CEM 141 Exam #3-Question with Correct Answers/ Verified
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Course
Cem 141
Institution
Cem 141
3-D extended continuous network (shown on tables in notes in red); continuous materials have
higher boiling and melting points than discrete -️️What are continuous materials?
small atoms/molecules (shoes on tables in notes in black) -️️What are discrete materials?
H2: LDF
He: LDF
Li: m...
CEM 141 Exam #3-Question with Correct Answers/ Verified
3-D extended continuous network (shown on tables in notes in red); continuous materials have
higher boiling and melting points than discrete - ✔️✔️What are continuous materials?
small atoms/molecules (shoes on tables in notes in black) - ✔️✔️What are discrete materials?
H2: LDF
He: LDF
Li: metallic bonds (no LDFs in metals) move metal cores relative to one another
c: covalent - ✔️✔️Using the table found on lecture 25 slide 13 what kinds of interactions must be
overcome in order to melt: H2, He, Li, and C?
diamond, graphite, lonsdaleite (not common), buckminsterfullerene, C540, C70, amorphous (soot),
carbon (nanotube) - ✔️✔️What are the allotropes of carbon?
high melting point; hard; brittle (breaks alongs planes); translucent (lets light through); does not
conduct electricity - ✔️✔️What are some characteristics of diamond?
high melting point; soft; slippery; grey, shiny; conducts electricity; opaque - ✔️✔️What are some
characteristics of graphite?
because the arrangement of atoms is different the bonding is different - ✔️✔️Diamond and
graphite are made out of carbon atoms only--how can they have such different properties?
-each carbon forms 4 bonds to 4 identical carbons
-the bonds arrange themselves towards the corners of a 4 sided figure (a tetrahedron)
-tetrahedral geometry
-the C-C-C bond angle is 109 degrees - ✔️✔️Explain the bonding in diamond.
all of those covalent bonds would have to be overcome; would require a lot of energy, therefore,
diamond does not really melt it decomposes - ✔️✔️If you wanted to "melt" diamond (tetrahedral
carbon) what would have to happen?
,metals melt because they have metallic bonds and a sea of electrons that can easily move about -
✔️✔️why do metals melt and diamonds do not?
bonds; localized - ✔️✔️To explain bonding in carbon, we use valence bond theory to assume that
the atomic orbitals overlap to form _________________.
we consider the bonds to be ___________________ between the two bonded atoms (not like in MO
theory).
-atomic orbitals combine to form an equal number of molecular orbitals
-each orbital can contain up to two electrons
-electrons in bonding orbitals stabilize the system
-electrons in anti-bonding orbital make it less stable
-electrons are delocalized - ✔️✔️Explain the molecular orbital model of bonding.
-atomic orbitals overlap to form a bond
-the greater the overlap, the stronger the bond
-each bond made up of two electrons
-electrons are localized in the bond - ✔️✔️Explain the valence bond model of bonding.
[He] 2s2 2p2 (1 s orbital and 3 p hybridize to make 4 sp3 orbitals) - ✔️✔️What is the electron
configuration of carbon?
using the electron configuration we know a C atom has 4 valence electrons in different types of
orbitals; electrons in a filled s orbital and two half-filled p orbitals
-should give different types of bonds, but the evidence indicates that all four bonds are identical,
therefore, it uses hybrid orbitals - ✔️✔️How can carbon form 4 identical bonds in diamond?
hybrid - ✔️✔️# atomic orbitals in = # ______________ orbitals out
4 - ✔️✔️How many atomic orbitals (on one carbon) should combine to make 4 hybrid orbitals?
-atomic orbitals hybridize (mix up) to form bonding orbitals that then combine with orbitals from
other atoms to form a bond
, -to form 4 bonds-carbon hybridizes the 4 orbitals that are used for bonding
-since one s and three p orbitals combine we call them sp3 orbitals
-they naturally assume a tetrahedral geometry
-when the hybrid orbitals combine there is a large gap between the bonding and anti-bonding
molecular orbitals - ✔️✔️Explain the bonding in diamond using the valence bond model.
strong directed bonds; these bonds give rise to high mp/decomposition temperature--because these
bonds have to be broken to melt diamond
*these bonds are sigma bonds* - ✔️✔️What do hybridized atomic orbitals (sp3) give rise to?
- ✔️✔️For a better understanding of sigma bonds in diamond refer to diagram on slide 20 of
lecture 26.
3D network of strong bond, you would have to break bonds to disrupt a crystal - ✔️✔️Why is a
diamond hard?
-electrons are located in bonds between atoms-not free to roam
-there is a large "band gap" between the bonding and anti-bonding orbitals - ✔️✔️why does
diamond have a high mp and does not conduct electricity?
light passes through or is reflected; to absorb light an electron must be promoted to a higher energy
level; there is a large "band gap" between the bonding and anti-bonding orbitals' diamond cannot
absorb a photon of many wavelengths - ✔️✔️why is a diamond translucent?
into many sheets or layers that are stacked together by LDFs
3
3 - ✔️✔️How is graphite arranged on the molecular level? In one sheet, how many carbons is each
carbon attached to? How many orbitals should "mix up" (hybridize) to form 3 bonds?
-one s and two p orbitals hybridize to give 3 sp2 orbitals (there is a p orbital left over)
-geometry is called trigonal planar
-C-C-C bond angle is 120
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