1. The Structure of the Atom And Electrons in Atoms
Chapters 4 and 5
The Structure of the Atom
And
Electrons in Atoms

2. Early Theories of Matter
Democritus ( B.C.)
Named atom (atomos)
What holds atoms together?

3. Early Theories of Matter
Aristotle ( B.C.)
Atoms move through empty space?
There is no “nothingness”

4. Early Theories of Matter
John Dalton ( )
First Atomic Theory
Schoolteacher
Atoms are divisible into subatomic particles
Atoms of the same element have slightly different masses

5. Defining an Atom
The smallest particle of an element that retains the properties of the element.
About 1 X m in diameter.
Can be seen with a scanning tunneling microscope.
Nanotechnology – molecular manufacturing – atom-by-atom building of machines the size of molecules

6. Discovering the Electron
William Crookes (1800’s)
Vacuum tube – saw flash of light in a darkened room
Flash due to light-producing coating – led to television and computer monitors
Flash called a cathode rays – travel from cathode to anode & negatively charged (late 1800’s)
Changing electron and gas inside tube doesn’t change ray – these negative particles are found in all matter.

7. Discovering the Electron
J.J. Thomson (late 1890’s)
Determined the charge-to-mass ratio
Mass must be less than a hydrogen atom
Plum Pudding Model of atom
Plum pudding Model

8. Discovering the Electron
Robert Millikan (1909)
Determined charge of
electron
1/1840 mass of a
hydrogen atom

9. The Nuclear Atom
Ernest Rutherford (1911)

10. The Nuclear Atom Atom contains: Creates nuclear model of atom
Mostly empty space
Tiny, dense nucleus which is positively charged
Creates nuclear model of atom

11. Other Subatomic Particles
Rutherford (1920)
Concluded nucleus contains proton
Proton as equal but opposite charge of electron
James Chadwick (1932)
Discovered neutron
Neutron has no charge
Chadwick was Rutherford’s coworker

12. Subatomic Particles

13. How Atoms Differ Moseley (shortly after Gold Foil)
Atoms of each element contain a unique number of protons
Atomic Number= #protons
Identifies the atom
For neutral atoms only: # protons = # electrons
Atomic number is always a whole number

14. Isotopes
Isotopes – atoms that contain the same number of protons but different number of neutrons.
Most elements contain a mixture of isotopes.
The relative abundance of each isotope is constant.
Ex: potassium 93.25% have 20 neutrons, % have 22 neutrons, % have 21 neutrons

15. Isotopes
Mass Number = #protons + #neutrons

16. Simple Practice Mg 25 12 Zn 30 35 Be 4 9 Hg 120 80 Atomic Number Mass
# of
Protons
# of neutrons
# of electrons Mg 25 12 Zn 30 35 Be 4 9 Hg
120 80

17. Mass of Atoms Atomic mass unit – 1/12 of a carbon-12 atom.
Atomic Mass – weighted average mass of the isotopes of that element.
1 proton = amu
1 neutron = amu
1 electron = amu

18. Calculating Atomic Masses
6X has mass of amu and abundance of 7.50%. 7X has mass of amu and abundance of 92.5%.
(6.015)(.0750) + (7.016)(.925) = 6.94 amu

19. More Challenging Problems!
Cu-63 has a mass of amu and an abundance of 69.17%. Find the mass and abundance of the other isotope.
Boron has two isotopes with the masses of amu and amu. Find the abundance of each isotope.

20. Radioactivity Nuclear Reactions – changes an atom’s nucleus.
Atom changes into a new element
Due to unstable nuclei
Radiation contains rays and particles emitted from a radioactive material.
Radioactive decay is the spontaneous emission of radiation.

21. Types of Radiation

22. Types of Radiation Radiation Type Symbol Mass (amu) Charge Alpha  or4 2+
Beta
e- or
1/1840 1-
Gamma
00

23. Nuclear Reactions
Mass numbers and Atomic numbers on both sides of the reaction must be equal
Practice Problem:

24. Chapter 5
Electrons in Atoms

25. Electromagnetic Radiation
Electromagnetic Radiation is a form of energy that has wave-like behavior.
4 properties of waves: wavelength, amplitude, speed and frequency.
Scientists need to explain behaviors of elements: emit light (color) when heated.

26. Properties of Waves
Frequency()- number of waves that pass a given point per second. (hertz or 1/s or s-1)
Speed (c)- is constant for all waves. 3 x 108 m/s

27. Calculating Properties of Waves
What is the frequency of light with a wavelength of 5.80 x 10-7 m?
A radio station broadcasts with a frequency of MHz. What is the wavelength of the broadcast?

28. Particle Nature of Light
Max Planck (1900) discovered that matter can gain or lose energy in small, specific amounts called quanta.
Equantum= h
Planck’s Constant (h)=6.626 x 10-34J·s
Wave model of light can not explain why heated objects only emit certain frequencies of light.

29. Practice Problems
What is the energy of a wave with a frequency of 6.25 x 1019Hz?
What is the frequency of a wave that contains 8.64 x 10-18J of energy?
A wave contains 4.62 x 10-15J of energy. Determine its wavelength.

30. Photoelectric Effect
Photoelectric effect – electrons are emitted from a metal’s surface when light of a certain frequency shines on it.
Frequency (color) of light, not brightness of light determines if electrons are emitted.
Einstein (1905)- light has wave-like properties but is also a stream of tiny particles or bundles of energy called photons.
Photon – a piece of EM with no mass and carries a quantum of energy.

31. Atomic Emission Spectrum
When atoms absorb energy they become excited.
Atomic Emission Spectrum- unique set of frequencies emitted by excited atoms.

32. Bohr Model of the Atom
Bohr (1913) proposed why the emission spectrum of hydrogen is not continuous.
Electrons can have only certain “energy states”
Ground State - the lowest allowable energy state.
Excited State – energy state of an electron when it gains energy

33. Bohr Model of the Atom
Model did not work well for atoms other than hydrogen.

34. Electrons as Waves
Louis de Broglie (1924) thought Bohr’s model had electrons having similar properties to waves.
de Broglie equation:
Predicts that all moving particles have wave properties.

35. Heisenberg Uncertainty Principle
When viewing an electron, a photon of light hits it and changes the velocity and position of the electron.
It is impossible to know precisely both the velocity and position of a particle at the same time.

36. Quantum Mechanical Model of the Atom
Schrödinger (1926) derived an equation that treated hydrogen’s electron as a wave.
Allows electron to have only certain energy but does not give path of electron.
Atomic orbital – a 3-D region around the nucleus in which the electron can be found 90% of the time.