1. Henry R. Kang (1/2010)
General Chemistry
Lecture 7
Atom

2. Henry R. Kang (1/2010)
Contents
• Atomic Theory
• Law of Mass Conservation
• Structure of Atom
• Atomic Number
• Mass Number
• Isotopes
• Periodic Table

3. Henry R. Kang (1/2010)
Atomic Theory
of
Matter

4. Henry R. Kang (1/2010)
Brief History of Atomic Theory
• Greek philosopher Democritus (460-370 B.C.) expressed
the belief that all matter consists of tiny indivisible
particles, which he named “atomos” (meaning
indivisible).
 However, this view of matter was not a mainstream
philosophy.
• Plato (427?-347 B.C.) and Aristotle (384-322 B.C.)
believed that there can be no ultimately indivisible
particles.
 Therefore, the “atomic” view of matter faded for several
millenniums.
• The modern era of atomic theory started from the work
of John Dalton (1766-1844).

5. Henry R. Kang (1/2010)
Dalton’s Atomic Theory (1808)
• All matter is composed of indivisible atoms.
 No longer true because atoms can be split into subatomic particles.
• Elements are composed of extremely small particles called atoms.
 All atoms of a given element are identical, having the same size, mass and
chemical properties.
 No longer true because of isotopes.
 The atoms of one element are different from the atoms of other elements.
• Compounds are composed of atoms of more than one element.
 The ratios of the numbers of atoms from all elements in a compound are
integers or simple fractions (Law of Definite Proportions).
• Chemical reactions only involve the separation, combination, or
rearrangement of atoms.
 Atoms are not created or destroyed in chemical reactions (Law of mass
conservation).

6. Henry R. Kang (1/2010)
Atomic Symbols and Models
• Atom is represented by one- or two-letter taken from its
name.
 The first letter is capitalized from the name of the element.
 H (hydrogen), C (carbon), O (oxygen), N (nitrogen), etc.
 Sometimes, the first letter is followed by an additional letter
from the name to distinguish elements having the same first
capital letter.
 Cl (chlorine), Ca (calcium), and Cu (copper from cuprum)
 Ne (neon), Na (sodium), and Ni (nickel)
• Models
 Dalton used spheres of different sizes to represent atoms and
combinations of theses spheres to represent compounds.
 Dalton’s approach has been evolved to three-dimensional
models.

7. Henry R. Kang (1/2010)
3
Li
lithium
37
19
K
potassium
87
55
20
Ca
calcium
10
Ne
neon
3231 3635
Br
bromine
3433
As
arsenic
15
P
phosphoru
s
16
S
sulfur
17
Cl
chlorine
18
42
14
Si
silicon
38
89
41
57
4039
88
56
Ba
barium
49 50
Sn
tin
51 52 53
I
iodine
54
105
73
104
72 74
W
tungsten
106
43 44 45 46 47
Ag
silver
48
109
76 79
Au
gold
108107
83
Bi
bismuth
84 85 8675 80
Hg
mercury
78 8177 82
Pb
lead
1
1A
2
2A
5
5B
4
4B
6
6B
3
3B
8
8B
7
7B
9
8B
10
8B
11
1B
12
2B
13
3A
14
4A
15
5A
16
6A
17
7A
18
8A
24
Cr
chromium
232221
12
Mg
magnesium
4
Be
beryllium
1
H
hydrogen
9
F
fluorine
2
He
helium
30
Zn
zinc
29
Cu
copper
28
Ni
nickel
27
Co
cobalt
26
Fe
iron
25
Mn
manganese
5
B
boron
13
Al
aluminum
6
C
carbon
7
N
nitrogen
8
O
oxygen
110 111 114112 118116
11
Na
sodium
Symbols & Names of
Common Elements

8. Henry R. Kang (1/2010)
Law of
Conservation of Mass

9. Henry R. Kang (1/2010)
Law of Conservation of Mass
• The total mass remains the same
during a chemical change (or
chemical reaction).
 Proposed by A. Lavoisier (1743-1794)
• Mass of reactants = mass of products
 Reactants are the original matters
before the chemical change.
 Products are the matters formed after
the chemical change.

10. Henry R. Kang (1/2010)
Law of Conservation of Mass:
Illustration
• Atoms are not created or destroyed in chemical
reactions (Law of conservation of mass)
8 X2Y16 X 8 Y+
Atoms of
element X
Atoms of
element Y
Compound of
elements X and Y

11. Henry R. Kang (1/2010)
Law of Conservation of Mass: Example
• A sample of 1.28 grams magnesium is burned in air
and 2.12 grams of a white ash-like residue (magnesium
oxide) is produced at the completion of the reaction.
What is the mass of oxygen that reacts?
 Magnesium + Oxygen → Magnesium oxide
 2 Mg + O2 → 2 MgO
• Answer:
 (mass of magnesium) + (mass of oxygen)
= (mass of magnesium oxide)
 1.28 g + (mass of oxygen) = 2.12 g
 (mass of oxygen) = 2.12 g – 1.28 g = 0.84 g

12. Henry R. Kang (1/2010)
Law of Definite Proportions
• Law of definite proportions was proposed by Joseph Proust
(1754-1826) in 1799.
 The law can be expressed in two ways:
1. Different samples of the same compound always contain elements
in the same proportion by mass, regardless where they come
from.
 Example: samples of carbon dioxide gas obtained from different sources
contain exactly the same ratio by mass of carbon to oxygen.
 Mass ratio = 12/32 = 3/8 = 0.375
1. The relative number of atoms of each element in a given
compound is always the same.
 Example: samples of carbon dioxide gas obtained from different sources
contain exactly the same ratio of carbon atom to oxygen atom.
 C/O atom ratio = 1/2 = 0.5

13. Henry R. Kang (1/2010)
Laws of Multiple Proportions
• If two elements can combine to form more than one
compound, the masses of one element that combine with
a fixed mass of the other element are in ratios of small
whole numbers.
Nitrogen monoxide
Nitrogen dioxide
Oxygen in NO and NO2 has a ratio of 1/2.
Oxygen in NO, NO2 and N2O5 has a ratio of 2/4/5.
O
C
O
C
1
1
2
1
= =
= =

14. Henry R. Kang (1/2010)
Structure
of
the Atom

15. Henry R. Kang (1/2010)
Structure of Atom
• Atom is the basic unit of an element that can
enter into chemical combination.
 However, atom is not indivisible; it can be split into
subatomic particles.
• Atom possesses internal structure that consists
an inner core, nucleus, and surrounding
electrons.
 Nucleus
Proton (p)
Neutron (n)
 Electron (e–
)

16. Henry R. Kang (1/2010)
Subatomic Particles: Leptons & Hadrons
• There are two broad categories of the subatomic particles
• Leptons (Greek for “light” or “small”)
 They can be viewed as a point particle with very little size or no size at all
 They have no internal structure
 They are not affected by the strong force interaction
 Example:
 electron, positron, neutrino, quarks
• Hadrons (Greek for “heavy” or “strong”)
 They have definite sizes
 They have internal structure
 They are subject to the strong force interaction
 Examples:
 Proton, neutron, etc.
 Proton consists of one d quark (-1/3 e) and two u quarks (+2/3 e)
 Neutron consists of one u quark and two d quarks
d
(-1/3)
n (0)
p (1)
Proton
Neutron
d
(-1/3)
d
(-1/3)
u
(+2/3)
u
(+2/3)
u
(+2/3)

17. Henry R. Kang (1/2010)
Discovery of Electron
• J.J. Thomson used cathode-ray tube to demonstrate the
existence of the charged particles and measured
charge/mass ratio of e–
(1906 Nobel Prize in Physics)
• The cathode ray is attracted by the plate bearing
positive charges and repelled by the plate bearing
negative charges.
 Thomson concluded that cathode rays are streams of
negatively charged particles.
• Using electromagnetic theory, Thomson determined the
charge (Ce) to mass (me) ratio of an electron.
 Ce/me = -1.76×108
coulomb/gram

18. Henry R. Kang (1/2010)
Millikan’s Experiment
• Millikan (1868-1953) used the oil drop experiment for
measuring charge of e–
(1923 Nobel Prize in Physics)
• He determined the value of the electronic charge by
monitoring the motions of charged oil drops under an
electric field.
 The charge on each electron is exactly the same.
 Ce = -1.6022 ×10-19
coulomb
• Knowing the charge and charge/mass ratio (Thomson’s
result), he calculated the mass of the electron
 Ce/me = -1.76×108
coulomb/gram (Thomson’s result)
 me = -1.6022 ×10–19
coulomb / (-1.76×108
coulomb/gram)
= 9.10×10–28
gram

19. Henry R. Kang (1/2010)
Radioactivity & Fundamental Particles
• Henri Becquerel (1852-1908) and Marie Curie (1845-
1923, 1903 Nobel Prize in Physics with husband, Pierre,
and Becquerel; and 1911 Nobel Prize in Chemistry)
coined the name radioactivity to describe the emission of
particles and radiation from some radioactive elements.
• Three types of rays are produced by the decay of
radioactive elements such as uranium.
 Alpha (α) ray is positively charged and is identified as the
helium nuclei.
 Beta (β) ray is negatively charged and is identified as the
electron.
 gamma (γ) ray has no charge and is identified as the high
energy photons.

20. Henry R. Kang (1/2010)
Thomson’s Atomic Model
• Thomson proposed (in 1904) that an atom is a uniform,
positive sphere in which electrons are embedded like
raisins in a cake; Therefore, this model is sometimes
referred to as the “raisin pudding” model.
Positive charge
spread over the
entire sphere –
–
–
–
–
–
–
–

21. Henry R. Kang (1/2010)
Geiger-Marsden-Rutherford’s
Experimental Design & Observations
• Marsden under the supervision of Geiger bombarded a
thin gold foil with α particles (velocity is about 1.4×107
m/s, 5% speed of light).
• Majority of α particles penetrated the foil un-deflected
or with a slight deflection.
• Occasionally, a few (about 1 in 8000) α particle was
scattered (or deflected) at a large angle.
• In rare instances, α particle was actually bounced
backward.
• Thomson’s model contradicted to this phenomenon.

22. Henry R. Kang (1/2010)
Rutherford’s Atomic Model
• Based on the results from Ernest Marsden and
Hans Geiger, Rutherford (1871-1937) proposed
(in 1911) that the majority of the mass and
positive charges of the atom was located in a
small, dense region, the nucleus, with negatively
charged electrons occupying a much larger
volume outside of the nucleus.
 The positive charge of atoms is concentrated
in the nucleus.
 Proton (p) has positive charge as opposite to
the negative charge of electron.
 Mass of proton is about 1840 times of the
electron (1.67×10-24
g).
 Nuclei have diameters of about 10-15
m,
whereas atomic diameters are about 10-10
m.
 If the nucleus is represented by a golf ball,
then the atom would be about 3 miles in
diameter.
A region of mostly empty
space where electrons reside
Dense, positively charged
nucleus at the center
⊕
–
–
–
–
–
–
–
–

23. Henry R. Kang (1/2010)
Comparison of Atom & Nuclear Radii
• For hydrogen atom
 Atomic radius = 3.1×10-11
m (31 pm)
 Radius of hydrogen nucleus (proton) = 8.768×10-16
m
 The ratio of atomic radius to nuclear radius is about 35,000.
• For a larger atom:
 Atomic radius ~ 100 pm = 1×10-10
m
 Nuclear radius ~ 5×10-3
pm = 5×10-15
m
 The ratio of radii is about 20,000.
• Imagine “If an atom has the size of the Houston
Astrodome, then the nucleus is a marble in the center.”

24. Henry R. Kang (1/2010)
Chadwick’s Experiment (1932)
• After the discovery of electron and proton, a problem arose that
the mass ratio of hydrogen to helium did not add up:
 H atoms - 1 proton
 He atom - 2 protons
 The mass ratio of (He mass) to (H mass) should be 2.
 But, the measured mass ratio was 4.
• Chadwick (1891-1974) bombarded a thin sheet of beryllium with
α particles, a very high-energy radiation was emitted by the
metal.
 α + 9
Be → 1
n + 12
C + energy
• Later, it was shown that the rays were a third type of subatomic
particle named “neutron” by Chadwick.
• Neutron (n) is neutral (charge = 0) with a mass slightly higher
than proton.
 n mass ~ p mass = 1.67×10-24
g

25. Henry R. Kang (1/2010)
Mass & Charge of Subatomic Particles
• (mass p) / (mass n) = 1.67262×10-24
/ 1.67493×10-24
= 0.998621
• (mass p) / (mass e-
) = 1.67262×10-24
/ 9.10939×10-28
= 1836
• (mass n) / (mass e-
) = 1.67493×10-24
/ 9.10939×10-28
= 1839
 They differ by about 3 electron-masses
• (mass p) ≅ (mass n) = 1840 × mass e-
Particle Mass
(gram)
Charge
(Coulomb)
Charge
unit
Electron 9.10939×10-28
-1.6022×10-19
-1
Proton 1.67262×10-24
+1.6022×10-19
+1
Neutron 1.67493×10-24
0 0

26. Henry R. Kang (1/2010)
Nuclear Structure
and
Isotopes

27. Henry R. Kang (1/2010)
Nuclear Structure
• Nucleus consists of protons and neutrons.
 Except the hydrogen nucleus
Hydrogen nucleus has only one proton and no
neutron.
• For any neutral atom, the number of proton
equals the number of electrons.
 #proton = #electron
• The number of protons is called “atomic
number”.

28. Henry R. Kang (1/2010)
Atomic Number, Mass Number & Isotopes
• All atoms can be identified by two numbers
 The number of protons and the number of neutrons
• Atomic Number (Z)
 The number of protons in the nucleus: Z = #protons
 In a neutral atom, the number of protons is equal to the number of electrons.
• Mass Number (A)
 The total number of neutrons and protons in the nucleus
 A = #protons + #neutrons
• Nuclide
 A nuclide is an atom characterized by atomic number and mass number,
represented by a symbol, A
ZX. Example: 32
16S
• Isotopes
 Atoms have the same atomic number but different mass numbers (or
different number of neutrons).

29. Henry R. Kang (1/2010)
Isotopes: Definition & Examples
• Isotopes are the same element (same number of protons)
with different numbers of neutrons in their nuclei.
 Atoms have the same atomic number but different mass
numbers.
1 proton
H1
1 H (D)2
1 H (T)3
1
1 proton
1 neutron
1 proton
2 neutron
Hydrogen Deuterium Tritium
U235
92 U238
92
XA
Z
Uranium-235 Uranium-238
Element Symbol
Mass Number
Atomic Number
1
1H
2
1H 3
1H

30. Henry R. Kang (1/2010)
Isotopes - Computation
• How many protons, neutrons, and
electrons are in 14
6C?
6 protons, 8 (14 - 6) neutrons, and
6 electrons
• How many protons, neutrons, and
electrons are in 11
6C?
6 protons, 5 (11 - 6) neutrons, and
6 electrons

31. Henry R. Kang (1/2010)
Examples of Nuclide Symbol
• Give the number of protons, neutrons, and electrons in
the following atoms:
 Number of protons = Number of electrons = Atomic number
 Number of neutrons = mass number – atomic number
 17
8O Number of protons = Number of electrons = 8
Number of neutrons = 17 – 8 = 9
 199
80Hg Number of protons = Number of electrons = 80
Number of neutrons = 199 – 80 = 119
 200
80Hg Number of protons = Number of electrons = 80
Number of neutrons = 200 – 80 = 120
 63
29Cu Number of protons = Number of electrons = 29
Number of neutrons = 63 – 29 = 34

32. Henry R. Kang (1/2010)
Periodic Table
of
Elements

33. Henry R. Kang (1/2010)
Periodic Table
• Many elements show strong similarities to one another.
 They process periodic regularities in physical and chemical properties.
• Periodic table is a chart in which elements, having similar
chemical and physical properties, are group together.
 Horizontal rows are called period.
 Vertical columns are called group or family.
• Three categories
 Metal
 Elements are good conductor of heat and electricity
 Nonmetal (17 elements)
 Usually, poor conductor of heat and electricity.
 Metalloid (8 elements)
 Intermediate elements between metals and nonmetals
 From left to right across any period, the properties of the elements change
gradually from metallic to nonmetallic.

34. Henry R. Kang (1/2010)
Modern Periodic Table
1
1A
18
8A
1
H
1.008
2
2A
13
3A
14
4A
15
5A
16
6A
17
7A
2
He
4.003
3
Li
6.941
4
Be
9.012
5
B
10.81
6
C
12/01
7
N
14.01
8
O
16.00
9
F
19.00
10
Ne
20.18
11
Na
22.99
12
Mg
24.31
3
3B
4
4B
5
5B
6
6B
7
7B
8
8B
9
8B
10
8B
11
1B
12
12B
13
Al
26.98
14
Si
28.09
15
P
30.97
16
S
32.07
17
Cl
35.45
18
Ar
39.95
19
K
39.10
20
Ca
40.08
21
Sc
44.96
22
Ti
47.88
23
V
50.94
24
Cr
52.00
25
Mn
54.94
26
Fe
55.85
27
Co
58.93
28
Ni
58.69
29
Cu
63.55
30
Zn
65.39
31
Ga
69.72
32
Ge
72.59
33
As
74.92
34
Se
78.96
35
Br
79.90
36
Kr
83.80
37
Rb
85.47
38
Sr
87.62
39
Y
88.91
40
Zr
91.22
41
Nb
92.91
42
Mo
95.94
43
Tc
(98)
44
Ru
101.1
45
Rh
102.9
46
Pd
106.4
47
Ag
107.9
48
Cd
112.4
49
In
114.8
50
Sn
118.7
51
Sb
121.8
52
Te
127.6
53
I
126.9
54
Xe
131.3
55
Cs
132.9
56
Ba
137.3
57
La
138.9
72
Hf
178.5
73
Ta
180.9
74
W
183.9
75
Re
186.2
76
Os
190.2
77
Ir
192.2
78
Pt
195.1
79
Au
197.0
80
Hg
200.5
81
Tl
204.4
82
Pb
207.2
83
Bi
208.9
84
Po
(209)
85
At
(210)
86
Rn
(222)
87
Fr
(223)
88
Ra
(226)
89
Ac
(227)
104
Rf
(257)
105
Db
(260)
106
Sg
(263)
107
Bh
(262)
108
Hs
(265)
109
Mt
(266)
110
Ds
(271)
111
Uuu
(272)
112
Uub
(277)
114
Uuq
(296)
116
Uuh
(298)
118
Uuo
(?)
58
Ce
140.1
59
Pr
140.9
60
Nd
144.2
61
Pm
(147)
62
Sm
(150.4)
63
Eu
152.0
64
Gd
157.3
65
Tb
158.9
66
Dy
162.5
67
Ho
164.9
68
Er
167.3
69
Tm
168.9
70
Yb
173.0
71
Lu
175.0
90
Th
232.0
91
Pa
(231)
92
U
(238)
93
Np
(237)
94
Pu
(242)
95
Am
(243)
96
Cm
(247)
97
Bk
(247)
98
Cf
(249)
99
Es
(254)
100
Fm
(253)
101
Md
(256)
102
No
(254)
103
Lr
(257)
Lanthanides
Actinides
Metals
Nonmetals
Metalloids
Halogen
NobleGas
AlkaliMetal
AlkalineEarth
Metal
Period
Group
Note that the table is organized in order of the atomic number.

35. Henry R. Kang (1/2010)
Distribution of Elements on Earth & Body
• Natural abundance of elements in earth’s crust
 Oxygen: 45.5%
 Silicon: 27.2%
 Aluminum: 8.3%
 Iron: 6.2%
 Calcium: 4.7%
 Magnesium: 2.8%
 All others: 5.3%
• Natural abundance of elements in human body
 Oxygen: 65%
 Carbon: 18%
 Hydrogen: 10%
 Nitrogen: 3%
 Calcium: 1.6%
 Phosphorus: 1.2%
 All others: 1.2%