1. Ch. 4
ATOMIC STRUCTURE

2. Sizing up the Atom
Elements are able to be subdivided into smaller and smaller particles – these are the atoms, and they still have properties of that element
If you could line up 100,000,000 copper atoms in a single file, they would be approximately 1 cm long
Despite their small size, individual atoms are observable with instruments such as scanning tunneling (electron) microscopes

3. An STM image of nickel atoms placed on a copper surface.
Source: IBM Research

4. Red ridge is a series of Cesium atoms

5. Image of a ring of cobalt atoms placed on a copper surface.
Source: IBM Research

6. Atom
- smallest particle making up elements
One teaspoon of water has 3 times as many atoms as the Atlantic Ocean has teaspoons of water!

7. Think about the technological advances of
the past 100 years! They have been nothing
short of miraculous!
Radios Calculators
Televisions Computers
Automobiles Cell phones
Jet airplanes Ipods
Plastic Velcro
Refrigerators Internet (thanks, Al Gore)
Penicillin CD’s & DVD’s
Insulin and, of course -
Electric guitars Sliced Bread!

8. Development of Atomic Theory
This explosion of technology occurred
once we had a better understanding
of the atom and how it behaves!

9. Where did it all begin?
The word “atom” comes from the Greek word “atomos” which means indivisible.
The idea that all matter is made up of atoms was first proposed by the Greek philosopher Democritus in the 5th century B.C.

10. Then came the idea of “The 4 Basic Elements” Earth, Air, Fire, & Water
After that came Alchemy.
The change to “real” Chemistry didn’t occur until the first true element was discovered! (1774)
The first element discovered was

11. Karl Scheele (1771) (German) first to prepare and describe oxygen
The discovery of oxygen is attributed to 3 scientists (working independently)
Karl Scheele (1771) (German)
first to prepare and describe oxygen
Joseph Priestley (1774) (British)
isolated oxygen gas from mercuric oxide.
observed accelerated burning
Antoine Lavoisier (1784) (French)
made accurate measurements and interpreted Priestley’s results
Discovery of Oxygen gas is attributed to three persons
Karl Scheele who first prepare and discribed oxygen
Joseph Priestly isolated oxygen gas from HgO. Collected the gas and observed that accelerated burning and increased activity of mice.
Connected oxygen with combustion and plant and animal metabolism
Also prepared many gases
hydrogen chloride, ammonia, sulfur dioxide
Founder of the ACS
Antoine Lavoiser who made accurate measurements and interpreted Priestly’s results.
He determined that mass of oxygen release from HgO is equal to mass loss from HgO.
And correctly calculated the change in mass of oxygen durning a combustion reactions.
(1784 wrote the Law of Conservation of Mass)

12. Carl Wilhelm Scheele beat Priestley to the discovery but published afterwards.
Too bad! – So sad!

13. Priestley Medal Priestley gets the main credit for discovering oxygen!
Source: Roald Hoffman, Cornell University

14. 2HgO(s) → 2Hg(l) + O2 (g)
Priestley produced a gas (oxygen) by using sunlight to heat mercuric oxide kept in a closed container. The oxygen forced some of the mercury out of the jar as it was produced, increasing the volume about five times.

15. Priestley: Scientific Contributions
DISCOVERY OF 8 GASES
Oxygen
Nitrogen
Carbon Dioxide
Carbon Monoxide
Sulfur Dioxide
Nitrous Oxide
Nitric Oxide
Hydrogen Chloride

16. Priestley: Additional Scientific Contributions
Discovered the interconnection between photosynthesis and respiration
Discovered carbonated water
Discovered that India rubber removed graphite pencil marks - the first rubber eraser
Now we can make mistakes!!

17. Lavoisier: the Founder of Modern Chemistry
Lavoisier continued the investigations of Priestly
Quantitative experiments led to:
Law of Conservation of Matter.
He systematized the language of chemistry, its nomenclature and rhetoric.
He was beheaded during the Reign of Terror for his role as a tax “farmer” prior to the Revolution (Priestley escaped to America!)
Antoine-Laurent Lavoisier

18. 2Hg(l) + O2 (g) → 2HgO(s)
Lavoisier heated a measured amount of mercury to form the red mercuric oxide. He measured the amount of oxygen removed from the jar and the amount of red oxide formed. When the reaction was reversed, he found the original amounts of mercury and oxygen.

19. Properties of Oxygen Colorless P Odorless P P P P C C Tasteless
Gas at room temperature
Slightly soluble in water
Inflammable (does NOT burn)
Only part of air that supports combustion P
P P P P C C
Physical Property or Chemical Property?

20. These properties of oxygen were later used
to determine the properties of other
substances.
By the late 18th century, scientists finally came
to the conclusion that Oxygen was truly an
element (can’t be broken down into simpler
forms without losing its properties)
Scientists began to search for & test other
new elements.

21. Sometimes, when they tried to react
substances together, nothing happened!
Substances that DO NOT react are Inert
They found that most materials will react to form new substances. These elements are said to be chemically active (reactive)
Oxygen is very reactive, so is hydrogen which we will look at next!
Oxygen
hydrogen
inert
Increasing chemical reactivity

22. Discovery of Henry Cavendish (1766) Reacted various metals with acids
producing a salt and hydrogen gas
Acid + metal → hydrogen gas + salt
Zinc + sulfuric acid → Hydrogen + zinc sulfate
Zn(s) + H2SO4(aq) → H2 (g) + ZnSO4 (aq)
While testing the properties of Hydrogen he
found that water is a compound
(1731 – 1810)
Word
Equation
Chemical
equation

23. Hydrogen + Oxygen Water
2H O H2O

24. Antoine Lavoisier
Named Priestly’s newly discovered gas - “oxygen” - meaning “acid former”
Named Cavendish’s new gas “hydrogen” -meaning “water former”

25. Dalton’s Atomic Theory
John Dalton ( )
While his theory was not completely correct, it revolutionized how chemists looked at matter and brought about chemistry as we know it today (instead of alchemy)
So, it’s an important landmark in
the history of science.

26. Dalton’s Modern Atomic Theory (experiment based!)
All elements are composed of tiny indivisible particles called atoms
Atoms of the same element are identical. Atoms of any one element are different from those of any other element.
Atoms of different elements combine in simple whole-number ratios to form chemical compounds
In chemical reactions, atoms are combined, separated, or rearranged – but never changed into atoms of another element.

27. Law of Definite Proportions
Each compound has a specific ratio of elements by mass.
Ex: Water is always 8 grams of oxygen for each gram of hydrogen.

28. Discovery of the Electron
Began with the invention of the Crooke’s Tube
(cathode ray tube) c. 1875

29. - + Cathode Ray Tube gas Voltage source
Electric current sent through gases sealed in tube at low pressure
Anode- positive electrode
Cathode- negative electrode
Metal Disks - electrodes

30. Modern Cathode Ray Tubes
Television
Computer Monitor
Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

31. In 1897, J.J. Thomson used a
cathode ray tube to study gases.

32. - + Thomson’s Experiment Voltage source
Passing an electric current makes a beam appear to move from the negative to the positive end – so the ‘beam’ was called a “Cathode Ray”

33. Thomson’s Experiment Voltage source -
Thomson found that cathode rays were deflected from a negatively-charged plate.

34. Thomson’s Experiment Voltage source + Does light bend like this?
and that cathode rays were attracted to plates with a positive charge
Does light bend like this?

35. Light doesn’t ‘bend’ so the cathode ray
must be made of particles rather than
Light!
Since they are attracted to a positive
plate & repelled by a negative one the
particles aren’t neutral –
What charge must they have?
That’s right! NEGATIVE!!
Thomson called these negative particles –
ELECTRONS

36. Mass of the Electron
Mass of the electron is
9.11 x g
The oil drop apparatus
1916 – Robert Millikan determined the mass of the electron: 1/1840 the mass of a hydrogen atom; and, has one unit of negative charge

38. Conclusions from the Study of the Electron:
Cathode rays have identical properties regardless of the element used to produce them. Therefore, all elements must contain identically charged electrons.
Atoms are neutral, so there must be a positive substance in the atom to balance the negative charge of the electrons
Electrons have so little mass that atoms must contain other particles that account for most of their mass

39. Thomson’s Atomic Model
J. J. Thomson
Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.

40. Plum-Pudding Model
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 56

41. Henri Becquerel discovered radioactivity
In 1903, An important discovery leading to further understandings of atomic structure happened by accident.
Henri Becquerel discovered radioactivity
Radioactivity is the spontaneous emission
of energy from an object
1903: Shared a Nobel Prize with Pierre and
Marie Curie for discovering radioactivity.

42. The Nobel Prize in Chemistry 1908
Ernest Rutherford ( )
The Nobel Prize in Chemistry 1908
Studied under J. J. Thomson

43. 3 Types of Radiation discovered by Ernest Rutherford
Alpha (ά) – a positively charged helium nucleus 42 He+2
Beta (β) – fast-moving electrons e
Gamma (γ) – like high-energy
x-rays

44. Ernest Rutherford’s Gold Foil Experiment - 1911
Shot alpha particles at a thin sheet of gold foil
Particles that hit on a detecting screen (film) were recorded

45. Flourescent
Screen
Lead block
Polonium
Gold Foil

46. He Expected:
The alpha particles to pass through the foil without changing direction very much.
Because…
The positive charges were spread out evenly (according to Thomson’s atomic theory). Alone they were not enough to stop the alpha particles.

47. What he expected

48. Again, because he thought the mass was evenly distributed in the atom

49. What he got

50. Rutherford’s Observations
“Like howitzer shells bouncing off of tissue paper!”
Most of the particles went straight through the foil (what he expected)
A few particles were slightly deflected
Still fewer actually bounced back towards the source!
Astonishing!!!
Rutherford said it was like firing a Howitzer shell at a piece of tissue paper & having it bounce back & hit you!

51. +

52. Rutherford’s Conclusions+
Since most of the particles went through the foil - atoms are mostly empty space.
Because a few + particles were deflected they must have come close to a positively charged core.
Since a very few particles were deflected straight back, the positively-charged core must be very dense.
This small dense positive area is the nucleus.

53. The Rutherford Atomic Model
Based on his experimental evidence:
The atom is mostly empty space
All the positive charge, and almost all the mass is concentrated in a small area in the center. He called this a “nucleus”
The electrons are distributed around the nucleus, and occupy most of the volume
His model was called a “nuclear model”

54. Discovery of Protons
Eugen Goldstein in 1886 observed particles with a positive charge passing through a perforated cathode.

55. In 1920, Rutherford studied these
particles & called them protons.
They have a charge of positive 1
and a mass of 1.7 x grams.
This is not a ‘handy’ number to work
with so we use a mass of 1 amu.
Amu stands for “atomic mass unit”

56. Discovery of the Neutron
Rutherford predicted the existence of the neutron in 1920.
Twelve years later, his assistant found it!
1932 – James Chadwick confirmed
the existence of the “neutron”
– a particle with no charge,
but a mass nearly equal to a proton
(1 amu).
So now we have a more complete picture of an atom!

57. Subatomic Particles Particle Charge Mass (g) Location Electron (e-) -1
9.11 x 10-28g
(virtually 0)
outside nucleus
Proton (p+)
(H+) +1
1 amu
(1.7 x 10-24g)
in nucleus
Neutron
(no)
(1.67 x 10-24g)

58. Elements are the new building blocks
Nitrogen-7
Hydrogen
Carbon-6
Oxygen-8

59. Henry Moseley
(1887 – 1915)
Between 1912 and 1914, the physicist H.G.J. Moseley
conducted a series of experiments where he bombarded targets
made out of different kinds of
metals with cathode rays. Each
metal he studied emitted X-rays
of a characteristic frequency,
almost like a set of "fingerprints".

60. The pattern that emerged when the
observed X-rays were organized in
order of increasing frequency
suggested to Moseley a regular
increase in the positive charge on
the nuclei of the atoms.
He called this positive nuclear charge-
the Atomic Number of the element

61. Since all atoms are neutral - the # protons in an atom = # electrons
Atomic Number
Henry Moseley – used x-ray spectra
& came up with the idea of the
Atomic Number
Elements are different because they contain different numbers of PROTONS
The “atomic number” of an element is the number of protons in the nucleus
Since all atoms are neutral - the
# protons in an atom = # electrons

62. Atomic Number, Z
All atoms of the same element have the same number of protons in the nucleus, Z 13
Atomic number Al
Atom symbol
26.981
AVERAGE Atomic Mass

63. Mass # = # protons + # neutrons
Mass Number
Mass number is the number of protons
and neutrons in the nucleus of an
isotope:
Mass # = # protons + # neutrons

64. Subatomic Particles equal in a neutral atom Atomic Number
NUCLEUS
ELECTRONS
equal in a neutral atom
PROTONS
NEUTRONS
NEGATIVE CHARGE
Atomic Number
equals the # of...
POSITIVE CHARGE
NEUTRAL CHARGE
Most of the atom’s mass.

65. Isotopes
Frederick Soddy ( ) proposed the idea of isotopes in (worked with Rutherford)
Isotopes are atoms of the same element having different mass numbers, due to varying numbers of neutrons.
Soddy won the Nobel Prize in Chemistry in 1921 for his work with isotopes and radioactive materials.

66. Isotopes
Atoms of the same element (same Z) but different mass number (A).
Boron-10 (B-10) has 5 p and 5 n Boron-11 (B-11) has 5 p and 6 n
10B
11B

68. Isotopes
Radioisotopes (radioactive isotopes) - unstable isotopes that spontaneously decay emitting radiation
They play an important part in the technologies that provide us with food, water and good health.
Radio-carbon dating of fossils
In medicine, diagnosis, treatment, and research
Sterilization of meat Disinfestation of grain and spices Increasing shelf life (eg, fruits)

70. Nuclear Symbols
Contain the symbol of the element, the mass number and the atomic number (represent isotopes of elements)
Mass
number X
Superscript →
Element
symbol
Atomic
number
Subscript →
REMEMBER! number of electrons = number of protons
So all atoms are neutral!

71. Rhenium
Protons: 75
Neutrons: 111
Electrons: 75 Re
186 75

72. Br Nuclear Symbols 80 35 Find each of these: number of protons
number of neutrons
number of electrons
Atomic number
Mass Number 80 Br 35

73. Nuclear 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
Write the complete symbol 78 34 Se 34 44 34

74. Naming Isotopes
We can name isotopes by placing the mass number after the name of the element:
carbon-12
carbon-14
uranium-235
Mass numbers

75. ISOTOPES Isotope p+ n0 e- Mass # Oxygen - 10 - 33 42 - 31 15 18 8 8
Arsenic 75 33 75
Phosphorus 16 15 31

76. The element hydrogen has 3 isotopes
Protons
Electrons
Neutrons
Nucleus
Hydrogen–1
(protium) 1
Hydrogen-2
(deuterium)
Hydrogen-3
(tritium) 2

77. Examples of Isotopes

78. Learning Check – Counting
Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of these carbon atoms.
12C C 14C
#p+ _______ _______ _______
#no _______ _______ _______
#e- _______ _______ _______

79. Answers
12C C 14C #p
#no #e

80. Learning Check An atom has 14 protons and 20 neutrons.
A. Its atomic number is
1) ) ) 34
B. Its mass number is
C. The element is
1) Si 2) Ca 3) Se
D. Another isotope of this element is
1) 34X 2) 34X 3) 36X

81. Atomic Mass How heavy is an atom of oxygen?
It depends, because there are different kinds of oxygen atoms.
We are more concerned with the average atomic mass.
This is based on the abundance (percentage) of each variety (isotope) of that element in nature.
We don’t use grams for this mass because the numbers would be too small –

82. Measuring Atomic Mass
Instead of grams, the unit we use is the Atomic Mass Unit (amu)
It is defined as one-twelfth the mass of a carbon-12 atom.
Carbon-12 chosen because of its isotope purity.
Each isotope has its own mass number, so we determine the average atomic mass from the element’s percent abundance.

83. To calculate the average atomic mass:
Multiply the mass of each isotope by it’s abundance, then add the results.
Abundance may be expressed as a decimal or a %, (Divide by 100 if using %’s)
Avg.
Atomic
Mass

84. Composition of the nucleus
Atomic Mass is the weighted average of all the naturally occurring isotopes of an element. on the Periodic Table
Isotope
Symbol
Composition of the nucleus
% in nature
Carbon-12
C-12
6 protons
6 neutrons
98.89%
Carbon-13
C-13
7 neutrons
1.11%
Carbon-14
C-14
8 neutrons
<0.01%
(98.89 x 12) + (1.11 x 13) + (0.01 x 14)
100 =

85. D. Average Atomic Mass
EX: Calculate the avg. atomic mass of oxygen if its abundance in nature is 99.76% 16O, 0.04% 17O, and 0.20% 18O.
Avg.
Atomic
Mass
16.00
amu

86. Sub-atomic Particles - Summary
Protons p+ - positive charge, in nucleus , mass of 1 amu
Neutrons n0 – no charge, in nucleus, mass of 1 amu
Electrons - e- negative
charge, orbiting nucleus, “no mass”