1. Atoms and their structure
Chapter 4
Atoms and their structure

2. History of the atom
Not the history of atom, but really the idea of the atom
The original idea - Ancient Greece (400 B.C..)
Democritus and Leucippus Greek philosophers who were debating whether matter was continuous or discontinuous.
Continuous – divide matter forever and always have a smaller piece of matter.
Discontinuous – divide matter only so far and can go no farther.

3. History of Atom
Start with a box of marble divide in half eventually you get down to one marble which if you divide again you no longer have a marble.
The Greek word for “can not cut” is atomos – thus atom.

4. Another Greek Aristotle – another famous Greek philosopher
All substances are made of 4 elements
Fire - Hot
Air - light
Earth - cool, heavy
Water - wet
Aristotle and others believed in 4 elements of matter, combined in different proportions rather than indivisible particles

5. Who Was Right?
Greek society was slave based and so it was beneath the famous to work with their hands.
The Greeks did not experiment, they settled disagreements by argument (debates).
Aristotle was more famous so his ideas carried through to the middle ages.
During the middle ages Alchemists tried to change lead to gold

6. Re-emergence of the Atomic Theory
Late 1700’s - John Dalton- England (a major contributor to today’s Atomic Theory)
A teacher who summarized results of his experiments and those of other’s
In Dalton’s Atomic Theory he combined ideas of elements with that of atoms

7. Dalton’s Atomic Theory
All matter is made of tiny indivisible particles called atoms.
Atoms of the same element are identical, those of different atoms 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.

8. Law of Definite Proportions
Each compound has a specific whole-number ratio of elements; ratio is by mass [Definite Proportions]
Water
H2O
Carbon dioxide
CO2
Methane
CH4
8.0 g oxygen reacts with 1.0 g hydrogen (H2O)
Ratio = 8:1 by mass

9. Law of Multiple Proportions
if two elements form more that one compound, the ratio of the second element that combines with 1 gram of the first element in each is a simple whole number.
In hydrogen peroxide 16.0 g oxygen reacts with 1.0 g hydrogen (H2O2)
Ratio of the masses of oxygen in hydrogen peroxide and water is 16:8 = 2:1 Therefore H2O2 contains twice as many oxygen atoms per hydrogen atom than H2O [Multiple Proportions]

10. What? Water is 8 grams of oxygen per gram of hydrogen. (H2O)
Hydrogen Peroxide is 16 grams of oxygen per gram of hydrogen. (H2O2)
16 to 8 is a 2 to 1 ratio
Therefore H2O2 contains twice as many oxygen atoms per hydrogen atom than H2O [Multiple Proportions]
True because you have to add a whole atom, you can’t add a piece of an atom.

11. Parts of Atoms J. J. Thomson - English physicist. 1897
As scientists began to develop methods for more detailed probing of the nature of matter, the atom (supposedly indivisible) began to show signs of a more complex structure
J. J. Thomson - English physicist. 1897
Made a piece of equipment called a cathode ray tube which was used to study the electrical conductivity of gasses.
The cathode ray tube is a vacuum tube - the air has been pumped out.

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

13. Thomson’s Experiment
Voltage source - +

14. Thomson’s Experiment
Voltage source - +

15. Thomson’s Experiment
Voltage source - +

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

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

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

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

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

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

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

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

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

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

26. Thomson’s Experiment Voltage source + -
By adding an electric field he found that the moving pieces were negative because the rays were attracted to the positive electrode in the external field.

27. Cathode Ray Tube

28. Thompson concluded that:
Cathode rays consist of beams of particles
The particles have a negative charge
Based on his findings, a new fundamental particle of matter was discovered – The Electron!

29. Thomsom’s Model Couldn’t find positive (for a while)
Said the atom was like plum pudding or (blueberry muffin)
A bunch of positive stuff, with the electrons embedded (able to remove the embedded electrons)

30. Other pieces
Proton - positively charged pieces 1840 times heavier than the electron
Neutron - no charge but the same mass as a proton.
Where are the pieces?

31. Rutherford’s experiment
Ernest Rutherford English physicist. (1910)
Believed in the plum pudding model of the atom.
Used radioactivity
Alpha particles - positively charged pieces given off by uranium
Shot them at gold foil which can be made a few atoms thick

32. Rutherford’s experiment
When the alpha particles hit a florescent screen, it glows.
Here’s what it looked like

33. Florescent
Screen
Lead block
Uranium
Gold Foil

34. He Expected
Rutherford believed that the a particles would pass through unhindered. (The alpha particles to pass through without changing direction very much.)
Because the positive charges were spread out evenly within the atom. The positive charge in the atom was not enough to stop the alpha particles

35. What he expected

36. Because

37. Because, he thought the mass was evenly distributed in the atom
a particles

38. Because, he thought the mass was evenly distributed in the atom

39. What he got
A small percentage of the particles were being reflected at unexpected angles, inconsistent with the “muffin model”

40. 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 +

41. + a particles Rutherford explained his observations as follows:
Atom is mostly empty space
Small, dense, and positive at the center
Alpha particles were deflected if they got close enough +
a particles

42. Modern View The atom is mostly empty space Two regions
Nucleus- protons and neutrons
Electron cloud- region where you might find an electron

43. Density and the Atom
Since most of the alpha particles went through, the atom is mostly empty.
Because the alpha particles turned so much, the positive region of the atom is heavy.
Small volume, big mass, big density
This small dense positive area is the nucleus

44. 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 nº 1
1.67 x 10-24

45. Structure of the Atom There are two regions
The nucleus – with protons and neutrons so that it has a Positive charge and almost all the mass
Electron cloud- Most of the volume of an atom and is the region where the electron can be found (extra nuclear)

46. Size of an atom
Atoms are small and are measured in picometers, meters
Hydrogen atom, 32 pm radius
Nucleus tiny compared to atom. If the atom was the size of a stadium, the nucleus would be the size of a marble.
Radius of the nucleus near 10-15m.
Density near 1014 g/cm3

47. Counting the Pieces Atomic Number = number of protons (p+)
The number of protons determines kind of atom – 2 protons in the nucleus means that this is a Helium atom.
Chemists use Z as a symbol for atomic number.
In a neutral atom there is the same number of electrons (e-) and protons (atomic number)
Mass Number = number of protons + neutrons [Sum of p+ and nº (p+ + nº)] The symbol used for mass number is A.
The neucleons (p+ and nº) make up the mass of the atom.

48. Isotopes
Dalton was wrong when he said that all atoms of one element are the same.
Atoms of the same element can have different numbers of neutrons and therefore have different mass numbers and different masses.
The atoms of the same element that differ in the number of neutrons are called isotopes of that element.

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

50. E Z Nuclear Symbols A Contain: the mass number the atomic number
the symbol of the element
the atomic number Z

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

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

53. 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 nuclear symbol

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

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

56. Naming Isotopes Put the mass number after the name of the element
carbon- 12
carbon -14
uranium-235

57. Atomic Mass How heavy is an atom of oxygen?
There are different kinds of oxygen atoms.
More concerned with average atomic mass.
Based on abundance of each isotope in nature.
Don’t use grams because the numbers would be too small

58. Measuring Atomic Mass Unit is the Atomic Mass Unit (amu)
One twelfth the mass of a carbon-12 atom.
Each isotope has its own atomic mass we need the average from percent abundance.

59. Calculating averages
You have five rocks, four with a mass of 50 g, and one with a mass of 60 g. What is the average mass of the rocks?
Total mass = x x 60 = 260 g
Average mass = 4 x x 60 = 260 g
Average mass = 4 x x 60 = 260 g

60. Calculating averages Average mass = .8 x 50g + .2 x 60g = 52g
Average mass = 4 x 50g + 1 x 60g = 260 g = 52g
Average mass = .8 x 50g + .2 x 60g = 52g
80% of the rocks were 50 grams
20% of the rocks were 60 grams
Average = % as decimal x mass1+ % as decimal x mass2 + % as decimal x mass3 +…

61. Atomic Mass
Calculate the atomic mass of copper if copper has two isotopes. 69.1% has a mass of amu and the rest (30.9%) has a mass of amu.
0.691 x 62.93amu
amu
x 64.93amu =
amu = amu

62. 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

63. Atomic Mass
is not a whole number because it is an average (are the decimal numbers on the periodic table).
Isotopes - atoms of the same element can have different numbers of neutrons and therefore have different mass numbers
When naming, write the mass number after the name of the element H 1
Protium 2
Deuterium 3
Tritium