Difficulty: Intermediate | Prerequisites: Electronegativity, bond types (see Part 1 notes), VSEPR basics (see Part 2 notes)
Lewis structures show how valence electrons are distributed in a molecule, and formal charge helps you pick the best structure. Resonance applies when a single Lewis structure cannot capture the true electron distribution. Metals are held together by a "sea" of delocalised electrons, which is why they are malleable and ductile. Alloys are mixtures of metals (or metals with small nonmetals like carbon) that alter the properties of pure metals by disrupting the regular lattice.
Lewis structure (electron dot diagram)
A diagram showing the arrangement of valence electrons around atoms in a molecule, using dots for lone pairs and lines for bonding pairs.
Octet rule
Most atoms bond in a way that gives them eight valence electrons. In simple terms, atoms want a full outer shell. Exceptions include hydrogen (2 electrons), boron (often only 6), and expanded octets for elements in Period 3 and below.
Formal charge
The charge an atom would have if bonding electrons were shared equally. Calculated as: valence electrons minus lone pair electrons minus half of bonding electrons. The best Lewis structure minimises formal charges.
Resonance
When more than one valid Lewis structure can be drawn for a molecule by moving electrons (not atoms). The real molecule is a blend (hybrid) of all the resonance structures.
Bond order
The average number of bonds between two atoms. For resonance structures, bond order = (total bonds across all structures) / (number of structures). A bond order of 1.5 means the bond is intermediate between a single and a double bond.
Metallic bond
The attraction between a lattice of metal cations and a "sea" of delocalised valence electrons. In simple terms, metal atoms share their outer electrons freely with all neighbours.
Alloy
A mixture of a metal with one or more other elements (usually metals, sometimes small nonmetals like carbon). The added atoms disrupt the regular lattice, changing properties like hardness and malleability.
Substitutional alloy
An alloy where the added atoms replace some host atoms in the lattice, typically because they are similar in size. Example: brass (Cu and Zn, both about 135 pm radius).
Interstitial alloy
An alloy where small atoms fit into the gaps (interstices) between the larger host atoms. Example: steel (Fe with small C atoms).
Isomer (structural isomer)
Molecules that share the same molecular formula but have different structural arrangements of atoms. Example: CH3OCH3 (dimethyl ether) and CH3CH2OH (ethanol) are both C2H6O.
Drawing Lewis structures: the method
Count total valence electrons (adjust for charge on ions).
Place the least electronegative atom in the centre.
Draw single bonds to all surrounding atoms.
Distribute remaining electrons as lone pairs, starting with outer atoms.
If the central atom lacks an octet, form double or triple bonds by converting lone pairs from outer atoms.
Octet exceptions
BH3 (and BF3): boron has only 6 electrons around it. This is a stable electron-deficient molecule.
Molecules with expanded octets (e.g. PCl5, SF6) can have more than 8 electrons around the central atom because they are in Period 3 or below and can use d orbitals.
Formal charge and choosing the best Lewis structure
Formal charge = (valence electrons) minus (lone pair electrons) minus (1/2 bonding electrons).
The best structure has formal charges as close to zero as possible.
Negative formal charges should sit on more electronegative atoms.
N2O example
N2O can be drawn several ways. The best Lewis structure (by formal charge) is N triple bond N single bond O, with a lone pair on the terminal N and two lone pairs on O.
This gives formal charges of -1 on the terminal N, +1 on the central N, and 0 on O. The alternative with the triple bond on the other side puts a negative formal charge on N (more electronegative placement) rather than on O.
Molecules with a 120-degree bond angle from Lewis structure
Acetic acid (CH3COOH): the carboxyl carbon (C bonded to one O by double bond and one OH by single bond) has three electron domains = 120 degrees. The Lewis structure showing the double bond to O is the one with the 120-degree angle.
Lewis structures for H2S
S is the central atom with two bonding pairs (to H) and two lone pairs = four electron domains. Two unshared pairs of electrons on the central atom.
When resonance is needed
Resonance applies when a single Lewis structure cannot describe the actual electron distribution. The molecule has delocalised electrons that are spread across multiple bonds.
SO2: two resonance structures. The S=O double bond can be drawn on either side. Each S-O bond has a bond order of 1.5. The molecule is bent (lone pair on S).
O3: two resonance structures, bent geometry. Bond order of 1.5 for each O-O bond.
NO3-: three equivalent resonance structures. All three N-O bonds are identical, each with a bond order of 4/3. The ion is trigonal planar and is not polar (supporting the claim of three equivalent resonance forms, since a polar species would require unequal bonds).
Benzene (C6H6): two resonance structures (alternating single/double bonds). All six C-C bonds are equivalent with a bond order of 1.5. This is why all C-C bonds in benzene have the same length.
Glycine (amino acid): the resonance structures of the carboxylate group show both C-O bonds with a bond order of 1.5.
Bond order from resonance
For glycine's two C-O bonds: one resonance structure shows C=O and C-O; the other shows C-O and C=O (swapped). Averaging: each bond is (2 + 1) / 2 = 1.5.
How resonance affects properties
Equal bond lengths: if all resonance structures are equivalent (like NO3- or benzene), all the relevant bonds are the same length.
Intermediate bond strength: resonance bonds are stronger than single bonds but weaker than double bonds.
Molecules needing resonance: SO2, O3, NO3-, CO3 2-, benzene. Molecules that do not need resonance: H2S, CO2 (has two equivalent double bonds in a single Lewis structure), OF2, PF3.
CO2 does not need resonance
A common source of confusion. CO2 has two equivalent C=O double bonds, but these are already captured in a single Lewis structure. There is no need to invoke resonance.
The sea of electrons model
In a metal, the valence electrons are delocalised: they are not bound to any one atom but flow freely throughout the lattice of metal cations.
The electrons are equally shared and form nondirectional bonds. This is the key reason metals are malleable and ductile.
Because the bonding is nondirectional, metal atoms can slide past each other without breaking bonds. The electron sea simply rearranges. Compare this with ionic crystals, which shatter when displaced because like charges align.
Substitutional alloys
Formed when atoms of similar size replace host atoms in the lattice.
Cu-Zn (brass): copper and zinc both have an atomic radius of 135 pm. In the correct diagram, two types of same-sized atoms are randomly distributed throughout the lattice.
Au-Cu vs Au-Ag: Cu atoms (128 pm) are smaller than Ag atoms (144 pm, same as Au). The smaller Cu atoms interfere more with the displacement of Au atoms, making the Au/Cu alloy harder than Au/Ag.
Interstitial alloys
Formed when much smaller atoms fit into the spaces between the larger host atoms.
Steel (Fe + C): iron atoms are large (125 pm), carbon atoms are small (77 pm). The small C atoms sit in the gaps between Fe atoms.
In the correct diagram, large Fe atoms form the main lattice and small C atoms fill some of the interstices. Fe atoms greatly outnumber C atoms (steel is mostly Fe by mass).
Higher carbon content makes steel harder and more brittle because the C atoms block the Fe atoms from sliding past each other.
Steel-chromium alloy
Cr (127 pm) is nearly the same size as Fe (125 pm), so Cr substitutes for some Fe atoms in the lattice.
On the surface, Cr reacts with O2 to form a chromium oxide layer. In the correct diagram, the surface shows Cr and O atoms, while the interior shows mostly Fe with scattered Cr and small C atoms.
Ni-B alloy
Nickel is a large atom; boron is very small. This is an interstitial alloy. The correct diagram shows large Ni atoms with small B atoms in the gaps, similar to steel's Fe-C arrangement.
Structural isomers have the same molecular formula but different connectivity.
CH3OCH3 (dimethyl ether) and CH3CH2OH (ethanol) are isomers. Both have the formula C2H6O, but the atoms are connected differently.
CH3COCH3 (acetone, a ketone) and CH3CH2CHO (propanal, an aldehyde) are isomers. Both have the formula C3H6O.
CH3CH2CH2OH (1-propanol) and CH3COCH3 (acetone) share the same molecular formula and are isomers.
How to spot isomers on the exam
Count the atoms in each formula. If two compounds have identical atom counts but different structural formulas, they are isomers. The functional groups will typically differ (e.g. an alcohol vs an ether, or a ketone vs an aldehyde).
Students sometimes think CO2 needs resonance structures. It does not. Both C=O double bonds are already equivalent in a single Lewis structure.
Confusing bond order with the number of bonds in a single resonance structure. Bond order is the average across all valid resonance forms.
Thinking alloys are compounds. An alloy is a mixture, not a compound. The metals are physically mixed, not chemically bonded in fixed ratios.
Assuming all alloys are substitutional. Small atoms like carbon and boron form interstitial alloys, fitting into gaps rather than replacing host atoms.
Believing metals are malleable because metallic bonds are weak. Metallic bonds are strong, but they are nondirectional. The atoms can slide without breaking the overall bonding, which is what allows deformation.
Resonance and bond order questions appear frequently. Be ready to draw resonance structures for SO2, O3, NO3-, and benzene, and to calculate bond orders.
Formal charge is tested both as a standalone calculation and as a way to choose between competing Lewis structures (especially for N2O and similar molecules).
Alloy questions are becoming more common on the AP exam. Know the difference between substitutional and interstitial, and be able to identify which type from a particulate diagram.
Isomer identification is a reliable MCQ topic. Practise counting atoms to check whether two formulas are isomeric.
The connection between metallic bonding and physical properties (malleability, ductility, conductivity) is a standard exam question.
True or False: BH3 satisfies the octet rule. (False, boron has only 6 electrons.)
Fill in the blank: The bond order of each C-C bond in benzene is ______. (1.5)
True or False: Steel is an interstitial alloy of iron and carbon. (True)
Fill in the blank: CH3OCH3 and CH3CH2OH are ______ of each other. (structural isomers)
True or False: Metals are malleable because their bonding electrons are delocalised and nondirectional. (True)
Q: Draw the best Lewis structure for N2O and explain your choice using formal charges.
A: The best structure is N triple bond N single bond O (with a lone pair on terminal N and two lone pairs on O). This gives formal charges of -1 on terminal N, +1 on central N, and 0 on O. This is preferred because negative formal charge sits on the more electronegative atom (N is less electronegative than O, but the alternative structures place even larger formal charges on atoms).
Q: Why does the NO3- ion have three equivalent resonance structures? What evidence would support this claim?
A: The three equivalent resonance structures place the N=O double bond on each of the three oxygen atoms in turn. Evidence: all three N-O bond lengths are identical, all O-N-O bond angles are 120 degrees, and the ion is not polar. If one bond were different, the ion would be asymmetric.
Q: Explain why high-carbon steel is harder and more brittle than low-carbon steel.
A: The additional carbon atoms sit in the interstices between iron atoms. More carbon atoms mean more obstacles to the sliding of iron atom planes, making the alloy harder. The same blockage prevents deformation, making it brittle rather than malleable.
Q: Brass is an alloy of copper and zinc. Both atoms have a radius of 135 pm. Is brass a substitutional or interstitial alloy? Explain.
A: Brass is a substitutional alloy. Because Cu and Zn atoms are the same size, Zn atoms replace Cu atoms in the lattice positions rather than fitting into gaps between them.
Q: Which molecule needs resonance structures to describe its bonding: H2S, SO2, CO2, OF2, or PF3?
A: SO2. It has two resonance structures where the S=O double bond alternates between the two oxygen atoms. The other molecules can each be described adequately by a single Lewis structure.
Resonance connects to acid-base chemistry: the stability of a conjugate base often depends on resonance delocalisation of the negative charge (e.g. carboxylate ions, nitrate). Metallic bonding connects to conductivity and the properties of materials discussed in later units. Alloy composition and structure connect to stoichiometry (mass percent) and to materials science applications.
Lewis structure, Lewis dot diagram, electron dot structure, octet rule, octet exception, expanded octet, formal charge, resonance, resonance structures, resonance hybrid, bond order, delocalised electrons, metallic bond, sea of electrons, electron sea model, malleable, ductile, alloy, substitutional alloy, interstitial alloy, brass, bronze, steel, stainless steel, chromium oxide, Fe-C alloy, Au-Cu alloy, Ni-B alloy, isomer, structural isomer, constitutional isomer, CH3OCH3, CH3CH2OH, SO2, O3, NO3-, CO3 2-, benzene, C6H6, N2O, BH3, formal charge calculation, AP Chemistry Unit 2