Chemical Bonding Flashcards
VSEPR Models :(
What type of structures do ionic bonds favour?
Ionic bonding favours the formation of large stable spatial arrangements of ions:
Crystalline solids
Very briefly, what are covalent bonds and ionic bonds?
Ionic bonds: One or more electron transferred from atom to atom, leading to a cation and an anion, which are electrostatically attracted
Covalent bonds: two valence electrons shared by two atoms
How can the formal charge of an atom in a molecule be calculated? How about a whole molecule?
(# valence electrons in free atom) - (# non-binding electrons in molecule) - (1/2# bonding electrons in the molecule)
The formal charge of each atom is added to get the formal charge of a molecule.
How many electrons surround sulfur usually?
12 (expanded octet)
How many electrons generally surround elements form the 3A group (eg. Al, B)?
Less than 8
How is the dipole moment (D) calculated for a molecule?
D = (q)(d)
q = absolute value of partial charge on the most EN/EP bonded atom
d = distance between the two atoms
For a molecule, all the dipole moments (D) are aded up as vectors. This may result in zero if they are all opposing.
In terms of accepting/donating electrons, what are Lewis Acids and Lewis Bases?
Lewis Acids: Accept electrons due to deficiency
Lewis Bases: Donate electrons from lone pairs
What are VSEPR (valence shell electronic pair repulsions) models useful for? What are they?
To predict molecular geometries
VSEPR models are the geometrical arrangements of atoms or groups of atoms bound to a central atom that are determined by the number of pairs of valence electrons around the central atom.
VSEPR procedure is based on the principle that these electronic pairs around the central atom are arranged in such a way that the repulsions between them are minimized.
How are VSEPR models determined?
- Determine number of electron pairs around A (central atom)
((Valence electrons in free atom of A) + (sigma bonds) - (pi bonds))/2 = electron pairs - By using the total number of electron pairs and the number of lone and bonding pairs, the arrangement of electron pairs and geometry around the central atom can be interpreted from a table (ie. table III.A.3.1 on CHM-32)
So far, I don’t think these have to be memorized for MCAT.. But practice will reveal that.
What are some rules that apply to VSEPR model forming? (5)
- Subtract the charge of cations from the total number of electrons
- Add the charge of anions to the total number of electrons
- A lone pair repels another lone pair or a bonding pair very strongly, this causes deformation in bond angles
- The previous rule also holds for a double bond
- The VSEPR model can be applied to polyatomic molecules, where each central atom has to be done individually
List the five arrangements of electron pairs for the following numbers of electron pairs around a central atom
2, 3, 4, 5, 6
2 electron pairs: linear 3 electron pairs: trigonal planar 4 electron pairs: tetrahedral 5 electron pairs: trigonal bipyramidal 6 electron pairs: octahedral
N2 has a lower boiling point than O2, what best accounts for this difference?
N2 has a lower molecular weight than does O2.
The forces holding these molecules together are London (dispersion) forces (AKA Van der Waals forces). Since the strength of a dispersion force increases with the ease of polarizability of a molecule, the strength also increases with increasing molecular mass. When dispersion is the most important intermolecular force, the heavier of two similar chemical species has the higher boiling point because it has more electrons and is more polarizable.
Since oxygen has a molar mass of 32 and nitrogen 28, oxygen’s boiling point is higher than nitrogen’s.
Why would nitrates decompose into Nitrite (NO2-) rather than O2-?
Even though oxygen gas would be a smaller molecule, it’s two anionic charges make it less stable than the singly charged nitrite.
When anions and cations are of comparable size, do they decompose more or less?
Less.
