1. Atoms and Their Structure

2. History of the Atom
Original idea (400 B.C.) came from Democritus, a Greek philosopher
Democritus expressed the belief that all matter is composed of very small, indivisible particles, which he named atomos.

3. Who’s Next?
John Dalton ( ), an English school teacher and chemist, studied the results of experiments by other scientists.

4. Dalton’s Atomic Theory
Dalton proposed his atomic theory of matter in 1803.
Although his theory has been modified slightly to accommodate new discoveries, Dalton’s theory was so insightful that it has remained essentially intact up to the present time.

5. 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.

6. Dalton’s Atomic Theory, cont.
Atoms of different elements combine in whole number ratios to form compounds
Chemical reactions involve the rearrangement of atoms. No new atoms are created nor destroyed.

7. Parts of the Atom
Because of Dalton’s atomic theory, most scientists in the 1800s believed that the atom was like a tiny solid ball that could not be broken up into parts.
In 1897, a British physicist, J.J. Thomson, made discoveries that required Dalton’s 1st postulate to be modified
He discovered that atoms are made of smaller (subatomic) particles.

8. Parts of the Atom Thomson’s experiments used a cathode ray tube.
It is a vacuum tube - all the air has been pumped out.

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

10. - + Thomson’s Experiment Voltage source
At each end of the tube is a metal piece called an electrode, which is connected through the glass to a metal terminal outside the tube.

11. - + Thomson’s Experiment Voltage source
When the electrodes are charged, rays travel in the tube from the negative electrode, which is the cathode, to the positive electrode, the anode.

12. - + Thomson’s Experiment Voltage source
Because these rays originate at the cathode, they are called cathode rays.

13. Thomson’s Experiment
Voltage source - +

14. Thomson’s Experiment
Voltage source - +

15. Thomson’s Experiment
Voltage source - +

16. Thomson’s Experiment
Voltage source - +

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

18. Thomson’s Experiment
Voltage source + -

19. Thomson’s Experiment Voltage source + -
Thomson found that the rays bent toward a positively charged plate and away from a negatively charged plate.

20. Thomson’s Experiment Voltage source + -
He knew that objects with like charges repel each other, and objects with unlike charges attract each other.

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

22. Thomson’s Experiment Voltage source + -
Thomson concluded that cathode rays are made up of invisible, negatively charged particles.

23. Cathode Ray Tube

24. Thomson’s Model
From Thomson’s experiments, scientists had to conclude that although atoms are neutral, some of the subatomic particles have a charge (positive or negative).

25. Thomson’s Model
Matter is not negatively charged, so atoms can’t be negatively charged either.
If atoms contained extremely light, negatively charged particles, then they must also contain positively charged particles — probably with a much greater mass than electrons.

26. Thomson’s Model
J.J. Thomson said the atom was like plum pudding, a popular English dessert.

27. Thomspon’s Model
In 1886, scientists discovered that a cathode-ray tube emitted rays not only from the cathode but also from the positively charged anode.
Years later, scientists determined that the rays were composed of positively charged subatomic particles.

28. Isotopes
In 1910, J.J. Thomson discovered evidence that atoms of the same type (elements) can have different masses.
Neon consisted of atoms of two different masses.

29. Isotopes
Atoms of an element that are chemically alike but differ in mass are called isotopes of the element.

30. Ernest Rutherford
In 1909, a team of scientists led by Ernest Rutherford in England carried out the first of several important experiments that revealed an arrangement far different from the plum pudding model of the atom.

32. Rutherford’s Experiment
The experimenters set up a lead-shielded box containing radioactive polonium, which emitted a beam of positively charged subatomic particles through a small hole.

33. Rutherford’s Experiment
The sheet of gold foil was surrounded by a screen coated with zinc sulfide, which glows when struck by the positively charged particles of the beam.

34. Florescent
Screen
Lead block
Polonium
Gold Foil

35. What Rutherford Expected
The alpha particles to pass through without changing direction very much.

36. Because he thought the mass was evenly distributed in the atom.

38. What Rutherford Observed

39. How Rutherford Explained It
To explain the results of the experiment, Rutherford’s team proposed a new model of the atom:
Because most of the particles passed through the foil, they concluded that the atom is nearly all empty space.

40. How Rutherford Explained It
Because so few particles were deflected, they proposed that the atom has a small, dense, positively charged central core, called a “nucleus.” +

41. Alpha particles are deflected by it if they get close enough to the nucleus.+

42. The Nuclear Model of the Atom
The new model of the atom as pictured by Rutherford’s group in 1911 is shown below.

43. Explanation of 4 Observations
Three subatomic particles were proposed to explain the four observations made by Thomson and Rutherford: protons, electrons, and neutrons.

44. The Electron
The first to be discovered, electrons have a negative charge and almost no mass (compared to protons and neutrons).
Electrons account for the volume of an atom.

45. The Proton
Protons are positively charged, neutralizing the charge of electrons.
They have mass, and are located in the nucleus.

46. The Neutron
Because of the discovery of isotopes, scientists predicted that a third particle would be discovered that exists in the nucleus, has a mass equal to that of a proton, but has no charge.
The existence of this neutral particle, called a neutron, was confirmed in the early 1930s.

47. Subatomic Particles Name Electron e- -1 1/2000 Proton p+ +1 1 Neutron
Relative
mass
Relative
mass
Name
Symbol
Symbol
Charge
Charge
Electron e- -1
1/2000
Proton p+ +1 1
Neutron n0 1

48. Modern View of the Atom
The atom has two regions and is 3- dimensional.
The nucleus is at the center and contains the protons and neutrons.

49. Modern View of the Atom
The electron cloud is the region where you might find an electron and most of the volume of an atom.

50. Atomic Number
The atomic number of an element is the number of protons in the nucleus of an atom of that element.
The number of protons determines identity of an element, as well as many of its chemical and physical properties.

51. Atomic Number
Because neutral atoms have no overall electrical charge, a neutral atom must have as many electrons as there are protons in its nucleus.
Therefore, the atomic number of an element also tells the number of electrons in a neutral atom of that element.

52. Masses
The mass of a neutron is almost the same as the mass of a proton.
The sum of the protons and neutrons in the nucleus is the mass number of that particular atom.
Isotopes of an element have different mass numbers because they have different numbers of neutrons, but they all have the same atomic number.

53. Masses
Isotopes of an element have different mass numbers because they have different numbers of neutrons, but they all have the same atomic number.
Isotopes are always identified by their mass number, and can be represented in two ways:
Two equivalent representations of the carbon – 12 isotope:
C C

54. Symbols
Elements can be represented by using the symbol of the element, the mass number and the atomic number. X
Mass
number
Atomic
number
The mass number is the number of protons + the number of neutrons.

55. X A Z Symbols Mass number is represented by the letter A. Mass number
Atomic
number Z
Atomic number is represented by the letter Z.

56. Symbols Example
Determine the following for the fluorine atom depicted below. 19 F
(9)
number of protons 9
number of neutrons
(10)
number of electrons
(9)
atomic number
(9)
e) mass number
(19)

57. Symbols Problem
Determine the following for the bromine atom depicted below. 80 Br
(35)
number of protons 35
number of neutrons
(45)
number of electrons
(35)
atomic number
(35)
e) mass number
(80)

58. Se or Se-78 Symbols Problem
If a neutral element has an atomic number of 34 and a mass number of 78 what is the
(34)
number of protons
number of neutrons
(44)
number of electrons
(34)
complete symbol (2 ways) 78
Se or Se-78 34

59. Pa or Pa-231 Symbols Problem
If a neutral element has 91 protons and 140 neutrons what is the
(91)
atomic number
mass number
(231)
number of electrons
(91)
complete symbol (2 ways)
Pa or Pa-231
231 91

60. Pt or Pt-195 Symbols Problem
If a neutral element has 78 electrons and 117 neutrons what is the
(78)
atomic number
mass number
(195)
number of protons
(78)
complete symbol (2 ways)
Pt or Pt-195
195 78

61. Information in the Periodic Table
The average atomic mass is the weighted average mass of all the naturally occurring isotopes of that element.
The unit is the Atomic Mass Unit (amu).

62. Calculating Atomic Mass

63. Calculating Atomic Mass
Copper exists as a mixture of two isotopes.
The lighter isotope (Cu-63), with 29 protons and 34 neutrons, makes up 69.17% of copper atoms.
The heavier isotope (Cu-65), with 29 protons and 36 neutrons, constitutes the remaining 30.83% of copper atoms.

64. Calculating Atomic Mass
To determine the average atomic mass, first calculate the contribution of each isotope to the average atomic mass, being sure to convert each percent to a fractional abundance.
Will the average atomic mass of copper be closer to 63 amu or 65 amu?

65. Calculating Atomic Mass
Mass contribution = mass of isotope x abundance of isotope
For Cu-63:
Mass contribution = amu x = amu
For Cu-65:
Mass contribution = amu x = amu

66. Calculating Atomic Mass
The average atomic mass of the element is the sum of the mass contributions of each isotope.
Atomic mass Cu = mass contribution Cu-63 +
mass contribution Cu-65
Atomic mass Cu = = amu

67. Ions
While changing the number of neutrons in an atom changes the mass and creates an isotope, changing the number of electrons changes the charge of an atom and creates an ion.

68. Ions
The charge is the sum of the charges contributed by the protons and electrons. Charge = (p+) – (e-)
The charge of an atom is written as a superscript on the right side of an element’s symbol:
Ex. Fluorine has 9 protons with 10 electrons: F1- or simply F-
Sodium has 11 protons with 10 electrons: Na1+or simply Na+

69. Ions O2- Be2+ Zr3+ Br- Write the symbol for the following ions:
oxygen (O) with 10 electrons
beryllium (Be) with 2 electrons
zirconium (Zr) with 37 electrons
bromine (Br) with 36 electrons
O2-
Be2+
Zr3+
Br-

70. Summary of Subatomic Particles
mass
charge
location
changing the number creates…
proton, p+ 1 +1
nucleus
a different element
neutron, n0
an isotope
electron, e- ~0 -1
electron cloud
an ion

71. End of Day 1