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NAME__Answers_____
Sci 101
3rd Midterm Exam
December 29th 2009
1.
(14 points)Fill in the blanks
a) Matter composed of two or more atoms is called Compound/Molecule and they have
a definite/fixed composition.
b) Inertness, flammability and acidity represents the chemical properties of matter
c) The electron that has the highest energy in the ground state of a sodium atom will
be in 3s orbital.
d) When atoms gain energy the electrons are excited
to higher orbitals.
e) The line emission spectrum of pure elements indicate that the atomic orbitals
are/have quantized/definite energies_.
f) The reactivity of alkali metals increases as one moves down the group.
g) Anions always have larger_ radius tan the corresponding neutral atom.
h) Isotopes are atoms that have the same number of protons (meaning they are the
same element) but having different numbers of neutrons (meaning they have
different mass number.
2. (6 points) Fill in the gaps in the following table:
Symbol
Protons
Neutrons
Electrons
Net charge
102
Rb3+
37
65
34
+3
129
Cs1+
74
54
+1
3. Multiple choice problems (3 points each) In each question (a – j) write the correct answer
in the space provided. DO NOT CIRCLE THE ANSWER!!! IF ANSWERS ARE
CIRCLED YOU WILL GET NO CREDIT!!
a) The bond with highest polarity. N-Br O-Br, P-Br. O-Br
b) The number of electron(s) in the 3p orbitals of Cl
5
c) How many lone pair electrons in the Lewis structure for NF3. 2/10 or 1/10
d). The most metallic element: Ag, Fe,
Sb,
Ti,
Rb Rb
e). The element with the greatest first ionization energy: Ge, Ca,
f) The element that has the least electronegativity: Se,
Sr,
I,
F, Na,
P, B, Cl. Cl
Cl Sr
g) The element that has the most negative electron affinity: K, Mg, Br, Ge, Sn Br
h) The atom that has only 4 valence electrons: Na,
Si,
Al,
P,
Ca
Si
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i) The elements that will show greatest similar chemical properties in the following list
Cr, Ca, Si, S, Te, Br
S and Te
j) The compound that have a chemical bonds with largest percentage of ionic character?
CO2,
IBr, H2O, HF, C (diamond) HF
4. (4 points) predict the formula of the ionic compound that will form between K and S.
K2S
5. (5 points) The following species are isoelectronic with the noble gas Krypton. Arrange
them in order of increasing size
Rb+ Y3+ Br- Kr Sr2+ Se2Se2- > Br-> Kr > Rb+> Sr2+> Y3+
6. (8 points) Draw the three-dimensional shape of H3O+ ion and write its geometric name.
Trigonal pyramidal (Tetrahedral)
7. (10 points each) Draw the Lewis structure of the following compounds. Explain if these
molecules have dipole moment or not.
a) Ozone (O3)
b) SF2
.
Bent Molecule
Will have dipole moment
Bent molecule
Will have dipole moment
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8. (6 points) Briefly explain the contribution to the atomic theory by the gold foil
experiment carried out by Rutherford.
This is the experiment where a thin gold foil is bombarded by heavy -particles. It was
expected that all of the-particles would go through the foil without any deflection.
However, certain number of -particles was deflected. It is thus concluded that the main
mass of atoms are concentrated at a very small nucleus and the majority volume of atoms
are empty space.
9. (7 points) Decide if the following statement is true or false: “All molecules that have
polar covalent bond(s) must be polar”. Explain why in either case. Give an example
if necessary.
Not true. If the dipole vectors oppose each other such that negative and positive
partial charges coincide, then the distance d = O. This will mean the molecule will not
have a dipole moment. CO2 is a good example. C O bond is highly polar. However in
CO2 which is a linear molecule, the opposing moments cancel each other.
O=C=O
10. (5 points BONUS!!!) What is chemical communications. Why do you think it is
important.
This is how rest of the living animals besides humans communicates. Each species
excrete special pheromones which signals danger, food, mating etc. within the
community.