Chemistry 2202 Unit 2 Section 2 Homework Portfolio (2006-2007)
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CHEMISTRY 2202
Unit 2 Section 2 Homework Portfolio -- Solution Key
Value
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1.
Why do atoms of low electronegativities normally form cations?
Atoms with low electronegativities do not attract their valence electrons strongly.
Therefore an atom with a high electronegativity has a greater attraction for these
valence electrons and a transfer of electrons occurs. The atom with low
electronegativity loses its valence electrons, and becomes a positively charged
ion, a cation, because it now has less electrons than protons.
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2.
Classify each species as stable or unstable and neutral or charged and explain
each of your choices.
(a) potassium atom: unstable and neutral. The potassium atom is unstable
because it has only one valence electron, but is neutral because it has an equal
number of electrons and protons.
(b) oxygen atom: unstable and neutral. The oxygen atom is unstable because it
has six valence electrons, but is neutral because it has an equal number of
electrons and protons.
(c) neon atom: stable and neutral. The neon atom is stable because it has eight
valence electrons and is neutral because it has an equal number of electrons and
protons.
(d) magnesium ion: stable and charged. The magnesium ion is stable because
its valence orbitals are filled. It has a 2+ charge because it lost 2 electrons.
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3.
(a)
Write a Lewis diagram equation for the reaction between each pair of
elements:
(i) sodium and oxygen
(ii) magnesium and nitrogen
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(b)
Write the empirical formula (formula unit) for each ionic compound formed in
3a.
(i) Na2O
(ii)
Mg3N2
Chemistry 2202 Unit 2 Section 2 Homework Portfolio (2006-2007)
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4.
(a)
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What is an ionic bond?
An ionic bond is the electrostatic attraction of positively charged ions and
negatively charged ions.
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(b)
When describing potassium chloride, KCl, why is the term “formula unit” used
instead of “molecule”?
An ionic compound such as KCl consists of many positively charged and
negatively charged ions arranged in a three dimensional structure. The term
“formula unit” describes the lowest ratio (or ‘empirical formula’ ) of these ions.
The term molecule is used to describe compounds that bond by sharing
valence electrons to form a stable group of atoms.
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(c)
Why do ionic compounds often fracture to form pieces with flat sides?
Consider the diagrams below. Ions are held rigidly in place in the crystal.
When a force causes ions within the crystal structure to move, it often results in
ions of similar charge repelling each other along the plane of movement and thus
creates a plane along which the crystal fractures.
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5.
(a)
Define covalent bond.
A covalent bond is the attraction of the nuclei of two atoms for a shared pair of
valence electrons.
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(b)
Which elements are most likely to bond covalently? Why?
Elements that have a high electronegativity are most likely to bond covalently
because both atoms will have a strong attraction for the pair of electrons.
Chemistry 2202 Unit 2 Section 2 Homework Portfolio (2006-2007)
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(c)
Use Lewis diagrams to illustrate the formation of a molecule of dihydrogen
sulfide (H2S).
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(d)
Are all atoms in a molecule of H2S stable? Explain.
By sharing electrons the atom of sulphur now has a stable octet of electrons
in its valence energy level. The Hydrogen atom, which has only one energy level,
is stable because the valence orbital is filled with two valence electrons.
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6.
Draw a Lewis diagram and a structural formula for each molecule. Be sure to
properly illustrate the single, double and triple bonds.
(a) CH2Br2
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7.
(b)
C2Br4
(c)
C2Br2
What are the properties expected of a compound that is held together by network
covalent bonding?
A network of covalent bonding creates structures in which the atoms are rigidly
held in place by numerous covalent bonds. These atoms require a tremendous
amount of force or heat to break these bonds and dislodge them from their
positions. Therefore network solids will be very hard, have very high melting
points and be poor conductors of heat. Because the valence electrons are shared
between two atoms and are not mobile it is also expected that network solids will
be poor conductors of electricity
Chemistry 2202 Unit 2 Section 2 Homework Portfolio (2006-2007)
8.
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Four solids are tested in order to determine the type of bonding in each sample.
These observations were made during each test.
Sample
Electrical
Conductivity
Hit with
Hammer
Melting Point
Appearance
A
did not conduct
under any
conditions
shattered into
many pieces
75°C
white, opaque,
soft texture
B
did not conduct
under any
conditions
did not break
3550°C
colorless,
shiny, opaque
C
conducted in
the liquid state
only
shattered into
many sharp
pieces
801°C
white, shiny,
transparent
D
conducted in
solid and liquid
states
dented,
changed shape
232°C
grey, shiny,
opaque
Identify the bond type within each sample. Justify each choice.
Sample A: Covalent Bonding: It’s low melting point and soft texture are the best
evidence to support that the substance consists of molecules.
Sample B: Network Covalent Bonding: Its very high melting point and its
hardness suggest that the substance is a network solid.
Sample C: Ionic Bonding: The high melting point, the sharpness of the pieces
and the fact that it conducted electricity only after being melted strongly
suggest that this compound is an ionic compound.
Sample D: Metallic Bonding: The fact that it conducted electricity as a solid was a
strong clue as well as its tendency to dent suggest this substance is a metal.