1. Unit 2 - Lecture 1: Structure of the Atom
Atomic Theory and what we understand today about…
Atoms, Molecules, and Ions

2. The Power of 10
Have you ever wondered your placement in the universe and the things that make it up? Picture this!

3. A HISTORY OF THE STRUCTURE OF THE ATOM
Unit 2 - Lecture 1: Structure of the Atom
A HISTORY OF THE STRUCTURE OF THE ATOM

4. Why weren’t Democritus’s ideas accepted?
Greek Philosopher Democritus ( B.C.):
all matter composed of small atoms
atomos = indivisible
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.
Why weren’t Democritus’s ideas accepted?
There was no evidence for against his prediction
It went against Aristotle’s popular beliefs

6. John Dalton
Demonstrated that atoms are the fundamental building blocks of matter
Performed many experiments to study how elements join together to form new substances
He found that they combine in specific ratios and he predicted it was because the elements are made of atoms.

7. Dalton's Postulates
1. Each element is composed of extremely small particles called atoms, which are indivisible and indestructible.
Figure 2.1 John Dalton ( )

8. Dalton's Postulates
2. All atoms of a given element are identical to one another in mass and other properties, but the atoms of one element are different from the atoms of all other elements.
Figure 2.1 John Dalton ( )

9. Dalton's Postulates
3. Atoms of an element are not changed into atoms of a different element by chemical reactions; atoms are neither created nor destroyed in chemical reactions.
Figure 2.1 John Dalton ( )

10. Dalton’s Postulates
4. Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms.

11. How do Dalton’s postulates relate to the Law of the Conservation of Mass?
Atoms of an element are not changed into different types of atoms by chemical reactions. Atoms are neither created nor destroyed in chemical reactions. This is the Law of Conservation of Mass.

12. John Dalton’s Atomic Theory
Unit 2 - Lecture 1: Structure of the Atom
John Dalton’s Atomic Theory
Almost right. A good start.
very small
Structure of the atom after Dalton (ca. 1810)

14. Unit 2 - Lecture 1: Structure of the Atom
J.J. Thomson (1897):
Cathode Rays
Atoms subjected to high voltages give off cathode rays

15. JJ Thomson: https://youtu.be/oddjdB0qfMg Teacher of Rutherford
Discovered the electron
A particle that has mass and a Negative Charge
The mass was 2000x smaller than the smallest known atom (Hydrogen)
Proposed the atom is divisible

16. Unit 2 - Lecture 1: Structure of the Atom
J.J. Thomson: Summarizing what he saw
Thomson concluded that the negative charges came from within the atom.
A particle smaller than an atom had to exist.
The atom was divisible!
Since the gas was known to be neutral, having no charge, he reasoned that there must be positively charged particles in the atom. But he could never find them.
Electrons are in atoms.

17. Unit 2 - Lecture 1: Structure of the Atom
J.J. Thomson – The Electron
“Plum pudding” model: Negative electrons are embedded in a positively charged mass.
Electrons (-)
Opposite electrical charges attract, and that is what holds the atom together.
Positively charged mass
Structure of the atom after Thomson (ca. 1900)

19. Radioactivity
Radioactivity is the spontaneous emission of radiation by an atom.

20. Radioactivity First observed by Henri Becquerel (1852-1908).
Marie and Pierre Curie also studied it.
Nobel Prize in 1903 (physics).

21. Unit 2 - Lecture 1: Structure of the Atom
Studies of Natural Radioactivity
Some atoms naturally emit one or more of the following types of radiation:
alpha (α) radiation (later found to be He2+ - helium nucleus)
beta (β) radiation (later found to be electrons)
gamma (γ) radiation (high energy light) α
Alpha particles
Electrons (-) γ γ
Positively charged mass α
Somehow gamma radiation is in there, too.
Structure of the atom after Becquerel (early 1900s)

22. Radioactivity
Three types of radiation were discovered by Ernest Rutherford:
 particles (positive, charge 2+, mass 7400 times of e-)
 particles (negative, charge 1-)
 rays (high energy light)
Figure 2.8

23. Unit 2 - Lecture 1: Structure of the Atom
Ernest Rutherford (1910)
Scattering experiment:
Rutherford’s fired a stream of tiny positively charged particles at a thin sheet of gold foil (2000 atoms thick)

24. The Nuclear Atom
Most of the positively charged “bullets” passed right through the gold atoms in the sheet of gold foil without changing course at all.
Some of the positively charged “bullets,” however, did bounce away from the gold sheet as if they had hit something solid. He knew that positive charges repel positive charges.
(video)
Figure 2.11

25. Rutherford’s ideas
Gold atoms in the sheet were mostly open space. Atoms were not a pudding filled with a positively charged material.
An atom had a small, dense, positively charged center that repelled his positively charged “bullets.”
He named this the“nucleus”
The nucleus is tiny compared to the atom as a whole.

26. Unit 2 - Lecture 1: Structure of the Atom
Ernest Rutherford
The Nucleus (and later the Proton)
The mass is not spread evenly throughout the atom, but is concentrated in the center, the nucleus.
The positively charged part in the atom is the nucleus
Electrons (-) are now outside the nucleus.
Structure of the atom after Rutherford (1910)