Dot Structures and VSEPR I
Match the following electronegativity ranges to the corresponding compound: 0-0.5 0.5-1.7 >1.7 Non polar Polar Ionic
0-0.5 = Non polar 0.5-1.7 = Polar >1.7 = Ionic
Draw CH4 (methane) using VSEPR Theory
8 v.e total Carbon has an octet 4 electron clouds total Therefore methane is a tetrahedral molecule 109.5 degree angle
What is the difference in electronegativity required for an ionic bond to form?
Above 1.7 on the Pauling scale
Odd number of electrons octet rule exception?
Any molecule with an odd number of v.e. cannot distribute those electrons to give eight to each atom; for example NO (nitric oxide) has eleven v.e.
Draw the geometry of Borine tetrafluroide based on the VSEPR theory (BF3) What is the formal charge for B? What is the geometric shape of this molecule?
B - 3 valence e- F - 7 valence e- (3) = 21 Total valence e- = 24 B = 3 v.e. - bonded v.e. = 0! Geometric shape: trigonal planar with bond angle of 120 degrees because there are 3 electron clouds around B
Compare bond strength, bond length, and bond energy
Bond strength = electrostatic attraction bt nuclei and electrons (multiple bonds aka high bond order increases strength) Bond length = is a consequence of the attractions, the shorter the bond the stronger the attraction Bond energy = the minimum amount of energy needed to break a bond
List the elements that almost always abide by the octet rule
C, N, O, F Na, Mg
Name that bond: An electron pair is shared between two atoms, typically non-metals, that have relatively similar values of electronegativity.
COVALENT BOND!
What are the 2 classifications of chemical bonds?
Covalent and ionic
Dipole moment units
Debye units (coulomb-meters)
What does ΔEN represent?
Difference in Electronegativity
Which periodic trend determines polarity in covalent bonds?
Differences in electronegativity between the two atoms involved
Dipole moment equation
Dipole moment is a vector quantity p is the dipole moment q is the magnitude of the charge d is the displacement vector separating the two partial charges
What are the steps in determining the shape of a molecule based on VSEPR theory?
Draw a dot structure to show the valence e- Count the number of "electron clouds" surrounding the central atom Predict the geometry of the electron clouds surrounding the central atom Ignore any lone pairs and predict the geometry of the ion/molecule
TRUE OR FALSE: If you spot a polar bond in a molecule, the molecule can be either polar or non polar, when you see a nonpolar bond in a molecule the molecule can be either polar or non polar
FALSE: polar bond can be polar or non polar nonpolar bond is ALWAYS nonpolar When in doubt, draw out relevant structures on your scratch paper on test day!
formal charge equation
FC = valence electrons - assigned electrons
What are some other physical properties of ionic bonds?
Good conductors of heat/electricity Solubility in water Crystal lattice arrangements
Name that bond: One or more electrons from an atom with low ionization energy, typically a metal, are transferred to an atom with a high electron affinity, typically a non-metal
IONIC BONDING!
These types of bonds have high boiling and melting points due to electrostatic attractions
IONIC BONDS
The degree to which the pair of electrons is shared equally or unequally between the two atoms determines the degree of POLARITY in the covalent bond. If the electron pair is shared equally the bond is _________; and if the pair is shared unequally, the bond is __________.
If the electron pair is shared equally the bond is NON POLAR; and if the pair is shared unequally, the bond is POLAR.
What type of structures do ionic bonds form?
Lattice structures These types of structures form to minimize repulsive forces
Do lone pairs or bonded electron pairs repel other electron pairs more strongly?
Lone pairs because the electrons reside closer to the nucleus creating greater coulombic forces!
How can you determine the most stable resonance structure of a anion molecule?
Minimize the formal charges, and then assign the negative formal charge to the most electronegative atom!
Draw out the Lewis Dot Structure of NH3 and then predict the molecular geometry of the molecule
Molecular geometry: Pyramidal BEWARE: One common mistake is a prediction that NH3 is trigonal planar when in fact it is pyramidal. NH3 is pyramidal because the lone pair of electrons contribute to coloumbic repulsion, which forms a pyramid instead of a plane.
When can atoms defy the octet rule?
Past period 3 in the periodic table atoms can expand their shell and accommodate more valence electrons
Draw XeF5+ dot structure
Remember to subtract one valence e- to account for the positive charge of this cation Notice Xe violates the octet rule because it is past period 3 in the periodic table
Draw the dot structure for Silicon tetra-fluoride https://youtu.be/p7Fsb21B2Xg?list=PLdonEKZrk3DnjGf7KvDQi-Da5MQzoLzQr
Si - 4 valence e- F - 7 valence e- 4 + 7(4) = 32 valence e- total Note that each covalent bond between F & Si accounts for 2 valence e- 32 - 8 = 24, which is why there are 24 valence e- drawn at the terminal atoms
What are resonance forms?
The different combinations of lewis dot structure forms. Only e- are moved, atoms are never moved.
Nitrate has a resonance hybrid structure, what does this mean?
The electrons are not staying in one place, they are being shared across the entire molecule
In a polar covalent bond, how can you determine which atom takes on the partial negative charge?
The more electronegative element will aquire a greater portion of the electron density, resulting in a partial negative charge
Draw the dot structure for formaldehyde (CH20)
There is a double bond from carbon to o2 to fill the octet rule for both atoms!
Bond length, bond energy, polarity, when are these characteristics important?
These are important characteristics for covalent bonds
What is the octet rule exception for helium, lithium, berylium, and boron?
They are more stable with an incomplete octet
Why do covalent compounds have lower boiling points and melting points?
They contain discrete molecular units with relatively weak intermolecular forces
Where are most of the common diatomic elements located on the periodic table? Visualize it.
They form the shape of a 7 on the periodic table, there are 7 of them...including hydrogen, most of them are found in group 7
What does VSEPR stand for?
Valence Shell Electron Pair Repulsion
Compare electron geometry vs molecular geometry
What is the difference between Electron Pair and Molecular Geometries? • When determining the electron pair geometry, lone pairs and bonds are considered and when determining molecular geometry only bonded atoms are considered. • If there aren't any lone pairs around the central atom, the molecular geometry is as same as the electron pair geometry. However, if there are any lone pairs involved both geometries are different.
Expanded octet rule?
When a central atom can have more than 8 shared electrons, this is possible for any elements in period 3 and greater including Phosporous(10), sulfur(12), chlorine (14), and many others
In a dot structure, how do you decide the central atom?
You select the atom that is the LEAST electronegative (except for H)
How are chemical bonds formed?
by the interactions between valence electrons of the combining atoms
What does the VSEPR theory state?
repulsion between the sets of valence-level electrons surrounding an atom causes these sets to be oriented as far apart as possible, this predicts the shape of molecules Like charges repel one another
What is Formal Charge?
the difference between the number of valence electrons in an isolated atom and the number of electrons assigned to that atom in a Lewis structure
