Week 3 content Flashcards
There are two main types of chemical bonds:
- Ionic bonds – result from the attractive force of oppositely charged ions
- Covalent bonds – results from the equal sharing of valence
electrons between two atoms.
Lewis electron-dot symbol:
a notation in which the electrons in the
valence shell of an atom or ion are represented by dots placed
around the chemical symbol of the element.
Boron Trifluoride: (Draw it)
The B atom has a share in only 6 pairs of
electrons (or 3 pairs). B atom is often electron
deficient because it can “hyperconjugate” to many
atoms.
Molecular geometry:
the general shape of a molecule, as
determined by the relative positions of the atomic nuclei.
VSEPR model predicts the…
… shapes of molecules and ions by
assuming that the valence-shell electron pairs are arranged about
each atom so that electron pairs are kept as far away from one
another as possible, thus minimizing electron-pair repulsions.
A higher number of lone
pairs leads to…
… a higher
repulsion, and thus a
smaller angle
e “density maps”
of a sulphur lone pair (lp) vs. a
sulphur-fluorine bonding pair
(bp). (draw?)
It is obvious that the lone pair
have a larger spatial domain than
the bonding pair, and thus will contort the geometry of a molecule by
decreasing the angle between bonding pairs:
Multiple Bonds are…
…slightly more repulsive than single bonds, but not as repulsive
as lone pairs
lp-lp > lp-bp > bp-bp
similar molecules - same terminal atoms - different central atom, the electronegativity of the central atom…
distorts the structure.
decreases in electronegativity as we go down the
group
central atom less electronegative…
…, the bonding electron
domains get smaller. The bond angle decreases with decreasing
electronegativity
effect of decreasing the repulsion between bonding pairs,
- repulsive effect of lone pairs unchanged - bond
angle decreases as the lone pairs push the bonding pairs closer
together.
more electronegative terminal atoms cause
bonding
pair to have a smaller domain than less electronegative terminal
atoms. Thus, the bond angle increases with decreasing
electronegativity.
Hybridization on an atoms are known as…
hybridized orbitals: orbitals which
maintain characteristics of both of its “parent” orbitals.
Hybridization of e orbitals on two (or more) different atoms, these are
called…
molecular orbitals.
- This is what forms a bond…when an orbital on one atom comes to
occupy a portion of the same region of space as an orbital on the
other atom. The orbitals are said to overlap. The greater the orbital
overlap, the stronger the bond.
Polar Bonds:
A polar bond is characterized by separation of electrical
charge. Polar molecules, therefore, have nonzero dipole moments.
Since opposite charges attract, an ion eill be attracted to a…
a polar molecule.
- This attraction is on the order of 40 – 600 kJ/mol and is
comparable to lattice stabilization. - About 10 - 25% the strength of covalent bonds
Some ionic solids are soluble in water because:
- There is an increase in entropy
- There is a compensation of enthalpy
- Lattice stabilization energy is compensated by ion-dipole
interactions
Attraction between ions and dipole depends on:
- Ion charge, magnitude of dipole and ion-dipole distance.
- Measured by H for Mn+ + H2O → [M(H2O)x]n+
Dipole-Dipole Forces:
- Depending on orientation can be attractive or repulsive.
- Such forces bind molecules having permanent dipoles to one another.
μ=?
= Q·R
(units of debyes D (3.336x10-30 C·m)):
where Q is the charge (δ-) and R is the bond length
Van der Waals or
London Dispersion
Forces:
Features:
- Small differences in
electronegativity - Very small dipole moments
- Essentially non-polar bonds
Ex: Just C and H
present, i.e. CH4
Dipole-dipole:
Features:
* Larger difference in
electronegativity causes the
bonds to be more polar
Ex:
C-O, C-X, C-N, e.g.
CH3Cl
Hydrogen bonding:
Features:
- Very large difference in
electronegativity which is
achieved by having a
hydrogen attached to
something other than
carbon - Two unshared electrons
must be attached to the
heteroatom - Very strong intermolecular
attractions
*important dipole-dipole interaction
*account for high bp
H bonds in H2O:
Abnormally high specific heat capacity of water (4.184 g/Kmol).
This is the reason water is used to put out fires, it is the reason
lakes/oceans control climate, and is the reason thunderstorms release
huge amount of energy.
Induced Dipole:
- Materials without a dipole moment can have one temporarily
induced by having electron density brought close by… - These interactions can be very weak: (0.05 – 40 kJ/mol)
Polarizability:
degree and ease of distortion of electron cloud (related to
size and number of electrons)
