Chemistry exam 3
the three concepts for VB Theory.
The atomic orbitals around the central atom are considered when predicting bonding between atoms Atomic orbitals combine to form hybridized orbitals Electrons are localized
When two orbitals combine in MO Theory, two new molecular orbitals are formed: one bonding orbital and one anti-bonding orbital.
The bonding orbital is ALWAYS lower in energy than the anti-bonding orbital. All anti-bonding orbitals are given the symbol, *
ion-ion interactions
These electrostatic interactions are relatively easy to identify because they exist in any lattice that contains cations and anions.
Can nonpolar compounds have dispersion forces?
Yes, all molecules have dispersion forces.
In Lewis structures, a solid line is used to represent...
a covalent bond
nonpolar covalent bond
a covalent bond in which the electrons are shared equally by the two atoms
Hydrogen bonding is a special case of extreme dipole-dipole forces. It can only occur when
a hydrogen atom is bonding directly to one of these three electronegative atoms: fluorine, oxygen, and nitrogen. A hydrogen bond is formed between the partially positive hydrogen atom from molecule A and the electronegative atom from molecule B.
Viscosity
a liquids resistance to flow
If the anions and cations can pack tightly together (i.e. small ions), it will require
a lot of energy to separate the solid into gaseous ions.
If the anions and cations have a strong electrostatic interaction (i.e. large charges), it will require
a lot of energy to separate the solid into gaseous ions.
paramagnetic
a molecule with at least one unpaired electrons
diamagnetic -
a molecule with no unpaired electrons
Ammonia (NH3) can form hydrogen bonds with neighboring molecules. This intermolecular force is formed from
a nitrogen atom in one molecule and a hydrogen atom in another molecule. This strong attraction is a direct result of the electronegativity difference in the covalent bond between nitrogen and hydrogen in a single ammonia molecule.
Surface tension -
a property of liquids that arises because the molecules at the surface of a liquid have higher energy than the molecules in the middle of the liquid
The electronegativity of H is...
about equal to that of C.
covalent solid -
all atoms bonded with covalent bonds
molecular solid
all molecules connected with IMFs
the guiding principle of lewis structures is that
atoms tend to arrange in ionic or molecular clusters to satisfy the octet rule (i.e. to try to achieve noble gas configuration).
The hotter the temperature, the more energy a system can use to
break the IMFs between neighboring molecules.
molecular solids characteristics
brittle low melting points
Name the compound CaBr2.
calcium bromide
Consider a situation in which you discover an unknown solid. After examining the solid, you know that all atoms are nonmetals and that they share their electrons equally. You determine that the melting point is 277 °C. What type of solid did you find?
covalent
S(g) + e- → S-(g)
electron affinity
The energy that is gained by forming the anion.
electron affinity
a large atom with a small effective nuclear charge will not be very
electronegative
the valence electrons for elements in each group are
essentially the same
Vapor pressure is a liquid property related to
evaporation
A small atom with a large effective nuclear charge will be
extremely electronegative
Dispersion forces
extremely weak electrostatic interactions between neighboring molecules and/or atoms. Every system can exhibit dispersion forces, which means that all molecules and atoms have dispersion forces. Dispersion forces are induced-dipole induced-dipole forces that arise from fluctuation in the arrangement of the electrons around a molecule. If there is a fluctuation that leads to the molecule having an instantaneous dipole, this dipole will induce a dipole in a neighboring molecule. This will in turn induce another dipole in another neighbor. The induced dipole effect will propagate throughout the whole system.
K(g) → K+(g) + e-
first ionization energy
Molecular Orbital (MO) Theory.
focuses on bonding throughout a molecule All atomic orbitals are considered when predicting bonding across the molecule Atomic orbitals combine to form molecular orbitals Electrons are delocalized
Intermolecular forces
forces between molecules contribute to the physical properties of a substance examples: ion-ion, dipole-dipole, hydrogen bonding, dispersion
Intramolecular forces
forces within the molecule that keep the molecule together the bonds between the atoms examples: covalent, ionic
Ionic bonds are chemical bonds in which we assume the electrons have
fully "moved" from one element in the compound to another.
Ionic solids characteristics
hard high melting point conducts electricity in water
covalent solids characteristics
hard high melting points insoluble in water
Lewis structures are used to
help correctly predict the structural formula of these compounds.
The electronegativity of nonmetals is relatively __________ as compared to the electronegativity of metals.
high
HOMO -
highest occupied molecular orbital
If someone is talking about the electrostatic interaction between two sodium chloride molecules, are they referring to the intramolecular or intermolecular force?
intermolecular
ionic solid
ion-ion interactions between the cations and anions
The energy required to form the cation.
ionization energy
a structure can show resonance if
it has at least one double or triple bond.
As an atom's radius decreases...
its ionization energy increases.
The degree to which a molecule has dispersion forces is measured by
its polarizability. The more electrons a molecule has and the farther they are from all the nuclei, the more likely a dipole can be induced in the electron cloud (aka: polarized).
The energy that is gained by bringing the cation and anion together.
lattice energy
What is the electronic geometry for a molecule with two RHEDs surrounding the central atom?
linear
If a molecule has strong intermolecular forces, would you expect for it to have a high or low vapor pressure?
low
would you expect for a nonpolar molecule to have a higher or lower boiling point than a polar molecule? For this question, assume that the molecules have the same relative size.
lower
LUMO -
lowest unoccupied molecular orbital
Metallic solids characteristics
malleable delocalized electrons thermal and electrical conductor
When the adhesive forces are greater than the cohesive forces, a liquid will tend to try to wet the surface. The liquid will maximize attractive forces by
maximizing the contact with the surface.
metallic solid
metal atoms connected by dative bonds
Which of the following solids is malleable AND has delocalized electrons?
metallic solid
When the cohesive forces are greater than the adhesive forces, a liquid will tend to bead on the surface. The liquid will maximize attractive forces by
minimizing the contact with the surface
Water molecules are often connected by strong hydrogen bonds. What type of solid would you expect ice to form?
molecular solid
In a structure, each atom is assigned a formal charge based upon the
number of valence electrons for that atom as well as the distribution of electrons in the structure. It can help us in two ways.
When you combine two s orbitals, The σ bonding orbital has zero nodes, whereas the σ* anti-bonding orbital has
one node
A phosphorous‒chlorine bond would be expected to be...
polar, with the chlorine end having a partial negative charge.
K+(g) → K2+(g) + e-
second ionization energy
When two p atomic orbitals combine, two different types of molecular orbitals can form:
sigma or pi. Head-on overlap causes two sigma molecular orbitals to form: σ and σ*. Side-by-side overlap causes two pi molecular orbitals to form: π and π*.
Given two isoelectronic ions, the one with more protons will be
smaller
We expect bromine to have a BLANK atomic radius than selenium because it has a BLANK effective nuclear charge.
smaller attomic radium, larger Zeff
All linear molecules have bond angles of 180°. What is the hybridization of the central atom with 2 RHEDs in a linear molecule?
sp
Did you know that a paperclip will float on water? This is because the force down from the gravity on the paperclip is not sufficient to overcome the
surface tension of the liquid
MO Theory always follows the
the Aufbau Principle, the Pauli Exclusion Principle, and Hund's Rule,
We used the valence bond and molecular orbital theories to help us understand
the bonding within a molecule.
if you have a cation and an anion that are isoelectronic, which will be smaller
the cation
Formal charge is a concept used to account for
the distribution of electrons in a compound.
What are periodic trends a direct result of?
the effective nuclear charge of an atom
Electron affinity is
the energy released from an atom upon the addition of an electron to form the anion.
Ionization energy is
the energy required to remove an electron from an atom in the gas phase.
The lattice energy of an ionic compound is defined as
the energy required to separate one mole of the solid into its component gaseous ions. This energy is directly proportional to the size of the atoms and inversely proportional to the charge they carry.
Dipole-dipole forces
the forces between molecules with permanent dipole moments. If a molecule is polar, then it will exhibit dipole-dipole forces.
Cohesive forces
the forces of attraction between a liquid and itself
Adhesive forces -
the forces of attraction between two different molecules.
intermolecular forces (IMFs)
the interactions between two different molecules
Unlike with Valence Bond Theory, we can use Molecular Orbital Theory to predict
the magnetism for a molecule.
Bond Order is
the number of chemical bonds between a pair of atoms Bond Order = 1/2 (bonding electrons - antibonding electrons)
Electronegativity describes
the relative pull of the electrons in a covalent compound.
the boiling point
the temperature at which IMFs break in order for a liquid to boil
the properties of the valence electrons are related to the strength of
their interaction with the nucleus.
Elements in the same group will have similar properties because
they have similar electron configurations.
HCN can never show resonance
true
The π bonding orbital has one node, whereas the π* anti-bonding orbital has
two nodes
When two s atomic orbitals combine,
two sigma molecular orbitals are formed: σ and σ*.
How many unshared electrons and bonding electrons exist around the central atom in ozone (O3)?
two, six
It is important to realize that it is difficult to make comparisons between very different types of compounds, but it is always possible to
understand trends among similar compounds.
Covalent compounds are generally made up of elements found in which part of the periodic table?
upper right
Did you know that molecules can become tangled when a liquid is poured? This is because molecules with branches or kinks have a greater
viscosity compared to straight-chain molecules. It is harder for them to "slide by" other molecules than it is for straight-chained molecules.
Resonance is a concept that describes the bonding in molecules...
where there is more than one choice of location for a double or triple bond as deduced from Lewis dot structures. The true bonding is the average over all possible multiple bond locations.
fluorine and fluorine
would share electrons equally in a nonpolar covalent bond
hydrogen and hydrogen
would share electrons equally in a nonpolar covalent bond
nitrogen and nitrogen
would share electrons equally in a nonpolar covalent bond
Formal Charge =
(# of valence electrons) - (# electrons touching the atom in the Lewis structure)
Calculate the formal charge on N in the molecule NH3
0
What is the total formal charge for neutral compounds?
0
The diameter of the electron density of an atom is roughly...
0.1 - 0.5 nm
How many double bonds are present in the 'best' resonance structure of the phosphate ion?
1
How many lone pairs of electrons are on nitrogen in NF3?
1
1 RHED is equal to..
1 lone pair of electrons 1 single bond 1 double bond 1 triple bond
How many total bonds and lone pairs exist in the Lewis structure for chlorine fluoride (ClF)?
1, 6
Lewis Structure Rules
1. Determine total # valence e- in molecule 2. Write skeletal structure and start with single bonds between elements 3. Fill in remaining electrons as lone pairs. Try for OCTET around all except H (and other exceptions, to be discussed). 5. If you can't satisfy octet (or exception) rule for all atoms, try double or triple bonds. 6. CHECK: Are the right number e- around each atom? Are all available electrons assigned (right # total)?
formal charge can help us
1. It can help us decide which of several Lewis structures is closest to representing the properties of the real compound. 2. It can help us envision where there might be regions of positive or negative charge in a molecule.
The strategy for classifying solids
1. Look at the atoms in the substance Metallic = Metals Ionic = Metals + Non-metals Molecular OR Covalent = Non-Metals 2. Molecular solids usually melt below 500K 3. Covalent network solids melt above 500K
In general we can think about the formation of an ionic bond as the sum of three processes.
1. The energy required to form the cation. 2. The energy that is gained by forming the anion. 3. The energy that is gained by bringing the cation and anion together.
Bond Order =
1/2 (bonding electrons - antibonding electrons)
The P2‒ anion has how many total electrons and how many valence electrons respectively?
17, 7
How many resonance structures can be drawn for N2O? The atoms are arranged in the following order: N-N-O. Disregard any structure with formal charges other than 0, +1, and -1.
2
How many RHEDs does the central carbon atom in CS2 have?
2 2 sets of double bonds
What is the bond order of the O‒O bond in O2?
2 oneDouble bond
How many resonance structures for nitrate ion
3
The carbonate ion (CO32-) has how many resonance configurations?
3
how many resonance structures for carbonate ion
3
What is the bond order of the C‒C bond in acetylene (ethyne, C2H2)?
3 one triple bond between C-C
How many total bonds and lone pairs exist in the Lewis structure for boron trichloride (BCl3)?
3, 9
How many valence electrons are in a Kr atom?
8
polar covalent bond
A covalent bond in which electrons are not shared equally
Regions of High Electron Density (RHEDs)
A region within a molecule where valence electrons can be found
Name the compound CaC2O4.
Calcium oxalate
Valence Bond (VB) Theory
Describes one way of picturing *how* the bonding occurs Describes the geometry predicted by VSEPR You can imagine the atoms combining their "pure" valence shell atomic orbitals (s, p, d, and f) to form a new set of orbitals to use in bonding. The process is called hybridization The resulting orbitals are called hybrid orbitals
the only periodic trend that assumes that the atom is within a molecule.
ELECTRONEGATIVITY
Valence Shell Electron Pair Repulsion (VSEPR) Theory
Electrons repel other electrons They require "elbow space" The Electronic Geometry is the resulting shape of the electrons that minimizes the repulsions between rheds The Molecular Geometry is the resulting shape of the atoms that minimizes the repulsions between rheds
Consider the elements lithium, oxygen, fluorine, and neon. Based on their position in the periodic table, which element would you expect to have the GREATEST tendency to attract a shared pair of electrons?
F
The most important property the Molecular Orbital Theory can predict is the
HOMO-LUMO gap.
Lattice Energy is defined as
MX(s) → M+ (g) + X- (g).
Choose the formula for the compound magnesium sulfide.
MgS
What is the formula of dinitrogen pentoxide?
N2O5
The molecule AX2 has two RHEDs surrounding the central atom, A. Is this molecule polar?
NO
Which of the following demonstrates the formation of an ionic compound involving the elements Na and S?
Na+ + Na+ + S2- ⟶Na2S
Give the formula for sodium nitrate.
NaNO3
Write an equation that represents the second ionization energy of nickel.
Ni+(g) ⟶Ni2+(g) + e-
Will carbon dioxide have dipole-dipole forces?
No because the molecule is not polar, only the bonds
