Unit 2: Molecules Flashcards
What are the atomic and physical properties of alkali metals?
tend to be soft and have low melting point
low ionization energy, lose s electrons easily (M+ formed)
How would you extract alkali metals?
M+ —> M
NaCl —> Na + Cl2
What is Chemical Bonding?
atoms bond together to form molecules or extended structures
electrostatic attraction between cations and anions (ionic solids) as well as between electrons and nuclei (in molecules)
What are the properties of metals?
few valence electrons
low ionization energy
less negative electron affinity
tend to lose e-
What are the properties of nonmetals?
many valence electrons
high ionization energy
more negative electron affinity
tend to gain e-
What is ionic bonding?
metals + nonmetals
electrons are transferred completely from one atom to another
form an extended structure
a great deal of energy is released when the gaseous ions combine to form the ionic solid
poor electrical conductors in the solid, but good conductors when molten or dissolved in solution
What is covalent bonding?
nonmetals + nonmetals
electrons are shared between atoms (located in the space between them)
bonded atoms form a molecule
typically exist as liquids or gases with low mp or bp
What is metallic bonding?
metals + metals
electrons are highly delocalized and shared by all atoms
bonded atoms are held together by a “sea of electrons” in an extended structure
typically malleable and ductile, have moderately high mp and bp, and conduct heat and electricity well
What is bond energy?
the bond energy (BE) of a bond A-B is the energy required to overcome the attraction of the two atoms A and B
bond breaking: A-B –> A + B (endothermic)
bond making: A + B –> A-B
(exothermic)
What does bond length have to do with bond strength?
Besides a single covalent bond, stronger multiple bonds can also form, the bond strength is related to its length and energy
shorter bond means a stronger bond
How can you tell if a fuel will release more energy?
fuels with more weak bonds (like C-C and C-H) yield more energy
fuel with fewer bonds to oxygen release more energy
What is electronegativity?
most bonds are intermediate, having both ionic and covalent character
electronegativity: the relative ability of an atom, bonded within a molecule, to attract shared electrons to itself
What are the trends of electronegativity?
across a period: EN increases
down a group: EN decreases
EN is inversely related to atomic size, smaller atoms are more electronegative
What are dipole moments?
molecules such as HF with partial charges, separated by a distance, possess a dipole moment
molecules with dipole moments are aligned in an electric field
molecules with more than two atoms may or may not have an overall dipole moment, depending on how the bond dipoles add vectorially
What are the features of Lewis Structures?
Only valence electrons are shown
A pair of bonding electrons (or a bond pair) between two atoms is a single covalent bond and shown by a line, a pair of non-bonding electrons on one atom is a lone pair and is shown by two dots
Electrons are distributed so that atoms acquire a stable electron configuration, usually an octet (8 e-) for most atoms (but 2e- for H atoms), multiple bonds (double or triple bonds) may need to be formed
What is the procedure of drawing Lewis structures?
- Determine the total number of valence electrons
- Determine how atoms are connected, identify the central atoms and terminal atoms
- Draw a skeletal structure by joining atoms with single bonds, subtract 2 electrons for each single bond
- Distribute the remaining electrons in pairs, first complete octets around the terminal atoms, then distribute the remaining electrons around the central atoms
- If there’s too few electrons, convert lone pairs from terminal atoms to form multiple bonds with central atoms (to attain octets)
What is resonance?
often, more than one plausible Lewis structure
the true structure is a resonance hybrid of the contributing Lewis structures, the concept of resonance accounts for the fact that bonding electrons density can be delocalized over more than two atoms
What are the most important structures a Lewis diagram can have?
complete octets
low formal charges
negative formal charges borne by more electronegative atoms
separated like charges
What is a formal charge?
each atom is assigned electrons which “belong” to them
the formal charge on an atom is the difference between the number of valence electrons in the free atom and the number of assigned electrons in the bonded atom in the molecule
for a neutral molecule, the sum of the formal charges is zero, for an ion the sum of the formal charge equals the charge on the ion
What are some exceptions to the octet rule?
- Odd-electron species (place unpaired electrons on the least EN atom)
- Incomplete octets (Be, B, Al my have less than an octet)
- Expanded valence shells (third period or heavier elements may have 10 or 12 electrons around them)
What is the difference between formal charge and oxidation number?
in formal charge, electrons in bond are split evenly in half, corresponding to the covalent extreme
in oxidation number, electrons are bond in assigned to the more electronegative atom, corresponding to the ionic extreme
What are molecular shapes?
Lewis structures portray the arrangement of electrons, but not the molecular shape
a simple model to predict shapes is valence-shell electron-pair repulsion (VSEPR) theory
electron pairs (bond pairs or lone pairs) are arranged around a central atom to minimize repulsions
What is the procedure for finding molecular shapes?
- Draw Lewis structures
- Establish electron-group arrangement AXE
- Determine the molecular shape
- Predict deviations
- Polar or nonpolar
What VSEPR shapes could a molecule with two electron groups have?
AX2, linear geometry
What VSEPR shapes could a molecule with three electron groups have?
AX3, trigonal planar
AX2E, bent, v-shaped, angular
What VSEPR shapes could a molecule with four electron groups have?
four groups around a central atom adopt a tetrahedral electron group arrangement, with bond angles of 109.5
AX4, tetrahedral, no polar
AX3E, trigonal pyramidal, polar
AX2E2, bent/v-shaped/angular, polar
What VSEPR shapes could a molecule with five electron groups have?
five groups around a central atom adopt a trigonal bipyramidal electron group arrangement, with two in axial and three in equatorial positions
AX5, trigonal bipyramidal, no polar
AX4E, see saw, polar
AX3E2, t-shaped, polar
AX2E3, linear, non polar
What VSEPR shapes could a molecule with six electron groups have?
six groups around a central atom adopt an octahedral electron group arrangement, all in equivalent positions
all bond angles are 90
AX6, octahedral, non polar
AX5E, square pyramidal, polar
AX4E2, square planar
What is valence bond theory?
the basic principle is that a covalent bond forms when orbitals of two atoms overlap, and a pair of electrons is localized in the region between the atoms
the pair of electrons has opposite spins
good, in-phase overlap leads to strong bonding interaction (if atomic orbitals overlap, the electrons have to be of opposite spin)
orbitals can hybridize to more appropriate shapes and orientations to maximize overlap
What is hybridization?
the four atomic orbitals on C can be reformulated as four new hybrid orbitals, directed in a tetrahedral geometry
hybridization is not an actual physical process, it is a hypothetical process that is invoked to explain molecular shapes that are observed experimentally
What is the process used to describe bonding using a hybridization scheme?
- Write a plausible Lewis structure
- From VSEPR, predict the electron-group arrangement (which is not necessarily the same thing as molecular shape)
- Choose a hybridization scheme for the central atom based on the electron group arrangement
- Indicate orbital overlaps
What are the two different ways orbitals can overlap?
sigma bond: end-to-end overlap of orbitals, with electron density along axis of bond
pi bond: side-to-side overlap of orbitals, with electron density above or below axis of bond
What is molecular orbital theory?
quantum mechanics is now applied to molecules
the Schrodinger equation is solved for electrons in a molecule, resulting in molecular orbitals (MOs), these MOs represent electron delocalization over an entire molecule
treats a molecule as a group of nuclei with orbitals that extend over the entire molecule
What are the features of MO diagrams?
- The number of MOs equals the number of AOs
- Combining two AOs gives a bonding MO and an antibonding MO
- In ground state configurations, electrons occupy lowest energy MOs upwards while respecting the Pauli Exclusion Principle and Hund’s Rule
- Bond strength can be measured by the calculated order, Bond order = 1/2(# of bonding electrons - # of nonbonding electrons)
Why is the splitting of energy between bonding and antibonding MOs greater for sigma bonds than pi bonds?
Pi-overlap is less effective (weaker interaction) than sigma-overlap
How can you relate energy of a MO to the number of nodes?
no electron density at nodes
more nodes means depletion of electron density so higher energy
What is the principle of Lewis structures?
within molecules, electron pairs are localized between atoms, which try to attain closed-shell configurations
What are the uses of Lewis structures?
relative bond strengths can be gauged by single, double, and triple bonds
What are the limitations of Lewis structures?
no consideration of molecular shape
no convincing explanation of why bonds form
What is the principle of valence bond theory?
electrons are localized as pairs between two atoms
bonds form by in-phase overlap of orbitals
atomic orbitals can “mix” (hybridize) to maximize overlap
What are the uses of valence bond theory?
rationalization of molecular geometrics, multiple bonds involve additional pi-overlap
What are the limitations of valence bond theory?
electron delocalization is modelled by invoking multiple resonance structures
many hybridization schemes are not supported by the environment
gives incorrect predictions of magnetic properties
limited to compounds of light elements (e.g. organic molecules)
What is the principle of molecular orbital theory?
electrons are delocalized over the entire molecule
molecular orbitals can be described as combinations of atomic orbitals
What are the uses of molecular orbital theory?
bond order is evaluated from the balance of bonding and antibonding interactions
gives accurate predictions in agreement with experiments
What are the limitations of molecular orbital theory?
for molecules more complex than diatomic ones it generally requires more computational resources
more intuitive and may be more difficult to interpret
What is the difference between second and third period elements?
the second period elements are often anomalous in their behavior, whereas the third period elements are more representative of their group
What can the anomalous behavior of second period elements be traced to?
small size and high electronegativity
maximum of four covalent bonds (no d orbitals for bonding
more common occurrence of multiple bonding (good pi overlap of 2p orbitals)
What are the properties and occurrence of alkaline earth metals?
these are harder and higher-melting than the alkali metals, and are somewhat less reactive
they occur naturally as ionic compounds of M2+
What is the preparation of alkaline earth metals?
the pure elements can be obtained by reduction of molten salts
What are the uses of alkaline earth metals?
non-magnetic and non-sparking tools
alloy wheels
cement and concrete
furnace lining
What is the reactivity of alkaline earth metals?
with valence configuration ns2 and low IE, alkaline earth metals form many ionic compounds containing M2+
only Ca, Sr, and Ba reduce water (Be and Mg form a protective coating)
Be is unique
How is Be unique?
unreactive in air and water
BeO is amphoteric
compounds have strong covalent character and conduct electricity poorly in the molten state
What are the properties and occurrence of the group 13 element boron?
elemental boron is black, hard, and has a high melting point (mp > 2000 degrees)
it is a metalloid with a covalent network structure
it occurs naturally in the mineral borax
What is the preparation of the group 13 element boron?
borax is converted to the oxide B2O3, and eventually reduced to elemental B with magnesium
What are the uses of the group 13 element boron?
pyrex glasses
color-safe bleach
insecticide
What is the reactivity of the group 13 element boron?
the characteristic feature of boron compounds is electron deficiency, as seen in BF3 and B2H6
When liquid benzene (C6H6) boils, explain if the gas consists of molecules, ions, or separate atoms.
Benzene is a compound consisting of C6H6 molecules. When energy is supplied to make the liquid boil, the strong intramolecular covalent bonds within each molecule are not broken, but only the weak intermolecular forces between molecules are disrupted. The gas still contains intact C6H6 molecules.
Explain why magnesium metal is deformed by an applied force, whereas magnesium fluoride is shattered.
When magnesium metal is deformed, the atoms are displaced over each other but are still tightly held together by the attraction to a delocalized “sea of electrons”. When MgF2, a solid consisting of Mg2+ and F- ions, is stuck by an applied force, the ions are displaced so that the like charges are brought into proximity throughout the crystal, generating repulsive forces that cause catastrophic shattering.
How do you rank lattice energy?
High ion charges and short separation between them lead to large lattice energies (an application of Coulomb’s Law describing electrostatic energies).
How do you rank bond strength?
The main factor is probably the bond length, as shorter separation between the atoms leads to a stronger bond.
How do you rank bond length between single and double bonds?
A C=O double bond is stronger, and thus shorter than a C-O single bond.
How do you determine ionic character?
A greater difference in electronegativity between atoms leads to more ionic character in a bond.
How do you determine if a substance is a better fuel than another substance?
Better fuels tend to have more C-C and C-H bonds which are weaker than C-O and O-H bonds.
How do oxidation number and formal charge differ?
In the definition of a formal charge, bonds are viewed as being completely covalent, so that bonding electrons are split evenly in half. In the definition of oxidation state, bonds are viewed as being completely ionic, so that bonding electrons are assigned completely to the more electronegative atom.
It seems that the ionic picture is probably less accurate because the charges estimated by the oxidation number are rather extreme. The covalent picture tends to give charges that are small (close to zero).
The true charges of course are somewhere in between these two pictures, because every bond has both covalent and ionic character.
In different resonance structures, the formal charges will be different, but the oxidation states will be the same.
Why is the statement: “Lewis structures with nonzero formal charges are incorrect.” a false statement?
Although we have learned the guideline that resonance structures with atom bearing non zero formal charges are generally less important contributors, this assumptions that some structures are “incorrect” is not true. For example, polyatomic ions, which carry an overall charge, at least one atom in the molecule must bear a positive or negative charge. But even in some neutral molecules, such ozone (O3), formal charges may be unavoidable.
Why is the statement: “Triatomic molecules have a planar shape.” a false statement?
Because three points define a plane, this statement seems to make sense at first glance. However, it is misleading because triatomic molecules can also be linear, such as HCN or CO2.
Why is the statement: “Molecules in which there is an electronegativity between the bonded atoms are polar.” a false statement?
The criterion for a molecule to be polar is that there is a net dipole moment, which is the vector sum which is the individual bond dipoles. Thus, there does exist molecules in which individual bonds may be polar, but the bond dipoles cancel out to give an overall zero dipole moment. Examples include CO2, BeCl2, CCl4, SF6.
