PA
Problem Set 3 ANSWERS
Due Tues Oct 15th
The Structure of the Atom
Conceptual questions
What are the major components of Democritus’s Atomic Philosophy? (see pg 88)
Matter is composed of empty space through which atoms move
Atoms are solid homogeneous indestructible and invisible
Different kinds of atoms have different sizes and shapes
The differing properties of matter are due to the size, shape and movement of
atoms
e. Apparent changes in matter result from changes in the groupings of atoms and not
from changes in the atoms themselves.
1)
a.
b.
c.
d.
2) What is wrong with Democritus’s ideas? There are several different types of atoms,
they are made of protons, neutrons and electrons (which are all made of smaller
particles called quarks, by the way). Atoms are heterogeneous, destructible and
visible, and atoms can change in a nuclear decay reaction.
3) What are the 5 major components of Dalton’s Atomic Theory?
a. All matter is composed of small particles called atoms
b. Atoms of the same element are the same. Atoms of a specific element are different
from those atoms of any other element.
c. Atoms cannot be created or destroyed or divided into smaller particles
d. Different atoms combine in simple whole number ratios to form compounds
e. In a chemical reaction, atoms are separated, combined or rearranged.
4) What is wrong with Dalton’s Atomic Theory? Isotopes exist… so atoms of the same
element are not all the same. Atoms can be created and destroyed and broken into
smaller particles. Nuclear reactions are a 4th type of reaction.
5) What is correct in Dalton’s Atomic Theory? The Law of Definite Proportions, atoms
do exist, Chemical bonds are formed, broken or rearranged during a chemical
reaction, and atoms of different elements are different.
Practice Questions
6) True False Questions – determine if the following statements are true or false
according to Democritus’s ideas about atoms.
a. Atoms are divisible. FALSE
b. Atoms are hard dense spheres FALSE
c. Changes in matter are due to the changes in atoms. TRUE
7) True False Questions – determine if the following statements are true or false
according Dalton’s Atomic Theory
a. All matter is made of small particles called atoms. TRUE
b. Atoms are divisible. FALSE
c. Atoms of the same element are identical TRUE
d. Atoms can be separated, combined or rearranged TRUE
Extension Questions
8) What is a Scanning Tunneling Microscope? Briefly describe what it does and
why it is an important tool for scientists. The STM is a high powered
microscope that beams electrons off of a surface to indicate the distance from
a probe to the surface. Three Dimensional models of the surface can be built
from these very accurate (but very small) outputs.
9) Why do you think Dalton’s Atomic theory is still taught in school, even though it
is over 200 years old? Answers will vary.. but it is taught because it is a law of
science
Sub-Atomic Particles and the Nuclear Atom: USE YOUR TEXTBOOK!!!
Conceptual Questions
10) The cathode ray tube was used to discover the electron. Re-Draw the diagram
from your notes and include what the ray looks like when the + end of a magnet is
brought near the ray.
MAGNET
The ray will travel toward the + pole
of the magnet because the particle of
the ray are negatively charges.
+
-cathode
anode
high voltage
11) What conclusion did Thomson draw from his observations? The atom is
divisible! There are negatively charged particles in the atom that can be
drawn out
12) Rutherford used radioactive material to fire charged particles at thin sheets of
metal.
a. What is the name of those particles? Alpha particles
b. What is the composition of those particles? 2 protons, 2 neutrons
13) Create a diagram of Rutherford’s Gold Foil Experiment. Be sure to include the
following details: the alpha particles, the radioactive polonium, the zinc sulfide
lining, the gold foil, and these 3 locations of hits: the most hits, some hits, and
only a few hits.
ZnS lined ring
Au
209
PoPo
Most hits
alpha particles ()
Very few hits
some hits
14) What were Rutherford’s two observations from his famous experiment?
a. Most of the sparks were located directly behind the gold foil
b. There were some (very few) sparks in front of the foil
15) What were the two conclusions that correspond to the Rutherford’s two
observations? Be sure to connect each conclusion with the appropriate
observation.
a. The atom is mostly empty space
b. There is a small dense positive nucleus to the atom
Practice Questions – see back of pset for answers
16) Who determined that electrons exist? JJ Thomson
17) Who determined the mass of an electron? Robert Millikan
18) What is the mass of an e-? 9.1 x 10-28 g p+: 1.67 x 10-24 g
no = 1.675 x 10-24 g
19) Who determined that the nucleus of an atom exists? Rutherford
20) Who determined the charge of nucleus? Rutherford
21) What is the charge of the nucleus? Positive
22) Who determined that neutrons exist? Chadwick
Extension Questions
23) Draw three historical pictures of an atom if we could take a picture of it…be as
detailed as possible
a
a. Dalton’s Perspective
b. Thomson and Millikan’s Perspective b
empty
c. Rutherford and Chadwick’s Perspective
c
empty
24) Why are we starting with the atom? To get a reference for all the reactions
25) Propose a reason for a neutron to be ever so slightly more massive than a proton.
Answers will vary, but the neutron is made of a proton and an electron…
think about how a beta particle is formed….
26) Diagram one of the experiments on the atom on one page of blank paper. Make it
colorful, big, label it… these will be posted in the room as we progress through
our study of the atom.- check out the ones that were submitted in the class
How Atoms Differ: USE YOUR TEXTBOOK!!!
Conceptual Questions
27) Define the following terms
a. Atomic number the number of protons in an atom -identifies the
element
b. Isotope – an atom of an element with a different number of neutrons
than another atom of the same element
c. Average Atomic Mass the weighted average of the isotopes of an
element – represented in the periodic table of elements
d. Percent Abundance the amount of each isotope that is found in nature
28) How are isotopes of the same element alike? They must have the same number
of protons and electrons to be the same element and to be an atom.
29) How are isotopes of the same element different? They must have different
number of neutrons.
30) What sub-atomic particle(s) identify/(ies) an element? protons
31) What sub-atomic particle(s) determine(s) the isotope of an element? neutrons
32) Use the data below to calculate the average atomic weight for compound X. Then
use your periodic table to identify element X.
Isotope 1 = 90. 00 %, 39 a.m.u.
Isotope 2 = 9.000 %, 40 a.m.u.
Isotope 3 = 1.000 %, 41 a.m.u.
39 amu x .9 + 40 amu x 0.09 + 41 amu x 0.01 = 39.11 amu  Potassium
33) One of the stable isotopes of tins is tin-119. The atomic number of tin is 50. How
many protons, neutrons and electrons are in one atom of this isotope of tin?
Tin has 50 protons and because it is a neutral atom, 50 electrons. The mass
number is 119, so there must be 69 neutrons.
34) What is the atomic mass of an isotope of potassium that has 21 neutrons?
Potassium has 19 p+, so an isotope with 21 no will have a mass of 40 amu
35)
Isotope
Name
tin - 118
Symbol
118
Sn
40
+1
K
19
# p+
#e -
#no
50
50
19
Mass
Number
118
Charge
68
Atomic
Number
50
18
21
19
40
+1
9
10
10
9
19
-1
7
10
7
7
14
-3
52
52
76
52
128
0
0
50
Potassium-40
Fluorine -19
19
F -1
9
Nitrogen -14
14
7
N
128
tellurium - 128
52
-3
Te