Download Observations Leading to the Nuclear Model of the Atom

Dalton’s Atomic Theory (1808)
1. matter is made up of microscopic, indivisible
particles (atoms) that cannot be created or destroyed
Our Understanding Today
2. all atoms of an element are identical in mass, and
have identical physical and chemical properties
3. atoms of different elements have different masses,
physical properties, and chemical properties
4. atoms of different elements combine in simple
whole numbers to form compounds
5. atoms of an element cannot be converted into atoms of other
elements; chemical reactions involve reorganization of the atoms
But Dalton’s Atomic Theory couldn’t explain it all—
 Why do atoms combine the ways they do?
 What about the electrically charged particles that were being observed?
Observations Leading to the Nuclear Model of the Atom
 Cathode Ray Tube Experiments (Figure 2.2): J.J. Thomson, 1897
The particles discovered were called electrons. Thomson determined the mass/charge ratio of the
electron in a related experiment. Another experiment was needed to determine the mass or charge.
 Millikan’s Oil Drop Experiment ( Figure 2.3), 1909
Control of oil drop by electric field allowed for calculation of charge of 1 electron:
e– charge = –1.602 H 10–19 C
Since the mass/charge ratio was already known, the mass could also be calculated:
e– mass = 9.109 H 10–28 g
So, where’s the positive charge? Speculation was that it was spread throughout the atom.
 Rutherford’s Alpha Particle Scattering Experiment ( Figure 2.6), 1910
To test this speculation, Rutherford shot α particles (positively charged and relatively massive) at
gold foil. Most of the α particles passed through the foil relatively unaffected, but a few bounced
back nearly right back at the source. Conclusion: Atoms have a dense, positively charged core
called the nucleus.
By 1932, Chadwick discovered the neutron (neutral, not electrically charged).
Figure 2.6
2-2
The chemistry of an atom is determined primarily by
its number and arrangement of electrons.
Describing Atoms
 atomic number, Z: # of protons in the nucleus of an atom
 mass number, A: sum of # of protons + # of neutrons in the nucleus of an atom

A
write as Z X or AX or “atomic name–A” (where X = atomic symbol)
e.g.,
 isotopes: atoms with same Z but different A. (atoms of same element but with different
masses)
e.g.,
Atomic Mass
standard: 12C atom mass = 12 atomic mass units (amu) exactly
1
the mass of a 12C atom [= 1 dalton (Da), often used in biochemistry and biology]
ˆ1 amu = 12
Using mass spectrometry, we can measure the masses of other atoms relative to 12C.
mass of 20 Ne atom
= 1.66604
e.g.,
mass of 12 C atom
 mass of 20Ne atom = 1.66604 H 12 amu = 19.9924 amu
Mass spectrometry also gives information about the relative
abundances (fractions) of each isotope of the element.
The weighted average of the masses of the isotopes is called the
atomic mass.
e.g.,
atomic mass of Ne = 0.904838 (19.9924 amu)
+ 0.002696 (20.9940 amu)
+ 0.092465 (21.9914 amu)
20.180 amu
PROTIP
atomic number (Z)
10
atomic symbol
Ne
atomic mass
Neon
20.180
atomic name
2-3
The atomic mass is
closest to the mass
of the most
abundant isotope of
the element.
The Periodic Table
Mendeleev proposed the first periodic table in 1871, similar to today’s table
1A
(1)
1
H
8A
(18)
2
He
3
Li
2A
(2)
4
Be
3A
(13)
5
B
4A
(14)
6
C
5A
(15)
7
N
6A
(16)
8
O
7A
(17)
9
F
11
Na
12
Mg
13
Al
14
Si
15
P
16
S
17
Cl
18
Ar
22.99
24.31
26.98
28.09
30.97
32.07
35.45
39.95
35
Br
36
Kr
1.008
6.941
9.012
10.81
4B
(4)
22
Ti
5B
(5)
23
V
6B
(6)
24
Cr
7B
(7)
25
Mn
12.01
14.01
16.00
19.00
4.003
10
Ne
20.18
19
K
20
Ca
3B
(3)
21
Sc
39.10
40.08
44.96
47.88
50.94
52.00
54.94
55.85
58.93
58.69
63.55
65.39
69.72
72.61
74.92
78.96
79.90
83.80
37
Rb
38
Sr
39
Y
40
Zr
41
Nb
42
Mo
43
Tc
(98)
44
Ru
101.07
45
Rh
102.91
46
Pd
106.42
47
Ag
107.87
48
Cd
112.41
49
In
114.82
50
Sn
118.71
51
Sb
121.76
52
Te
127.60
53
I
126.90
54
Xe
131.29
73
Ta
74
W
75
Re
76
Os
77
Ir
78
Pt
79
Au
80
Hg
81
Tl
82
Pb
83
Bi
84
Po
85
At
86
Rn
2B
(12)
30
Zn
31
Ga
32
Ge
33
As
34
Se
87.62
88.91
91.22
55
Cs
56
Ba
71
Lu
72
Hf
132.91
137.33
174.97
178.49
180.95
183.84
186.21
190.23
192.22
195.08
197.00
200.59
204.38
207.2
208.98
(209)
(210)
(222)
87
Fr
88
Ra
103
Lr
104
Rf
105
Db
106
Sg
107
Bh
108
Hs
109
Mt
110
Ds
111
Rg
112
Cn
113
Nh
114
Fl
115
Mc
116
Lv
117
Ts
118
Og
(267)
(268)
(271)
(270)
(269)
(278)
(281)
(281)
(285)
(286)
(289)
(289)
(293)
(294)
(294)
57
La
58
Ce
59
Pr
60
Nd
61
Pm
62
Sm
63
Eu
64
Gd
65
Tb
66
Dy
67
Ho
68
Er
69
Tm
70
Yb
138.91
140.12
140.91
144.24
(145)
150.36
151.96
157.25
158.93
162.50
164.93
167.26
168.93
173.04
89
Ac
90
Th
91
Pa
92
U
93
Np
94
Pu
95
Am
96
Cm
97
Bk
98
Cf
99
Es
100
Fm
101
Md
102
No
[227]
232.04
231.04
238.03
[237]
(244)
(243)
(247)
(247)
(251)
(252)
(257)
(258)
(259)
[226]



(262)
95.94
(10)
28
Ni
1B
(11)
29
Cu
85.47
(223)
92.91
(8)
26
Fe
8B
(9)
27
Co
METALS, METALLOIDS, NONMETALS
Columns: “Groups”; Rows: “Periods”
Group Names to LEARN
ELEMENTS IN THE
O 1A(1): Alkali Metals
SAME GROUP
O 2A(2): Alkaline Earth Metals
ARE USUALLY
O 6A(16): Chalcogens
CHEMICALLY SIMILAR
O 7A(17): Halogens
O 8A(18): Noble Gases (somewhat old-fashioned, but still in use)
Know/be aware of the A and B group designations as shown above. While now not official, they are
too popular and enduring to not be aware of. (They also have uses that we will take advantage of.)
 “A” group elements are “main group” or “representative” elements.
 “B” group elements are “transition metals.”
The 14-column-wide section at the bottom would fit in between Groups 2A and 3B if there were space.
Two numbering conventions are in use.
 American convention: “A” and “B” groups, as shown above
[Europeans reverse the “A” and “B” designations for 3 to 8!]
New International convention: Consecutive numbers 1 to 18 (future: 1 to 32?)
2-4