ch3 Flashcards
Do core electrons contribute to bonding? (Yes/No)
No.
Which elements are not limited to the octet rule?
Row 3 and below, because (n+1)s and nd orbitals are not exceedingly higher in their energy from the atoms regular valence orbitals.
Which orbitals can form an excited valence state?
Empty nd orbitals, since they can house excess valence e-s.
What is another term for an excited valence state?
hypervalent or expanded shell structures.
What are the steps in drawing Lewis structures?
- count total # valence e-s.
- identify central atom: usually lowest in electronegativity.
- make single bonds to connect peripheral atoms to central atom: keep track of e-s used.
- use remaining electrons in adding lone pairs to achieve octet: start with atoms in higher electronegativity because they pull harder on e-s and less likely to share.
- if central atom has less than 8 e-s form multiple bonds by converting lone pairs of least electronegative peripheral atoms to bonds, because they pull less hard on e-s and more likely to share.
- assign formal charges: repeat step 5 to minimize FCs.
How do you calculate formal charge of an atom?
valence e-s - (non-bonding e-s + 1 e- from each bond)
Define a canonical structure.
Canonical structures is a way of describing the delocalized e-s within molecules or polyatomic ions where the bonding cannot be expressed in terms of single lewis structure. the true structure is an average of 2 (or more) Lewis structures.
Which term is used for “true structure”.
resonance hybrid.
Why does a single Lewis structure fail to represent the true structure of a molecule (provide example)?
Because Lewis structures predicts a difference in bond lengths between identical pairs of atoms. For example, it predicts one double and one single bond in O3.
When does resonance occur? Energy of molecule vs contributing forms?
It occurs when e-s are delocalized over several overlapping orbitals. Also the energy of the molecule or ion ends up lower than the contributing forms.
Why does resonance increase stability?
Because resonance allows for delocalization of e-s, the overall energy of a molecule is lowered since its e-s occupy a greater volume.
What are three factors of Pauling’s principle of electroneutrality? Where does his theory apply?
1) FC’s as close to zero are preferred.
2) Any negative FC is most likely on the most electronegative atom(s).
3) Any positive FC is most likely on the least electronegative atom(s).
* *It applies when identifying the resonance hybrid.
Which structure(s) contribute most to the resonance hybrid?
The most stable canonical structure(s).
What does the most realistic Lewis structure have (contributes most to the resonance hybrid)?
- all atoms with full valence shell.
- minimum charge separation.
- formal charges agreeing with atoms electronegativity.
What is the proceeding step from establishing the most stable structure?
To deduce how many equivalent most-stable canonical structures there are (resonance contributors).
Do inequivalent resonance structures contribute to the resonance hybrid? (Yes, No)
Yes, but not as much.
How do you calculate average bond order?
(sum of individual bond orders) / (# of canonical structures)
What is the difference between resonance structures and isomerism?
Resonance structures have the same atoms in the same arrangement, however e-s can move around the molecule. Isomerism is the possibility of differing compounds having the same atoms, but in a different arrangement and e- arrangement.
Explain why some elements have more than the octet.
In row 3, empty d orbitals can house extra e-s , allowing elements like silicon, phosphorus, sulfur, chlorine, to have more than the octet.
Explain why some elements have less than the octet.
This exception applies to Be, B and Al which (remain “open-shell”). Although they’re stable, they’ll try to form a fourth bond to get eight electrons.
What is a unique feature of elements that have less than the octet?
They’re good Lewis acids because they can accept e-s.
For the molecule BF3, why is B-F(obsd)=131 pm and B-F(calc)=152 pm?
BF3 has 2 inequivalent canonical structures, 3 for B=F and 1 B-F. B=O has a full valence shell, but highly electronegative F pulls strongly on e-s, which is less likely to share e-s as a double bond. The overall B-F bond is less than predicted, because it shows partial double bond character.
Which is structure expected to be a Lewis acid for BF3, B=F or B-F?
B-F because it only has 6 e-s around B, can achieve 8 by accepting e-s which makes it a good Lewis acid.
Define oxidizing agent and rationalize the oxidizing ability of K2CrO7.
An oxidizing agent gains e-s and is reduced in the process. Since O is more electronegative than Cr it has a stronger pull on e-s, and is less likely to share. Cr hold 6 valence e-s and its oxidation state is very high because it is bonded to so many strong pulling atoms, so in reaction with reducing agent, Cr could be converted into a form in which it is more e- rich.
How is the geometry of molecules predicted?
Predicted by arranging regions of enhanced e- density most efficiently.
Define a steric number/electron grouping?
The number of atoms bonded to central atom.
Types of electron groupings.
- Bonds (single, double, triple).
- Stereochemically inactive lone pairs.
- One lone electron.
Provide steric number, basic geometry, molecular geometry, and predicted bond angles (up to 6).
2, linear, 180.
3, trigonal planar,
bent or angular (1 lone e- pair), 120.
4, tetrahedral, trigonal pyramidal (1 lone e- pair), bent/angular (2 lone e-pairs), 109.5.
5, trigonal bipyramidal,
seesaw (1 lone e- pairs), T-shape (2 lone e- pairs), Linear (3 lone e- pairs), 120&90.
6, octahedral, square pyramidal (1 lone e- pair), square-planar (2 lone e- pair), 90.
What is the geometry and predicted bond angle for steric numbers 7 & 8?
7, pentagonal bipyramidal, 72/90.
8, square antiprismatic, 70.5/99.6/109.5
Factors affecting bond length.
- Bond polarity, thus electronegativity.
- size.
Explain the effect(s) of bond polarity (electronegativity) on bond angles.
If peripheral atoms are more electronegative than central atom, there’s less e- density around central atom, lone pair has room to expand pushing bonds closer together.
How do electronegativity and size work against each other? Compare N(CH3)3 & N(CF3)3. Which dominates size or bond polarity?
CF3 has a bigger bond angle than CH3. Since CF3 is more electronegative than CH3, you expect less e- density near the central atom resulting in smaller bond angle. CF3 has a bigger radius than CH3, therefore you expect more steric repulsion between F atoms, thus bonds cannot squeeze closer together, lone pair must act smaller.
- Data shows that size dominates in this particular case.
Bond angle trends with change in peripheral atoms.
- > bond angle as electronegativity of group decreases.
- >bond angle as size of group increases.
Bond angle trends with change in central atom.
-
Which position do lone pairs prefer, equatorial or axial?
Lone pairs prefer equatorial position, because there’s less neighboring bonding pairs 90 degrees away and more e- repulsive forces than bonding pairs, therefore prefer their space.
Which information is provided and missing with the Lewis structure?
Provides # of lone pairs and types of bonds pbetween the atoms. Missing information about the actual arrangement of atoms in space.
Which information is provided and missing with the VSEPR Theory?
Provides the molecular geometry of molecules. Missing information about bond lengths.
A combination of VSEPR and a bonding model, such as Lewis electron structures, is necessary to understand the presence of multiple bonds (T/F).
True.
Limitations of VSEPR Theory.
Heavy p-block elements in their lower valence state have geometries that can’t be predicted by VSEPR, because they prefer to hold their lone pairs in an unhybridized s orbital.
What does LCP stand for?
Ligand Close-Packing, to predict bond lengths.
What bond lengths do we expect as bond angles get smaller?
Longer bond lengths.
