Atomic Theory and Structure: Review Solutions and Test Topics
1.
Atomic Theory Development
For each scientist, name (if applicable) and draw his atomic model’
Democritus (ca 400 BC) and John Dalton (1803) - smallest fundamental part of nature. Would look like a
small sphere. Dalton also stated that atoms combined in whole number ratios, that
atoms were indivisible (did not know about fission) and that atoms of the same
element were alike (did not know about isotopes)
2.
“plum-pudding model” The model had an atom filled with a positive mass (the
pudding) with negative electrons scattered throughout (the plums).
Nagaoka (1904) –
“Saturnian Model” The model had a very large nucleus filled with the positively
charged mass surrounded by a ring of negative electrons.
Rutherford (1911) –
Following the results of his Gold-foil experiment, Rutherford’s model had a very
tiny nucleus that contained all the positive mass while the electrons were scattered
throughout the relatively massive electron cloud. Rutherford concluded that an
atom was “mostly empty space.”
Bohr (1913) –
Similar to Rutherford’s model, but Bohr had filled the electron cloud with a series
of concentric rings (orbits) containing the electrons. We know the rings as energy
levels.
Fill in the blanks.
11
5
3.
Thompson (1903) –
B 3 p =
Ti 2
48
22
5
16
n = 48 – 22 = 26
n = 32 – 16 = 16
e = 5–3=2
e = 22 – 2 = 20
e = 16 + 2 = 18
35
17
36
17
Cl

S 2 p =
n = 11 – 5 = 6
Which atom is most likely to be radioactive and why?
n
p
32
16
p = 22
37
17
Cl
18
17
19
17
35
Cl
36
Cl
37
Cl
Cl
20
17
1
Furthest from 1 , so most likely to be radioactive
4.
Define half-life. The time required for ½ of a radioactive sample to decay .
5.
A radioactive sample has a half-life of 20.0 s. How long will it take a 50.0 g sample to decay to less than 5.0 g?
50.0 g half  life 25.0 g half  life
12.5 g half  life 6..25 g half  life
3.125g (now less than 5.0 g)
So, four half-lives are needed, or 4 x 20.0 s = 80.0 s
6.
A radioactive sample has a half-life of 0.50 yr. After 3 yrs what amount of the original sample remains?
3 yrs
Total time is 3 yrs, with each half-life being 0.5 yrs, so we get 0.5 yrs  6 half  lives . So if we cut something in half
six times, we would go from 100% down to 1.56%. or from 1.00 to a fraction of 0.00156 or from
1
1
1
1
2
2
1
4
3
1
8
4
1
16
5
1
32
6
1
64
1
1
to
1 th
64
.
Atomic Theory and Structure: Review Solutions and Test Topics
7. List the five types of radioactive decay.
Write the symbol for the radioactive particle.
Briefly describe the decay.
 Types of radioactive decay
4
, Alpha Particle, 2 He
o
,
223
87
Fr
4
2
He
219
+ 85
At
Beta Decay,
o
32
15
(n p + e-),
positive He nucleus ejected from the nucleus
0
1
high energy e- is ejected from the nucleus
e
32
P  10 e + 16
S
o , Gamma Rays
high energy photon emitted as nucleus moves from excited to lower energy
state
232
90
Th *  132
90Th +  (*=excited state)
0
o Positron Emission, 1 e positive particle ejected from nucleus (p
1
0
n+
0
1
30
30
e ), 15
Si
P  10 e + 14
e- falls into nucleus combining with a proton and forming a neutron,
o EC-electron capture
Tl + 10 e  202
80 Hg
202
81
8.
Complete the following equations.
Note:
Atomic mass (top #) and atomic number (bottom #) must be the same on both sides of the arrow.
Note:
The atomic number (bottom #) determines which symbol to write; except when the value is zero, 0, in
which case it could be n or , so in that case look at the atomic mass (top #)
1.
202
81
+
0
1
2.
223
87

4
2
3.
30
15
P

0
1
4.
232
90
Th *

5.
32
15
P

Tl
Fr
e

202
80
He
+
219
85
+
30
14
232
90
Th
+
0 *
0
0
1
e
+
32
16
e
Hg
At
Si

S
Topics for test include:

Nucleus
Central portion of the atom, positively charged and contains the mass .

Electron cloud
The bulk of the space of an atom, mostly empty, negatively charged.

Basic Subatomic Particles
electron
proton
neutron

Note that for an individual atom, the number of protons and neutrons never changes in ordinary reactions.
The number of electrons can change, which effects the charge of the atom, but the nucleus does not change.
negative charge (–)
positive charge (+)
neutral
( )
located in electron cloud
located in nucleus
located in nucleus
0u
1u
1u
Atomic Theory and Structure: Review Solutions and Test Topics

Atomic Mass, Y
the sum of the protons and neutrons. p + n
e.g. an atom with 6 protons and 7 neutrons has an atomic mass of 13

Atomic Number, Z
The number of protons. This defines the element. For example carbon
always has 6 protons, but is known to have 6 neutrons (Y=12) or 7 neutrons
(Y=13) or 8 neutrons (Y=14) See isotopes below.

Isotope
Atoms with the same number of protons (same element), but with different
number of neutrons.

Percent Abundance
the percentage of one isotope for an element

Average Atomic Mass, Yavg
a weighted average of all known isotopic masses for an element
Yavg    Y1    Y2  
where X = percent abundance as a decimal
Y1 and Y2 are isotopic masses

Charge
Some particles emit an electric force creating a field of force around the
particle. This field attracts opposite fields while repelling similar fields.
These fields are positive (+) or negative (-). The absence of the field is
neutral ( ).

Ion
a charged atom or molecule

Cation
positive ion, lost electrons

Anion
negative ion, gained electrons

To calculate charge
atom – number of excess protons or electrons. e.g. if an atom contains 6
protons and 7 electrons, thus 6 + charges and 7- charges with a net of 1charge

Radioactivity
the release of energy and/or particles resulting from an unstable nucleus
There is no set rule for stability, but from experiment stability is based on the
neutron to proton ration, the further the value of np is from 1, the more likely the
isotope is radioactive.
12
6
13
6
C
n
p
C
= 66
7
6
14
6
C
8
6
furthest from 1, so most likely radioactive
 Nuclear Transformations
a change in the number of protons and /or neutrons in the nucleus as a result
of radioactive decay
 Half-life
The time it takes for half of a sample of a radioactive isotope to decay. For
example, the half-life of 32P is 14 days. So after 14 days a 50 g sample of
32
P is now 25 g of 32P and 25 g of 32S. (see beta decay below)
 Types of radioactive decay
4
o , Alpha Particle, 2 He positive He nucleus ejected from the nucleus ,
o Beta Decay,
0
1
Fr  24 He + 219
85 At
high energy e- is ejected from the nucleus (n p + e-),
e
o , Gamma Rays
32
15
32
P  10 e + 16
S
high energy photon emitted as nucleus moves from excited to lower energy
state
232
90
Th *  132
90Th +  (*=excited state)
0
o Positron Emission, 1 e positive particle ejected from nucleus (p
o EC-electron capture
223
87
1
0
n+
0
1
30
30
e ), 15
Si
P  10 e + 14
e- falls into nucleus combining with a proton and forming a neutron,
Tl + 10 e  202
80 Hg
202
81