Structure
Glenn Molecular V. Lo Department of Physical Sciences Nicholls State University
Covalent Bonding
Covalent bonding: sharing of electrons Molecule: an electrically neutral group of two or more atoms sharing electrons
Covalent bonding tends to occur between nonmetal atoms. Electron transfer, instead of sharing, is more likely between a metal and a nonmetal. Why?
If the group is not electrically neutral, it's called a polyatomic ion
Covalent Bonding
Covalent bonding tends to occur between nonmetal atoms. Electron transfer, instead of sharing, is more likely between a metal and a nonmetal. For which pair is covalent bonding likely to occur?
Na and Cl Cl and Cl H and Br C and O
Lewis dot symbol
Covalent bonding behavior of atoms depends on valence configuration
Lewis dot symbol: shows valence electrons as dots around the symbol for an atom. Example:
same group same valence configuration similar pattern
N: [He] 2s2 2p3 (five valence electrons) Lewis symbol:
N
Lewis dot symbol
Main groups (the "A" groups): number of dots = number of valence electrons = group #; indicate nonzero charge outside square brackets Which of the following are correctly drawn Lewis dot symbols?
Lewis structures
Lewis dot symbol for noble gases:
Lewis structure: a sketch showing how atoms in a molecule or polyatomic ion are sharing valence electrons
Draw eight dots (representing valence electrons) Exception: He (two dots)
Follow octet rule: share valence electrons so that each atom appears surrounded by eight dots. Follow duet rule for H Atoms acquire a noblegaslike configuration by following the octet/duet rule.
Interpreting Lewis Structures
Dots = valence electrons Dash = same as a pair of dots; a shared pair of electrons Lewis structure for polyatomic ions are enclosed in brackets and charge indicated as a superscript outside the brackets
Interpreting Lewis Structures
Consider the Lewis structure for urea. How many valence electrons are
shared? not shared? shared between C and O? shared between C and N? shared between N and H?
Terminologies
Single (covalent) bond: one shared pair between two atoms Double (covalent) bond: two shared pairs between two atoms Triple (covalent) bond: three shared pairs between two atoms Lone pair: unshared pair of valence electrons In the structure shown above, identify the single bonds, double bond, and lone pairs.
Diatomics
Shown above are five possible ways of drawing a Lewis structure for a diatomic molecule or ion. "A" and "X" are generic symbols draw brackets and indicate charge for ions For which of the five structures must "A" or "X" be a hydrogen atom? Which type of structure would you use to represent a diatomic molecule (or ion) with
14 electrons? 12 electrons? 10 electrons? 8 electrons? 2 electrons?
Diatomics
Draw Lewis structures for the following: H2, HCl, CO, O2, O22, N2, F2, PF, CN, OH
Triatomics
Draw Lewis structures for the following: HCN, HOH, HOCl, NH2
Triatomics
Draw Lewis structures for the following: CO2, OCS, Cl2O, SO2, O3, N3
Resonance Structures
Two or more ways of distributing valence electrons; atoms connected same way Example: a triatomic molecule or ion with 16 valence electrons can have the following resonance structures
Resonance Structures
Reality = hybrid of resonance structures. Some resonance structures are better representations of reality; they "contribute" more to the hybrid Which of the resonance structures of H2 shown above is more significant?
Resonance resonance Structures
Several structures may be needed to adequately represent reality. Example: the three Lewis structures for nitrate (below) are equally significant.
Expect double bonded atoms to be closer together than if they were singly bonded. Experimental data on nitrate suggest that all NtoO distances are equal
Isomeric Structures
Two or more ways of connecting the atoms. Isomers represent different compounds. Example: C4H8
Isomeric or Resonance
Consider the structures shown below for C2H6O. Would you call them isomeric or resonance structures?
Formal Charges
Question: How do we know how to connect the atoms? Example: how do we know that in CO2, the atoms are connected OCO and not COO? Answer:
Nature seems to like symmetry. Connectivity is more plausible if you can draw structures with very small formal charges (0, +1, and 1).
Formal Charges
To figure out formal charge for an atom in a structure:
Note: all formal charges in a structure should add up to the overall charge of the structure. (use as a final check)
Count valence electrons that atom appears to "own" in the structure. Assume atom owns all electrons in its lone pairs and half of electrons it is sharing. Subtract from valence electrons that atom would have if it were alone and neutral
Formal Charges
Determine the formal charges of the atoms in the structure above.
C appears to own 6 valence electrons
4 in the lone pairs Half of electrons shared with O (2)
Is this structure plausible?
C by itself, neutral, would have 4 valence electrons Formal charge of C = 4 6 = 2 C appears to have gained 2 electrons. Middle O: 6 4 = +2; Right O: 6 6 = 0
Formal Charges
Determine the formal charges of the atoms in the structure above.
C appears to own 5 valence electrons
2 in the lone pair Half of electrons shared with O (3)
Is this structure plausible?
C by itself, neutral, would have 4 valence electrons Formal charge of C = 4 5 = 1 C appears to have gained 1 electron. Middle O: 6 4 = +2; Right O: 6 7 = 1
Formal Charges
Formal charges can also be used to determine resonance structure(s) that are more significant.
Which of the 3 resonance structures shown for CO2 is most significant?
Formal charges of neighboring atoms should generally be consistent with their relative electronegativities. O is more electronegative than C; O should have a more negative formal charge.
Formal Charges
Formal charges can also be used to determine resonance structure(s) that are more significant.
Which of the 3 resonance structures shown for NO2+ is most significant?
Formal charges should be as small as possible (0, +1, 1) Formal charges of neighboring atoms should not have the same sign.
Exceptions to Octet Rule
Some molecules have an odd number of valence electrons.
Example: NO
Not possible to draw a Lewis structure Generally reactive; called "radicals" Experimental evidence for unpaired electron: paramagnetism Which resonance structure shown on the right is more significant?
Exceptions to Octet Rule
Compounds of B, Be, and Al. Example: BF3 Only structure B follows octet rule Electronegativities suggest structure C should be more significant than structure B(why?) Boron atom in BF3 readily forms coordinate covalent bond suggests structure A is also significant.
Exceptions to Octet Rule
Expanded octets possible with molecules or ions of type AXn, where A (the "central atom") is an atom from row 3 and below
additional electrons accommodated by d orbitals Examples: PCl5, SF6, SO42