Chapter 3 - Atomic Structure

Question 1

Octet rule

Answer 1

8 valence electrons (s2p6) forms a particularly stable arrangement.

Exception: Be (4 electron is best) and B (6 electron is best)

Question 2

Hypervalent

Answer 2

Expanded shells (for elements of n≥3)

Question 3

Resonance

Answer 3

more than one possible way in which valence electrons can be placed in a Lewis structure

Question 4

Formal Charge

Answer 4

apparent electronic charge of each atom in a molecule

formal charge = #valence electrons - #lone pair electrons - #bonds

Question 5

How can you distinguish better Lewis structures?

Answer 5

• smaller magnitudes of formal charges
• place negative formal charges on more electronegative atoms
• smaller separation of charges

Question 6

VSEPR

Answer 6

Valence shell electron pair repulsion

• electron pair repulsion –> predict shape of molecules
• AXmEn (X = atom/group, E=lone pair of electrons)
• Steric number (SN) = m + n

Question 7

Steric number

Answer 7

for AXmEn, SN = m + n

Question 8

Ranking of electron pair repulsion

Answer 8

LP-LP > multiple bonds > LP-BP > BP-BP

Question 9

Lone pairs/electron-heavy groups like what positions for the following:

1. ) SN=5, trigonal bipyramidal
2. ) SN=6, octahedral
3. ) SN=7, pentagonal bipyramidal

Answer 9

1. ) equatorial (120º > 90º)
2. ) none, all positions equivalent
3. ) axial (90º > 72º)

Question 10

How can we determine feasibility of VSEPR structures?

Answer 10

How much destabilizing interactions (e.g. LP–LP, LP–BP) are there? (especially if ≤90º)

Question 11

Molecules with the most electronegative outer atoms have the ____ bond angles

Answer 11

smallest

Question 12

Molecules with the most electronegative central atoms have the ____ bond angles

Answer 12

biggest

Question 13

Ligand Close Packing (LCP) model

Answer 13

• the ligands have a specific radius when bound to a certain central atom
• nonbonded atom–atom distances in molecules remain approximately the same, even if the bond angles around the central atom are changed

Question 14

Dipole moment

Answer 14

µ = (∆Q)r

∆Q = charge difference
r = distance between the centers of + and - charge

Question 15

Oxidation states/numbers

Answer 15

The oxidation state of an uncombined element is zero.

The sum of the oxidation states of all the atoms or ions in a neutral compound is zero.

The sum of the oxidation states of all the atoms in an ion is equal to the charge on the ion.

The more electronegative element in a substance is given a negative oxidation state. The less electronegative one is given a positive oxidation state. Remember that fluorine is the most electronegative element with oxygen second.

Some elements almost always have the same oxidation states in their compounds:

Group 1 metals always +1
Group 2 metals always +2
Oxygen usually -2
Hydrogen usually +1 except in metal hydrides where it is -1 (see below)
Fluorine always -1
Chlorine usually -1 except in compounds with O or F (see below)