2006 Brooks/Cole - Thomson Writing Lewis Formulas: The Octet Rule • The octet rule states that representative elements usually attain stable noble gas electron configurations in most of their compounds. • Lewis dot formulas are based on the octet rule. • We need to distinguish between bonding (or shared) electrons and nonbonding (or unshared or lone pairs) of electrons. • N - A = S rule – Simple mathematical relationship to help us write Lewis dot formulas. • N = number of electrons needed to achieve a noble gas configuration. – N usually has a value of 8 for representative elements. – N has a value of 2 for H atoms. • A = number of electrons available in valence shells of the atoms. – A is equal to the periodic group number for each element. – A is equal to 8 for the noble gases. • S = number of electrons shared in bonds. • A-S = number of electrons in unshared, lone, pairs.
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Writing Lewis Formulas: The Octet Rule• The octet rule states that representative elements usually attain stable noble gas electron
configurations in most of their compounds.• Lewis dot formulas are based on the octet rule.• We need to distinguish between bonding (or shared) electrons and nonbonding (or
unshared or lone pairs) of electrons.
• N - A = S rule– Simple mathematical relationship to help us write Lewis dot formulas.
• N = number of electrons needed to achieve a noble gas configuration.– N usually has a value of 8 for representative elements. – N has a value of 2 for H atoms.
• A = number of electrons available in valence shells of the atoms.– A is equal to the periodic group number for each element. – A is equal to 8 for the noble gases.
• S = number of electrons shared in bonds.• A-S = number of electrons in unshared, lone, pairs.
Writing Lewis Formulas: The Octet Rule1. For ions we must adjust the number of electrons available, A.
a. Add one e- to A for each negative charge.
b. Subtract one e- from A for each positive charge.
2. The central atom in a molecule or polyatomic ion is determined by:
a. The atom that requires the largest number of electrons to complete its octet goes in the center.
b. For two atoms in the same periodic group, the less electronegative element goes in the center.
3. Select a reasonable skeletona. The least electronegative is the central atomb. Carbon makes 2,3, or 4 bondsc. Nitrogen makes 1(rarely), 2,3, or 4 bondsd. Oxygen makes 1, 2(usually), or 3 bondse. Oxygen bonds to itself only as O2 or O3, peroxides, or superoxidesf. Ternary acids (those containing 3 elements) hydrogen bonds to the oxygen, not
the central atom, except phosphatesg. For ions or molecules with more than one central atom the most symmetrical
• What kind of covalent bonds, single, double, or triple, must this ion have so that the six shared electrons are used to attach the three O atoms to the S atom?
• There are three possible structures for SO3.– The double bond can be placed in one of three places.
O S
O
O·· ······ ··
······
OS
O
O·· ···· ·· ··
··
······
O S
O
O·· ····
·· ··
····
oWhen two or more Lewis formulas are necessary to show the bonding in a molecule, we must use equivalent resonance structures to show the molecule’s structure.
oDouble-headed arrows are used to indicate resonance formulas.
Writing Lewis Formulas:Limitations of the Octet Rule
• There are some molecules that violate the octet rule.– For these molecules the N - A = S rule does not apply:
1. The covalent compounds of Be.2. The covalent compounds of the IIIA Group.3. Species which contain an odd number of electrons.4. Species in which the central element must have a share of
more than 8 valence electrons to accommodate all of the substituents.
Writing Lewis Formulas:Limitations of the Octet Rule
• Write dot and dash formulas for AsF5.
– This is an example of rule 4; Species in which the central element must have a share of more than 8 valence electrons to accommodate all of the substituents.
• Covalent bonds in which the electrons are shared equally are designated as nonpolar covalent bonds.– Nonpolar covalent bonds have a symmetrical charge
distribution.• To be nonpolar the two atoms involved in the bond
• Covalent bonds in which the electrons are not shared equally are designated as polar covalent bonds– Polar covalent bonds have an asymmetrical charge
distribution• To be a polar covalent bond the two atoms involved in the bond
must have different electronegativities.• Some examples of polar covalent bonds.• HF
Dipole Moments• Molecules whose centers of positive and negative charge do not coincide, have an
asymmetric charge distribution, and are polar.
– These molecules have a dipole moment.
• The dipole moment has the symbol . is the product of the distance,d, separating charges of equal magnitude and opposite
sign, and the magnitude of the charge, q.
units Debye0.38 units Debye1.91
I- H F- H
--
• There are some nonpolar molecules that have polar bonds.• There are two conditions that must be true for a molecule to be polar.
1. There must be at least one polar bond present or one lone pair of electrons.2. The polar bonds, if there are more than one, and lone pairs must be arranged
so that their dipole moments do not cancel one another.
• Frequently, we will describe two geometries for each molecule. 1.1. Electronic geometryElectronic geometry is determined by the locations of regions of
high electron density around the central atom(s).2.2. Molecular geometryMolecular geometry determined by the
arrangement of atoms around the central atom(s).Electron pairs are not used in the molecular geometry determination just the positions of the atoms in the molecule are used.