1. History of the Atom Dalton’s Theory (1803)

2. John Dalton’s Theory
All matter is made up of tiny, indestructible particles called atoms
All atoms of a given element have identical physical and chemical properties
Atoms are neither created nor destroyed (law of conservation of mass)
Atoms of different elements form compounds in whole-number ratios
Example: the formula for water is H2O not H2.1O1.3

3. Dalton’s Theory Some of these postulates now have exceptions
Atoms can be broken apart in nuclear reactions
Atoms of a given element can have different physical and chemical properties (isotopes)

4. J.J. Thomson’s Plum-Pudding Model (1897)
His model portrays the atom as a big ball of positive charge that contains small particles of negative charge embedded in it

5. Thomson’s Model (Honors Only)
Discovered the charge of an electron by observing cathode rays in a cathode ray tube

6. Thomson’s Model (Honors Only)
From his observations, he concluded that cathode rays are streams of negatively charges particles with mass
Another scientist, Millikan, was able to determine the mass of an electron based upon Thomson’s work (1909)

7. Rutherford’s Model (1909)
Rutherford was able to make two key observations and conclusions based on his “gold-foil” experiment
He disproved Thomson’s model

8. Rutherford’s Model The Experiment
He bombarded a thin piece of gold foil with positively charged alpha particles (much smaller than the atom)

9. Rutherford’s Model Observations
Almost all the alpha particles passed through foil without deflection
Small percent slightly deflected
Some were largely deflected
A few even reflected back in the direction from where they had come
Demo

10. Rutherford’s Model of the Atom
Conclusions (1911)
Atom is mostly empty space
All the positive charge in an atom is concentrated in a small area (+ dense nucleus)
this area is positive since positively charges particles were deflected from it (like repels like)

11. Bohr Planetary Model 1913
Model displayed electrons traveling in orbits around the nucleus
Electrons are only found in orbits (principle energy levels) not in between

12. Bohr Model
The PEL approximates how far the electron is from the nucleus
PEL (n)
Shell
Max # of Electrons 2(n)2 1 K 2 L 8 3 M 18 4 N 32 5 O 50 6 P 72 7 Q 98

13. Bohr Model
The electrons distance from the nucleus is related to their specific amount of energy (quanta)
As you move away from the nucleus , the energy in each PEL increases

14. Bohr Model
Ground State – when electrons are in their lowest energy level
Quantum Leap
When electrons jump between energy levels
Electrons can only absorb a fixed amount of energy (quanta) to move to a higher energy level
Electrons can only jump to levels that are not completely filled with electrons
Heat, light, and electricity are all stimuli that can excite an electron

15. Bohr Model Excited State Electrons are in higher energy levels
Acquired when an electron absorbs energy and becomes unstable
Electrons quickly return to ground state, emitting the same amount of energy absorbed, usually in the form of light
Every element gives off a unique pattern of colors (line spectrum) which can be used to identify the element

16. Bohr Model
Spectrum of different elements

17. Bohr Model
Line spectrums are observed with an instrument called a spectroscope

19. Electron Configurations
An electron configuration tells you how many electrons there are in each energy level.
Mg (2-8-2) has 2 electrons in PEL 1, 8 electrons in PEL 2, and 2 electrons in PEL 3
The amount of numbers in the electron configuration tells you how many energy levels are occupied with electrons
Mg (2-8-2) has 3 energy levels containing electrons

20. Valence Electrons
Are the electrons in the outermost energy level of an atom (last # in the electron configuration).
Example: Al (2-8-3) has 3 valence electrons
Valence electrons determine the chemical properties of an element.

21. The Kernel Includes the nucleus and all non-valence electrons
What element is this?
What is the electron configuration?
How many valence electrons? Kernel?

22. Electron Configurations of Excited State Atoms
Cl Ground State
(17 e-)
Cl Excited State
2-7-8 (17 e-)
(17 e-)
Only jump one electron
Electrons can only jump into energy levels that aren’t filled
You are not losing or gaining any electrons, the total amount of electrons is the same

23. Excited State Electron Configurations
Determine a possible excited state electron configuration for: Mg Li S F
Answers:
Mg: or 2-7-3
Li: 1-2 or 2-0-1
S: or
F: or 1-8

24. Electron Configurations for Ions Vs. Atoms
Mg Atom 2-8-2
Mg Ion 2-8

25. Electron Configurations of Atoms vs. Ions
Find the electron configurations of the following ions
K+1
Al+3
P3-
Answers:
2-8-8
2-8