemical reactions only rearrange how atoms are combined in chemical compounds; the atoms themselves don’t change.

7. atom – The smallest particle that can
exist on its own.
compound – Substance made up of two or more different elements that are chemically joined together.
element – Substance made up
of only one type of atom.

8. Let’s See What you Know…
In our previous lesson we learned that everything is made of ____?
If you look closely at the tip of a sharpened pencil, you will see that it is made of a specific type of matter called _______.
The image shows a close up picture of the graphite. The graphite is made up of even smaller particles called _____.
These atoms particles contain even smaller particles called ______________.

9. Discovery of the Electron-Thomson 1897

10. Cathode Ray Tube J.J. Thomson, measured mass/charge of e-
(1906 Nobel Prize in Physics)

11. Cathode Ray Tube
Streams of negatively charged particles were found to emanate from cathode tubes.
Maybe atoms weren’t completely indivisible after all.

12. Deflection of Cathode Rays by an Applied Electric Field

13. Thomson Atomic Model (1903)
An atom consists of a diffuse cloud of positive charge with the negative electrons embedded randomly in it. This model is often called plum (or raisin) pudding model.
Observed cathode ray (produced at the negative electrode and repelled by the negative pole of an applied electric field.
Cathode ray was a stream of negatively charged particles now called electrons

14. The Plum Pudding Model of the Atom

15. Milikan measured charge of electron Univ. Chicago (1909).
Measured mass of e-
(1923 Nobel Prize in Physics)
e- charge = x C
e- mass = 9.10 x g

16. Types of Radioactivity
Studied by Marie and Pierre Curie
“rays” not particles
particles of some sort.
(uranium compound)

17. Rutherford’s Gold Foil Experiment
In Rutherford’s gold foil experiment ( ),positively
charged particles
were aimed at atoms of gold.
mostly went straight through the atoms.
were deflected only occasionally.
Conclusion:
There must be a small, dense, positively charged nucleus in the atom that deflects positive particles that come close.
Rutherford proposed that the atom must consist mainly of empty space with the mass concentrated in a tiny central core—the nucleus

18. Expected and Actual Results of Rutherford’s Experiment

19. Rutherford’s Gold Foil Experiment
In Rutherford’s gold foil experiment ( ),positively
charged particles
were aimed at atoms of gold.
mostly went straight through the atoms.
were deflected only occasionally.
Conclusion:
There must be a small, dense, positively charged nucleus in the atom that deflects positive particles that come close.
Rutherford proposed that the atom must consist mainly of empty space with the mass concentrated in a tiny central core—the nucleus

20. Protons were discovered by Rutherford in 1919
Protons were discovered by Rutherford in Have the positive charge in the atom.
Neutrons were discovered by James Chadwick in Have mass like proton, but no charge.

21. Atomic Structure: Protons and Neutrons
Chemistry: McMurry and Fay, 6th Edition
Chapter 2: Atoms, Molecules, and Ions
8/29/2019 4:37:40 PM
Atomic Structure: Protons and Neutrons
The charge of the proton is opposite in sign but equal to that of the electron.
mass p ≈ mass n ≈ 1840 x mass e-
Copyright © 2011 Pearson Prentice Hall, Inc.

23. How big is an atom?

24. Where Are the Electrons?
Bohr’s Electron Levels In 1913, Niels Bohr proposed that electrons move around the nucleus in certain paths, or energy levels.
Modern View of Atom – Electron Cloud
This is a bit more accurate because we have no idea of where the electrons are at any one time – they are in a cloud called the electron cloud.