VSEPR, Polarity, Intermolecular Forces Flashcards
What does VSEPR stand for?
Valence Shell Electron Pair Repulsion Theory
What does VSEPR predict?
VSEPR theory explains and predicts the geometry (shapes) of molecules.
(The 3-D molecular geometry is based on the number of valence shell electron bond pairs among the atoms in a molecule or ion. This model assumes that electron pairs will arrange themselves to minimize repulsion effects from one another.)
What are the three steps for determining the shape of a molecule?
STEP 1 - Draw the Lewis structure for the molecule
STEP 2 - Count the total number of bonding pairs and lone pairs of electrons around the central atom (AXE)
STEP 3 - Refer to reference table and use the number of pairs of electrons to predict the shape of the molecule
What does each letter of AXE stand for?
A represents the central atom
Each X represents an atom bonded to A.
Each E represents a lone pair of electrons on the central atom.
T/F:
LP-BP Repulsion are greater than BP-BP Repulsions
True!
Why does ammonia (NH3) have a geometry very similar to methane?
One “cloud of electrons” is not a bond – it is a lone pair yet it still created a tetrahedral shape (even though NH3 is a trigonal pyramidal)
What are lone pairs?
Lone pairs are orbitals of an atom that contain two electrons and as such do not form bonds with other atoms.
They must be taken into account because they are still regions of negative charge and repel other orbital clouds (bonded or lone pairs).
What must a molecule have to be polar?
- At least one polar bond
- Molecular shape, MUST have an asymmetrical geometry
because in all symmetrical molecules, the sum of the bond dipoles (vectors) is zero and the molecule is nonpolar.
What is the electronegativity difference for a polar molecule?
EN difference between 0.4-1.7
T/F:
INTERmolecular bonds are much STRONGER than INTRAmolecular bonds
False!
INTERRRmolecular bond are weaker!!
(e.g. takes much less energy to boil water (breaking intermolecular bonds) than it does to decompose water (breaking covalent bonds)
What are INTERmolecular forces?
Intermolecular forces occur between molecules to influence their physical and chemical properties.
What are INTRAmolecular forces?
Intramolecular forces occur between atoms within a molecule that hold the compound together. This is the basis of chemical bonding. (i.e ionic bonds, covalent bonds)
What is a vector?
An arrow pointing in a particular direction- it has a size and a direction.
Bond dipoles are used to represent the size (difference in electronegativities) and a direction, defined as pointing toward the negative end of the bond
