unit 3 Flashcards
When two atoms approach each other repulsions between each atom’s:
negatively charged electron clouds
Positively charged nuclei
Attractions between nucleus and approaching atoms electron cloud are strongest where
electron clouds overlap between adjacent nuclei
If attractive forces stronger than repulsive forces
Atoms have a lower energy than when apart
Chemical bond forms
ionic bond
electrostatic attraction between a metal cation and nonmetal anion
EN difference over 1.7
transfer of electrons
crystal lattice
oppositely charged ions form an ordered, solid, 3-D array with large numbers of interionic forces
crystal lattice properties
High melting and boiling points
Chemical formula of an ionic compound is the smallest whole-number ratio of ions
Different crystal structures depend on sizes and ratios of ions
Ratios depend on ionic charges in the compound
describe what happens when 2 H atoms approach each other
Electron cloud of one atom is attracted to nucleus of other atom, kinetic energy increases
Repulsive forces as nuclei approach each other slows atoms, kinetic energy becomes potential energy
what happens when electron clouds overlap?
Attractive forces exceed repulsive forces
Valence electrons move into space between the two nuclei where there is most attractive force between nuclei
covalent bond
sharing of electrons between 2 nonmetals
attraction between a pair of electrons and two nuclei
usually independent molecules
non polar covalent bond
EN difference = 0-0.3
atoms of the same element bonded
electrons equidistant between nuclei
polar covalent bond
EN difference = 0.4-1.6
bonding electrons pulled closer to more electronegative atom
partially positive end has less electron density
partially negative end has more electron density
number of electrons needed =
number of bonds formed in a covalent compound
ionic compound properties
Strong electrostatic attractions between charged ions must be broken to melt-high melting point
covalent compounds properties
Don’t always need to break bonds between atoms
Weak intermolecular attractions need to be broken to melt
Separation of molecules not breaking of bonds between atoms
network covalent solids
solid where all the atoms are covalently bonded in a continuous network
network covalent solids properties
Covalent bonds extend through entire sample
High melting points
For a chemical bond to form between two atoms:
How must the energy associated with the bonded atoms compare to the energy when the atoms are apart?
The energy of the bonded atoms must be lower than the energy of the atoms when they are apart
For a chemical bond to form between two atoms: What does this tell us about the attractive forces compared to the repulsive forces between them?
The attractive forces between the bonded atoms are stronger than the repulsive forces
What is an ionic crystal lattice and how does it explain the high melting points of ionic compounds?
An ionic crystal lattice is an ordered, solid, three-dimensional array of cations and anions
The large number of interionic forces in the crystal lattice locks the ions in place giving ionic compounds their high melting points
What are the attractive forces associated with
Ionic bonds
Electrostatic attractions between oppositely charged ions (cations and anions)
What are the attractive forces associated with Covalent bonds
The attractive force between the nuclei of the bonding atoms and the shared bonding electrons
What are 3 similarities between ionic and covalent bonds?
Both form when atoms try to achieve a noble gas configuration
Both are strong when compared with intermolecular attractions
The energy when both types of bonds form is lower than the energy of the atoms apart
What are 3 differences between ionic and covalent bonds?
Ionic bonds form between metals and nonmetals, covalent bonds form between nonmetals
In ionic bonds there is a complete transfer of electrons, while in covalent bonds there is a sharing (equal or unequal) of electrons.
Compounds with ionic bonds are crystalline solids at room temperature while compounds with covalent bonds are solids, liquids, or gases at room temperature.
Glucose is a covalent compound with the molecular formula C6H12O6. This and many other covalent formulas are not reduced down to their simplest whole-number ratio of atoms in the compound. Explain why.
Glucose is a covalently bonded molecule composed of discrete molecules. Each molecule contains six carbon atoms, twelve hydrogen atoms and six oxygen atoms.
In contrast to ionic compounds, glucose does not form a crystal lattice
Many covalent compounds have much lower melting points than ionic compounds. Why doesn’t this mean that covalent bonds are weaker than ionic bonds?
Melting points of covalent compounds usually do not require breaking the bonds between atoms. Instead, intermolecular attractions are broken when melting covalent compounds. The actual bonds between the atoms do not get broken as they are quite strong
Diamond is a form of pure carbon containing only covalent bonds. It is the hardest substance known and a melting point of 3550o C. Explain its hardness and high melting point.
Diamonds are network covalent solids that are held together by covalent bonds that extend throughout the entire sample.
Consider the nature of the covalent bonds present in HCl and in N2. Which substance would you expect to have the higher melting point? Explain your answer
HCl would have the higher melting point. HCl is polar (ΔEN = 0.9) while N2 is non-polar.
Because HCl is polar it acts as a dipole, the H end is ∂+ while the Cl end is ∂-
∂+ end of one molecule lines up with the ∂- end of a different molecule setting up an electrostatic attraction which need to be broken in order to melt HCl.
N2 doesn’t have these attractions.
how to draw a dot structure
put number of valence electrons around element symbol
lewis structure
2 dimensional representation of the molecular formula, usually for covalent compounds
single lines represent bonds
other pairs of electrons are non-bonding or lone pairs
steps in drawing a Lewis structure
- determine total number of valence electrons
- draw the bonds between the atoms
- subtract the number of valence electrons used for bonding (each counts for 2 electrons)
- arrange remaining valence electrons to obey the octet rule
bond energy
energy required to break a mole of bonds
radical
A molecule with one or more unpaired electron in its outer shell
dimer
a molecule or molecular complex consisting of two identical molecules linked together
resonance structure
molecule or ion that contains double bonds next to single bonds, often has several possible structures
delocalized electrons
not associated with any one pair of bonded atoms. Are spread out equally between the three pairs of atoms
formal charge
charge that that an atom would have if all bonding electrons are shared equally between the bonded atoms
ignored electronegativity
formal charge =
number of valence electrons - (number of nonbonding electrons + 1/2 number of bonding electrons)
sum of formal charges in a neutral molecule
0
sum of formal charges in a polyatomic ion
charge of ion
small/zero formal charges on individual atoms are better than
large formal charges
when formal charge cannot be avoided, _ forlmal charge should reside on the most _ atom
negative, electronegative
what do the dots represent in a Lewis structure?
valence electrons
what do the elements symbol represent in a Lewis structure?
nucleus and core electrons
What do the number of dots in main group metals tell us about the charges of the ions formed by these metals?
magnitude of the positive charge
ABn
a is central atom bonded to n atoms of b
AB2
linear
180
AB3
trigonal planar/pyramidal
120
AB4
tetrahedral
109.5
AB5
trigonal bipyramidal
120
90
AB6
octahedral
90
120
VSEPR
explains molecular shapes for representative elements
Negatively charged electron domains repel each other
electron domains
electron pairs in a covalent bond
Note: Each multiple bond in a molecule also represents a single electron domain.
nonbonding pair of electrons
electron geometries
Best arrangement of electron domains minimizes repulsions among them.
Shapes of different ABn molecules or ions depend on number of electron domains surrounding the central atom.
molecular geometry
arrangement of only the atoms in a molecule or ion.
Nonbonding pairs are not part of the description.
steps to use VSEPR to predict molecular shapes
- draw Lewis structure
- determine electron domain geometry
- determine molecular geometry
linear molecular geometry
2 bonding domains
0 nonbonding domains
trigonal planar molecular geometry
3 bonding domains
0 nonbonding domains
bent molecular geometry
2 bonding domains
1/2 nonbonding domains
tetrahedral molecular geometry
4 bonding domains
0 nonbonding domains
trigonal pyramidal molecular geometry
3 bonding domains
1 nonbonding domains
bond angles _ as nonbonding pairs increases
decrease
electron domains for nonbonding pairs exert _ repulsive forces on adjacent electron domains
greater
expanded valence shells are used when
there are 5 or 6 electron domains around the central atom
-central atom is in period 3 or above
-5 electron domains have one of four molecular geometries
-depends on number of nonbonding pairs and minimizing electron domain repulsions
trigonal bipyramidal electron domain
2 axial domains
3 equatorial domains
each axial domain forms 90 degree angle with any equatorial domain
seesaw electron domain
1 nonbonding domain
4 bonding
axial lone pair: 3 90 degree interaction with nonbonding pairs
equatorial lone pair: 2 90 degree interactions with bonding pairs
t-shaped electron domains
2 nonbonding domains occupy 2 of 3 equatorial positions
3 bonding
linear electron domains
3 nonbonding domains all occupy equatorial positions
octahedral electron domains
0 nonbonding
6 bonding
square pyramidal electron domain
1 nonbonding domain
4 bonding
square planar electron domain
2 nonbonding
4 bonding
bond polarity
measures how equally electrons in a bond are shared between 2 atoms of the bond
bond polarity and electronegativity proportion
increases with the other
dipole moment
measures the amount of charge separation in a diatomic molecule
dipole moment of a non-diatomic molecule depends on
-polarities of individual bonds
-geometry of the molecule
problems with Lewis Theory
-doesn’t give good :
-numerical predictions in property trends
-resonance predictions
-angle predictions
-correct magnetic behavior
valence bond theory postulates
-buildup of electron density between 2 nuclei
-Overlap of valence atomic orbitals of two atoms.
-Always an optimum distance between two nuclei.
-valence electrons reside in quantum mechanical atomic orbitals (s,p,d,f)
molecular orbitals
regions of high probability of finding shared electrons in the molecule
more stable than the separate atomic orbitals
chemical bond results from
-the overlap of 2 half-filled orbitals with spin-pairing of the 2 valence electrons
-a completely filled orbital with an empty orbital
geometry of overlapping orbitals determines
shape of molecule
hybridizing
mixing different types of orbitals in the valence shell to make a new set of degenerate orbitals
molecules use the hybridization that yields the lowest overall energy for the molecule
hybrid orbitals
have different shapes and energies from standard orbitals
still localized on individual atoms
total number of standard atomic orbitals =
number of hybrid orbitals formed
combinations of standard orbitals added determines
shapes and energies of hybrid orbitals
pi bonds
p orbitals overlap side by side
results in electron density above and below internuclear axis
sigma bonds
p orbitals that overlap end to end
double bond
one sigma and one pi bond
triple bond
one sigma and 2 pi bonds
which are stronger? sigma or pi bonds
pi
what types of bonds can you rotate around?
-single bonds are relatively unrestricted (if pi bond is broken)
-not double bonds
isomerism
phenomenon in which more than one compounds have the same chemical formula but different chemical structures
cis
same side
trans
opposite side
steps in predicting bonding in molecules
- draw lewis structure
- use VSEPR to predict electron geometry
What is valence bond theory?
describes covalent bonding as a buildup ofelectron density between two nuclei when the valence atomic orbital of one atom overlaps with the valence atomic orbital of another atom
What is a hybridized orbital?
A combination Q/ two standard (s, p, d or f) orbitals. Hybridized orbitals have different shapes from those of standard orbitals but are still localized on individual atoms
Explain how the Lewis model and the valence bond theory differ in their description of a chemical bond
Lewis model: covalent bonds occur when atoms share electrons to concentrate electron density between the two nuclei.
Valence bond theory: a buildup of electron density when the valence atomic orbital of one atom overlaps the valence electron orbital of another atom
In valence bond theory, what determines the shape of the molecule?
The combination of standard orbitals when added together
bond order
number of bonding pairs of electrons between two of atoms and indicates the stability of a bond
higher bond order =
greater stability of the molecule
more attraction between electrons (atoms held together more tightly)
bond order and length proportion
inverse
bond order and strength
direct
to determine bond order for diatomic molecules:
draw Lewis structure and determine types of bonds between atoms
bond order 0: no bond
bond order 1: single
bond order 2: double
bond order 3: triple
to determine bond order for non-diatomic molecules:
- draw Lewis structure
- count total number of bonds
- count number of bond groups between individual atoms
- divide number of bonds between atoms by the total number of bond groups in the molecule
multivalent
atoms that can have more than one charge
copper ions
1+ and 2+
iron ions
2+ and 3+
tin ions
2+ and 4+
manganese ions
2+ and 4+
lead ions
2+ and 4+
silver ion
Ag1+
Nickel ion
Ni2+
zinc ion
Zn2+
chromium
Cr3+
ammonium
NH4(1+)
hydrogen carbonate
HCO3(1-)
chlorite
ClO2(1-)
perchlorate
ClO4(1-)
nitrate
NO3(1-)
hydrogen sulfite
HSO3(1-)
hydrogen sulfate
HSO4(1-)
dichromate
Cr2O7(2-)
thiosulfate
S2O3(2-)
hydrogen phosphate
HPO4(2-)
acetate
C2H3O2(1-)
carbonate
CO3(2-)
hypochlorite
ClO(1-)
chlorate
ClO3(1-)
nitrite
NO2(1-)
sulfite
SO3(2-)
sulfate
SO4(2-)
permanganate
MnO4(1-)
chromate
CrO2(2-)
phosphate
PO4(3-)
dihydrogen phosphate
H2PO4(2-)
cyanide
CN(1-)
oxyanions
an element bonded to oxygen atoms
hydrates
ionic compounds that crystallize out of an aqueous solution and incorporate water molecules in a fixed ratio
anhydrous
removing water from a hydrate by heating
