1. Unit #3 Atomic Structure
What is an Atom?
Unit #3 Atomic Structure

2. Review from 8th Grade OBJECTIVES: Identify the subatomic particles
Explain how the atomic number identifies an element.
Use the atomic number and mass number of an element to find the numbers of protons, electrons, and neutrons
Review key terms

3. Subatomic particles Relative mass Actual mass (g) Name Symbol Charge
Electron e- -1
1/1840
9.11 x 10-28
Proton p+ +1 1
1.67 x 10-24
Neutron n0 1
1.67 x 10-24

4. Counting the Pieces Atomic Number = number of protons in the nucleus
# of protons determines kind of atom (since all protons are alike!)
the same as the number of electrons in the neutral atom.
Mass Number = the number of protons + neutrons.
These account for most of mass

5. Counting the Pieces Protons: equal to atomic number
Neutrons: Mass Number – Atomic Number
Electrons: In a neutral atom equal to atomic number

6. Symbols
Contain the symbol of the element, the mass number and the atomic number.

7. Symbols
Contain the symbol of the element, the mass number and the atomic number.
Mass
number X
Atomic
number

8. F Symbols 19 9 Find the number of protons number of neutrons
number of electrons
Atomic number
Mass Number 19 F 9

9. Br Symbols 80 35 Find the number of protons number of neutrons
number of electrons
Atomic number
Mass Number 80 Br 35

10. Symbols
if an element has an atomic number of 34 and a mass number of 78 what is the
number of protons
number of neutrons
number of electrons
Complete symbol

11. Symbols if an element has 91 protons and 140 neutrons what is the
Atomic number
Mass number
number of electrons
Complete symbol

12. Symbols if an element has 78 electrons and 117 neutrons what is the
Atomic number
Mass number
Number of protons
Complete symbol

13. What if Atoms Aren’t Neutral
Ions: charged atoms resulting from the loss or gain of electrons

14. What if Atoms Aren’t Neutral
Anion: negatively charged ion; result from gaining electrons
Take the number of electrons in a neutral atom and add the absolute value of the charge
Identify:
Number of Protons
Number of Neutrons
Number of Electrons 81 1- Br 35

15. What if Atoms Aren’t Neutral
Cation: positively charged ion; result from the loss of electrons
Take the number of electrons in a neutral atom and subtract the value of the charge
Identify:
Number of Protons
Number of Neutrons
Number of Electrons 27 Al 3+ 13

16. Distinguishing Between Atoms
OBJECTIVES:
Explain how isotopes differ, and why the atomic masses of elements are not whole numbers.

17. Distinguishing Between Atoms
OBJECTIVES:
Calculate the average atomic mass of an element from isotope data.

18. Isotopes
Atoms of the same element can have different numbers of neutrons
Different mass numbers
Called isotopes

19. Naming Isotopes
We can also put the mass number after the name of the element.
carbon- 12
carbon -14
uranium-235

20. Atomic Mass How heavy is an atom of oxygen?
There are different kinds of oxygen atoms
We are more concerned with average atomic mass
Average atomic mass is based on abundance of each element in nature.
We don’t use grams because the numbers would be too small

21. Measuring Atomic Mass Unit is the Atomic Mass Unit (amu)
It is one twelfth the mass of a carbon- 12 atom
Each isotope has its own atomic mass, thus we determine the average from percent abundance

22. Calculating averages
Convert the percent to a decimal (move the decimal 2 places to the left or divide by 100)
Multiply the atomic mass by it’s percent (expressed as a decimal), then add the results.

23. Calculating Averages
Calculating the Average Atomic Mass is similar to calculating your grade in this class
Percent of Grade
Student's Average
Tests 35% 89
Quizzes 25% 95
Labs 20% 35
Homework 20%
100

24. Calculating Averages
If Bromine-79 has an abundance of 50.69% and Bromine-81 has an abundance of % what is the average atomic mass of Bromine?
Why is it better to use the mass of an individual isotope for the mass number rather than round the number from the periodic table?

25. Atomic Mass
Magnesium has three isotopes % magnesium-24 with a mass of amu, 10.00% magnesium-25 with a mass of amu, and the rest magnesium- 26 with a mass of amu. What is the atomic mass of magnesium?
If not told otherwise, the mass of the isotope is the mass number in amu

26. Atomic Mass Is not a whole number because it is an average.
are the decimal numbers on the periodic table.

27. History of the atom Not the history of atom, but the idea of the atom.
Original idea Ancient Greece (400 B.C.)
Democritus and Leucippus- Greek philosophers.

28. Democritus and Leucippus
Leucippus was the first person to come up with the idea of the atom
Democritus was a student of Leucippus and expanded on his idea

29. Democritus Democritus added:
Matter is composed of atoms which move through empty space
Atoms are solid, homogeneous, indestructible, and indivisible
Different atoms have different shapes and sizes
The size, shape, and movement of atoms determine their properties

30. Greek Aristotle - Famous philosopher
All substances are made of 4 elements
Fire - Hot
Air - light
Earth - cool, heavy
Water - wet
Blend these in different proportions to get all substances

31. Aristotle
There was no scientific evidence to back up either Democritus or Aristotle, but people believed Aristotle based on reputation
Aristotle’s theory persisted for 2,000 years

32. Leading to the modern theory
Late 1700’s - John Dalton- England.
Teacher- summarized results of his experiments and those of others.
Dalton’s Atomic Theory
Combined ideas of elements with that of atoms.
Saw atoms as small solid spheres. Billiard Ball Model.

33. Dalton’s Atomic Theory
All matter is made of tiny indivisible particles called atoms.
Atoms of the same element are identical, those of atoms of different elements are different.
Atoms of different elements combine in whole number ratios to form compounds.
Chemical reactions involve the rearrangement of atoms. No new atoms are created or destroyed.

34. Laws Related to Atomic Theory
Law of Conservation of Mass- Antoine Lavoisier; states that matter is neither created nor destroyed in chemical reactions
Law of Definite Proportions- Joseph Proust; states that all compounds contain the same elements in the same ratio
Law of Multiple Proportions- John Dalton; elements combine in small whole number ratios

35. Just How Small Is an Atom?
Think of cutting a piece of lead into smaller and smaller pieces
How far can it be cut?
An atom is the smallest particle of an element that retains the properties of that element
Atoms-very small

36. Structure of the Nuclear Atom
OBJECTIVES:
Distinguish among protons, electrons, and neutrons in terms of relative mass and charge.
Describe the structure of an atom, including the location of the protons, electrons, and neutrons with respect to the nucleus.

37. Parts of Atoms J. J. Thomson - English physicist. 1897
Made a piece of equipment called a cathode ray tube.
It is a vacuum tube - all the air has been pumped out.

38. Thomson’s Experiment
Voltage source - +
Vacuum tube
Metal Disks

39. Thomson’s Experiment
Voltage source - +

40. Thomson’s Experiment
Voltage source - +

41. Thomson’s Experiment
Voltage source - +

42. - + Thomson’s Experiment
Voltage source - +
Passing an electric current makes a beam appear to move from the negative to the positive end

43. - + Thomson’s Experiment
Voltage source - +
Passing an electric current makes a beam appear to move from the negative to the positive end

44. - + Thomson’s Experiment
Voltage source - +
Passing an electric current makes a beam appear to move from the negative to the positive end

45. - + Thomson’s Experiment
Voltage source - +
Passing an electric current makes a beam appear to move from the negative to the positive end

46. Thomson’s Experiment
Voltage source
By adding an electric field

47. Thomson’s Experiment
Voltage source + -
By adding an electric field

48. Thomson’s Experiment
Voltage source + -
By adding an electric field

49. Thomson’s Experiment
Voltage source + -
By adding an electric field

50. Thomson’s Experiment
Voltage source + -
By adding an electric field

51. Thomson’s Experiment
Voltage source + -
By adding an electric field

52. Thomson’s Experiment
Voltage source + -
By adding an electric field he found that the moving pieces were negative

53. Plum Pudding Model Proposed by JJ Thomson
Said the atom had a uniform positive charge in which the negatively charged electrons resided

54. Other particles
Proton - positively charged pieces times heavier than the electron – by E. Goldstein
Neutron - no charge but the same mass as a proton – by J. Chadwick
Where are the pieces?

55. Millikan’s Oil Drop
JJ Thomson used the cathode ray to find the mass to charge ratio of an electron.
He did not know the mass or charge, only the ratio
Millikan’s Oil Drop experiment determined the charge of an electron

56. Millikan’s Experiment
Atomizer
Oil droplets - +
Oil
Telescope

57. Millikan’s Experiment
X-rays
X-rays give some electrons a charge.

58. Millikan’s Experiment
From the mass of the drop and the charge on
the plates, he calculated the mass of an electron
Some drops would hover

59. Rutherford’s experiment
Ernest Rutherford -English physicist. (1910)
Believed in the plum pudding model of the atom
Wanted to see how big they are.
Used radioactivity.
Alpha particles - positively charged pieces- helium atoms minus electrons
Shot them at gold foil which can be made a few atoms thick.

60. Rutherford’s experiment
When an alpha particle hits a fluorescent screen, it glows.
Here’s what it looked like

61. Fluorescent
Screen
Lead block
Uranium
Gold Foil

62. He Expected
The alpha particles to pass through without changing direction very much.
Because…?
…the positive charges were thought to be spread out evenly. Alone they were not enough to stop the alpha particles.

63. What he expected

64. Because

65. He thought the mass was evenly distributed in the atom

66. Since he thought the mass was evenly distributed in the atom

67. What he got

68. How he explained it + Atom is mostly empty.
Small dense, positive piece at center.
Alpha particles are deflected by it if they get close enough. +

69. +

70. Density and the Atom
Since most of the particles went through, it was mostly empty space.
Because the pieces turned so much, the positive pieces were heavy.
Small volume, big mass, big density.
This small dense positive area is the nucleus.

71. Bohr Model
Bohr worked with the concepts of energy, wavelength and frequency
Each color of light is associated with a different energy
Each atom gives off its own unique color so..
Electrons of diiferent atoms have different energies

72. Bohr Model Each atom has its own specific electron arrangement
Electrons are in Energy levels in the atom
When an electron goes from an excited state back down to its ground state the atom emits light

73. The Bohr Ring Atom
n = 4
n = 3
n = 2
n = 1

74. The Bohr Model Doesn’t work. Only works for hydrogen atoms.
Electrons don’t move in circles.
The quantization of energy is right, but not because they are circling like planets.
Back to the drawing board

75. The Quantum Mechanical Model
A totally new approach.
De Broglie said matter could be like a wave, like standing waves.
The vibrations of a stringed instrument.
More on this in Unit # 4

76. Models of the Atom to Know
Daltons
Thomsons
Rutherford
Bohr
Quantum