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Problem Set 1 ANSWERS
The Structure and History of the Atom
rd
Due Wed Jan 23
Test Friday Jan 25th
Conceptual questions
1) What is chemistry? Chemistry is the study of matter and the changes it undergoes.
Answers will vary for the professional chemists from teachers to environmental
activists to doctors to pharmacists to chefs…
2) Define physical change in your own words. a rearrangement of matter so that its
chemical properties are still the same, but some physical properties are changed.
(phase, surface area) Define chemical change in your own words. A rearrangement of
matter so that its chemical properties have changed. New bonds are formed, old bonds
are broken or rearranged.
3) Many of you did not yet e-mail me. Please do this.
4) The basics of any experiment should include: one variable to measure, one variable to change
in a controlled manner, a control run to compare the variable to, and all other potential
variables held constant. Multiple trials would be beneficial, as would recordings quantitative
and qualitative data. Washing one hand in water without any soap, then washing the other in
the new and improved soap. Compare the two hands… your answers will vary.
Conceptual questions
5) What are the major components of Democritus’s Atomic Philosophy? (see pg 88)
a. Matter is composed of empty space through which atoms move
b. Atoms are solid homogeneous indestructible and invisible
c. Different kinds of atoms have different sizes and shapes
d. 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.
6) 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.
7) 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.
8) 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.
9) 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
10) True False Questions – determine if the following statements are true or false according
to Democritus’s ideas about atoms.
Atoms are divisible. FALSE
Atoms are hard dense spheres FALSE
Changes in matter are due to the changes in atoms. TRUE
11) True False Questions – determine if the following statements are true or false according
Dalton’s Atomic Theory
All matter is made of small particles called atoms. TRUE
Atoms are divisible. FALSE
Atoms of the same element are identical TRUE
Atoms can be separated, combined or rearranged TRUE
Extension Questions
12) 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.
13) 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 basically it is taught because it is a law of
science
Sub-Atomic Particles and the Nuclear Atom: USE YOUR TEXTBOOK!!!
Conceptual Questions
14) 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
15) 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 with
electricity
16) Rutherford used radioactive material to fire positively charged particles at thin sheets of
metal. What is the name of those particles? Alpha particles
What is the composition of those particles? 2 protons, 2 neutrons
17) 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 three 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
18) What were Rutherford’s two observations from his famous experiment?
Most of the sparks were located directly behind the gold foil
There were some (very few) sparks in front of the foil
19) What were the two conclusions that correspond to the Rutherford’s two observations? Be
sure to connect each conclusion with the appropriate observation.
The atom is mostly empty space
There is a small dense positive nucleus to the atom
Practice Questions – see back of pset for answers
20) Who determined that electrons exist? JJ Thomson
21) Who determined the mass of an electron? Robert Millikan
22) 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
23) Who determined that the nucleus of an atom exists? Rutherford
24) Who determined the charge of nucleus? Rutherford
25) What is the charge of the nucleus? Positive
26) Who determined that neutrons exist? Chadwick
Extension Questions
27) Draw three historical pictures of an atom if we could take a picture of it…be as detailed
as possible
a
Dalton’s Perspective
empty
Thomson and Millikan’s Perspective b
Rutherford and Chadwick’s Perspective
c
empty
28) Why do you think we are starting with the atom in chemistry class?
Answers will vary, but defining the word “chemistry” is a good place to start…
it is the study of matter and the changes that matter undergoes. IF we can
understand elements on the atomic level, we will have a better understanding of
how they change, what changes about them and how humans have tried to
understand and organize nature.
29) Why do you think we are starting with the history of the atom? Again answers will
vary, but the historical evidence allows you a perspective of how humans have
improved what we understand to be true. It will also show that we are not always
right! We must make adjustments to what we understand based upon data,
observations and new ideas.
30) 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….
31) 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
32) Define the following terms
Atomic number the number of protons in an atom -identifies the element
Isotope – an atom of an element with a different number of neutrons than another atom
of the same element
Average Atomic Mass the weighted average of the isotopes of an element – represented
in the periodic table of elements
Percent Abundance the amount of each isotope that is found in nature
33) 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.
34) How are isotopes of the same element different? They must have different number of
neutrons.
35) What sub-atomic particle(s) identify/(ies) an element? protons
36) What sub-atomic particle(s) determine(s) the isotope of an element? neutrons
37) 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
38) 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.
39) 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
Practice Questions
40) Fill in this table for each atom or ion. You may write on the Problem Set… rather than
re-draw the table in your responses. –
Isotope
Symbol
# p+ #e - #no
Atomic
Mass
Charge
Name
Number
Number
118
tin - 118
68
0
50
50
50
118
Sn
+1
K
19
50
Potassium-40
40
19
18
21
19
40
+1
9
10
10
9
19
-1
7
10
7
7
14
-3
52
52
76
52
128
0
Fluorine -19
19
F -1
9
Nitrogen -14
14
7
N
128
tellurium - 128
52
-3
Te
Extension Questions
41) List the steps needed to calculate the average atomic mass if you are given the relative
abundance and mass of each isotope known to exist. Sum the products of the mass
times the relative abundance of each isotope.
42) Nitrogen has two naturally occurring isotopes: nitrogen-14 and nitrogen-15. The average
atomic mass of nitrogen is 14.0007 amu. Which isotope is more abundant? Explain.
Nitrogen– 14 is more abundant because the average atomic mass is much closer to
the mass of nitrogen–14 than it is to nitrogen-15.