1. Chapter 4:
Structure of the Atom

2. 4.1 Early theories and 4.2 Defining the atom
Historical Background:
Models of the Atom: -see reference chart
(On Pg. 10 of your packet)

3. 4.1 Early theories and 4.2 Defining the atom

4. 4.1 Early theories and 4.2 Defining the atom
1. Atomists and Democritus
Greeks approx 2,500 years ago
Matter was made up of atoms “atomos” or “Indivisible” particles
Seashell experiment—broken into smaller & smaller pieces

5. 4.1 Early theories and 4.2 Defining the atom
2. John Dalton
; returned to theory of atoms
Atoms are like billiard balls (solid spheres) which cannot be broken down further
4 major postulates
1) All elements are composed of atoms
2) Atoms of the same element are identical
3) Atoms can physically mix or chemically combine in simple whole number ratios
4) Reactions occur when atoms separate, join, or rearrange

6. 4.1 Early theories and 4.2 Defining the atom
3. William Crookes
developed Crookes tube (CRT) in 1870’s
first evidence for existence of electrons because you could “see” electrons flow and confirm their existence.
tube is precursor to today’s TV picture tubes

7. 4.1 Early theories and 4.2 Defining the atom
4. J.J. Thomson
discovered electron in 1897
discovered positively charged particles surrounded by electrons
found the ratio of the charge of an electron to its mass to be 1/1837

8. 4.1 Early theories and 4.2 Defining the atom
4. J.J. Thomson

9. 4.1 Early theories and 4.2 Defining the atom
4. J.J. Thomson
cathode ray tube experiments – advancement of Crookes tube
“plum-pudding model”

10. 4.1 Early theories and 4.2 Defining the atom
5. Ernest Rutherford
Discovered nucleus (dense core of atom) in 1911
Gold foil experiments
Quote from E.R.’s Lab Notebook
“It is about as incredible as if you had fired a
15-inch shell at a piece of tissue paper and it came back and hit you.” -ER

11. 4.1 Early theories and 4.2 Defining the atom
Video Clip: Rutherford Gold Foil Experiment

12. 4.1 Early theories and 4.2 Defining the atom
6. Robert Milliken
Oil drop experiment
determined the charge and mass of an electron
Video Clip: Milliken Oil Drop Experiment

13. 4.1 Early theories and 4.2 Defining the atom
7. James Chadwick
discovered the neutron (no charge, but same mass as proton)
Neutrons help disperse the strong repulsion of positive charges
Relative Sizes
Nucleus diameter = 10-5 nm
Atom diameter = 10-1 nm
Nucleus = basketball --> Atom = 6 miles wide!

14. 4.1 Early theories and 4.2 Defining the atom
8. Niels Bohr
improved on Rutherford’s work
“planetary model”- positive center is surrounded by electrons in defined orbits circling the center

15. 4.1 Early theories and 4.2 Defining the atom
defined the following:
energy level – the location where an electron is found at a set distance from the nucleus dependent on the amount of energy it has
ground state – the typical energy level where an electron is found; lowest energy
excited state – an energy level higher than the ground state for an electron; temporary condition

16. 4.1 Early theories and 4.2 Defining the atom
9. Quantum Mechanical Model
Erwin Schroedinger; Mathematical model
Electron locations are based on probability
Electrons are not particles, but waves!
Defined:
Orbital – region where an electron is likely to be found 90% of the time

17. 4.3 How atoms differ Atoms – vocabulary and classifications
Atom – the smallest particle of matter that retains its properties.
can “see” individual atoms with a scanning tunneling microscope.

18. 4.3 How atoms differ
Subatomic particles – the component parts of an atom: proton, neutron, and electron

19. 4.3 How atoms differ
Ion- atom with the same number of protons but a different number of electrons.
If the atom has a (+) charge it has fewer electrons than protons, If the atom has a (-) charge it has more electrons than protons.

20. 4.3 How atoms differ Subatomic Particle Mass and Abbreviation Charge
Location
Discoverer
Proton
1 amu, p+ +1
Nucleus
None
Neutron
1 amu, n
Chadwick in 1932
Electron
Almost zero, e- -1
Electron cloud
Thomson

21. 4.3 How atoms differ Calculations involving Subatomic Particles:
atomic number = # of protons
mass number = # of protons + # of neutrons
(neutral atom): # of protons = # of electrons
(charged ion): charge = #p+ - #e-

22. 4.3 How atoms differ Isotopes and Calculations:
Isotope – atoms of the same element with different numbers of neutrons
Atomic mass – weighted average of the masses of all the isotopes of an element

23. X 4.3 How atoms differ Isotope (Isotopic Notation) Mass # Z
Atomic Symbol A
Atomic #
Example: Uranium-238

24. 4.3 How atoms differ Isotope Problems:
Multiply the mass number of the isotope by the decimal value of the percent for that isotope
Add the relative masses of all of the isotopes to get the atomic mass of the element

25. 4.3 How atoms differ Example:
If 90% of the Beryllium in the world has a mass number of 9 and only 10% has a mass number of 10, what is the atomic mass of Beryllium?

26. 4.4 Unstable Nuclei and Radioactive Decay
Vocabulary
Radioactivity-the spontaneous emission of radiation from substances
Nuclear reactions- changes in an atom’s nucleus
Radiation-rays and/or particles emitted from radioactive material

27. 4.4 Unstable Nuclei and Radioactive Decay
Types of Radiation
Alpha radiation -stream of high energy alpha particles
alpha particles consist of 2 protons and 2 neutrons and are identical to helium-4 nucleus.
symbol He 2+ 2
not much penetrating power, travel a few centimeters, stopped by paper, no health hazard

28. 4.4 Unstable Nuclei and Radioactive Decay
mass number decreases by 4 atomic number by 2
alpha decay: Ra  222Ra + 4 He
Example: Uranium-238

29. 4.4 Unstable Nuclei and Radioactive Decay
Beta radiation
high speed electrons
To form beta radiation a neutron splits into a proton and an electron
The proton stays in nucleus and the electron propels out at high speed.
Symbol 0e- 0e- 0B
100 times more penetrating then alpha, pass through clothing to damage skin

30. 4.4 Unstable Nuclei and Radioactive Decay
Beta decay: 131I  Xe + 0B
Example: Astatine-220

31. 4.4 Unstable Nuclei and Radioactive Decay
Gamma radiation
similar to X rays
doesn’t consist of particles
symbol: 0
penetrates deeply into solid material, body tissue, stopped by Pb or concrete, dangerous
usually emitted with alpha and beta radiation
no mass or electrical charge
emission of gamma rays by themselves cannot result in the formation of a new atom

32. 4.4 Unstable Nuclei and Radioactive Decay
Practice:
What is the alpha decay of plutonium-250?

33. 4.4 Unstable Nuclei and Radioactive Decay
2. What is the beta decay of Carbon-14?

34. 4.1 Early theories and 4.2 Defining the atom

35. End of Chapter 4!