Chapter 8: Bonding and Molecular Structure
What is the molecular geometry of SF6?
- 6 bond pairs - no lone pairs - octahedral
How can an atom accommodate up to 12 electrons?
- currently being investigated - D-orbitals participate in bonding - terminal atoms withdraw electron density
What is the max possible bond order?
6
What is molecular geometry?
Geometry describing the arrangement of a central atom the atoms directly attached to it.
What is a formal charge?
The charge on an atom or ion calculated by assuming equal sharing of the bonding electrons
How can the number of bonds to form be predicted?
The number of unpaired valence electrons gives a general indication as to the number of bonds an atom will likely form.
________________ can not be measured.
electronegativity
What is Allred-Rochow definition of electronegativity?
force on the valence electrons due to the nuclear charge
Hydrogen never has __________________.
lone pairs
_________ pairs will squeeze the angles in the molecular geometry?
lone pairs
The molecular orbital theory explains ___________________________.
the stability of electrons
Forming bonds ______________ energy .
yields
What is the formula for bond dissociation enthalpy?
∆rxnH= ∑ ∆H (broken bonds) - ∑ ∆H (bonds formed)
What s the molecular geometry of XeF₂?
- 2 bond pairs - 3 lone pairs - linear
What is the molecular geometry of water?
- 2 bond pairs -2 lone pairs
What is the molecular geometry of an atom with 5 electron pairs?
- 3 120 interactions - 6 90 degree interactions - trigonal bipyramidal arrangement
What is the molecular geometry of Ammonia?
- 3 bond pairs - 1 lone pair - trigonal pyramidal
What is the molecular geometry of ClF₅?
- 3 bond pairs - 3 lone pairs - t-shaped
What is the bond order of the nitrate ion?
- 3 resonance structures - bond order is a result of averging over resonance structure - # of SP= 2+1+1= 4/3 - bond or 1 1/3
What is the molecular geometry of SF₄?
- 4 bond pairs - 1 lone pair - seesaw
What is the molecular geometry of XeF₄?
- 4 bond pairs - 2 lone pairs - square planar
What is the molecular geometry of methane?
- 4 bond pairs - no lone pairs - tetrahedral
What is the molecular geometry of PF₅?
- 5 bond pairs - no lone pairs - trigonal bipyramidal
What are lewis dot symbols?
- As you go down a group, the lewis dot is the same; therefore, they have similar reactivity
How do you represent a covalent bond in a lewis dot structure?
- The element's symbol represents the nucleus occluding the core elements. - Up to four valence electrons, indicated by dots, are placed one at a time around the symbol - If there is any remaining electrons, they are paired up with the ones already there or shared with other elements - Generally, only 8 electrons are allowed each atom.
What is a bond dissociation enthalpy?
- average bond dissociation energies can be used to estimate enthalpy change of energy
What kind elements can have an expanded octet?
- elements in the 3rd period or higher can form compounds in which they exceed the octet rule. - can accommodate up to 12 electrons
What are come examples of covalent substance held together by covalent bonds?
- gases from atmosphere - DNA - fuels
What is a molecule called when it has more than 8 electrons?
- hypervalent - hypercoordinate
what is a molecule called when it has less than 8 electrons?
- hypovalent - electron deficiency - incomplete octet
What are the periodic trends of the electronegativity?
- increases across a period - decrease down a group
How many bonds does group 6a usually form?
- it has 2 unpaired electrons, so 2
how many bonds does group 7a usually form?
- it has one unpaired electron, so one
What are molecular shapes?
- lewis structure show connectivity - generally does not convey information of structure - ?????
What is the molecular geometry of an atom with 4 electron pairs?
- max distance is 109.5 degrees - tetrahedral arrangement
What is the molecular geometry of an atom with 3 electron pairs?
- max distance is 120 degrees - trigonal planar arrangement
What is the molecular geometry of an atom with 2 electron pairs?
- max distance is 180 degrees - linear arrangement
what is the molecular geometry of an atom with 6 electron pairs?
- max distance is 90 degrees - octahedral arrangement
What are tabulated values?
- properties depend on the chemical environment - ex. bond length and bond strength
What are covalent bonds?
- sharing go valence electron pairs between atoms. - overlap of valence orbitals
What are the common bond orders?
- single bond - double bond - triple bond
What is the preferred lewis structure of an atom formal charge?
- smaller formal charges - negative formal charges on atoms with larger electron affinity - positive formal charges on atoms with smaller electron affinities
What is a resonance structure?
- sometimes, more than one lewis structure can be drawn for a molecules - lewis structure differs only in the assignment of electrons - each of these lewis structures are resonance structures - The actual structure is a hybrid of those resonance structures.
What is ionic bonding?
- the electron transfer from one substance t another - each atom has an "octet" of electrons
What things must you consider pertaining to chemical bonding?
- what hold the atoms in a molecule or ionic compound together? - What causes the angular distribution of atoms in a molecule? - Can we predict the structure of a compound? - How does structure relate to chemical and physical properties?
What is the range of electronegativity for a non polar covalent bond?
0 ≤ x ≤ 0.4
What is the range of electronegativity for a polar covalent bond?
0.5 ≤ x≤ 1.8
What are the steps of drawing a lewis dot structure?
1. determine the arrangement of atoms. central atoms generally have lowest electron affinity 2. Determine the total number of valence electrons in the molecule or ion. 3. Place one pair of electrons between each pair of bonded atoms to form single bonds 4. Use the remaining pairs as lone pairs to complete the octet of each terminal atom. The remaining electrons are assigned to the central atom. If the central atom has fewer than 8 electrons, change one or more of the lone pairs on the terminal atoms in a bonding pair between the central and terminal atom to form a multi-bond
What are the steps for determining molecular geometry?
1. determining the electron-pair geometry (base geometry 2. Place atoms and LPE and vertices of the base geometry so as to minimize interactions 3. Molecular geometry is given by the final placement of the atoms only.
What is the range of electronegativity for a ionic bond?
1.8 ≤ x
Boron often form only ______ bonds.
3
What are the different exceptions to the octet rule?
3 exceptions: 1. an old number of electrons; ex radicals 2. less than 8 electrons 3. more than 8 electrons
What is the molecular geometry of BrF₅?
5 bond pairs - 1 lone pair - square pyramidal
The sum of all formal charges ALWAYS = what?
ALWAYS equal the net charge of the molecule
How do you determine the total number of valence electrons for an anion?
Add the number of electrons equal to the negative charge.
What is the result of a chemical reaction?
Chemical reactions are a result of a gain, loss, or rearrangement of the valence electrons
What formula is used to calculate forma charge?
FC = NVE - [LPE + 1/2BE]
What is electron-pair geometry?
Geometry describing the arrangement of all of the valence electron pairs urn the central atom.
What is a chemical bond?
One or more electrons are transferred from one atom to another.
How do you determine the total number of valence electrons for a neutral molecule?
Sum of valence electrons in each atoms.
Which elements can exceed the octet rule?
The 3rd period and beyond
What is bond order?
The order of the bond is the number of bonding electron pairs shared by 2 atoms in a molecule
What does VSEPR stand for?
Valence shell electron pair repulsion
When calculating a formal charge of a resonance structure, how do you calculate it?
a formal charge is calculated for each individual structure but the actual formal charge is the average over all resonance structures.
To form an oxyacid from an anion, what should we do?
add a proton(H atom) to one or more oxygen atoms.
What is Mulliken's definition of electronegativity?
an average of ionization potential and electron affinity
What is the Allen definition of electronegativity?
average of valence subshell energies.
What mist be determined to predict the molecular geometry?
determining the electron pair geometry
What is Pauling's definition of electronegativity?
difference in electronegativity are related to dissociation energies.
What is the guiding principle for predicting molecular geometry?
electron pairs will arrange themselves around a central atom so as to minimize their interactions.
What determines the type of bond between atoms?
electronegativity
What is used to determine charge distribution?
electronegativity
What is electronegativity?
is an indicator of the ability of an atom to attract electrons in a covalent bond - not a unique number
What are isoelectric molecules and ions?
molecules and ions having the same number of valence electrons and similar lewis structures are said to be isoelectric
When determining which atom is the negative formal charged atom of a molecule?
negative formal charge, if necessary, should be on the most electronegative atom.
Does methane have a resonance structure and what is the bond order?
no; for each C-H bond, there is only one SP so the bond order is 1
In molecules made up from only ___________ atoms, the atoms are attached by covalent bonds.
nonmetal
What is the formula for bond order?
number SP in X-Y bonds / number of X-Y bonds
Stable molecules can form with an _______________ number of electrons.
odd
Wha is the H exception of the octet rule?
only 2 valence electrons on H configurations
Generally, bonds are ___________.
polar covalent; unequal sharing of electrons
Breaking bonds _________ energy.
requires
Noble gas configuration are ___________ configurations.
stable
How do you determine the total number of valence electrons for a cation?
subtract the number of electrons equal to the positive charge.
Average values are _____________.
tabulated
What is molecular polarity?
the extent to which molecules align in a electric field depends on the dipole moment.
As the difference in electronegativity increases, so does _____________?
the ionic character of the bond
What is charge distribution?
the way the electrons are distributed in an atom or molecule.
Are fractional bond orders possible?
yes; results from resonance structures