Structure 2.1 Flashcards

Models of Bonding & Structure. Select a sub-topic below

2.1 The Ionic Model: read all 25 questions and answers

How do main-group metals and non-metals form ions?
Metals lose valence electrons to form positively charged cations. Non-metals gain electrons to fill their valence shell, forming negatively charged anions. Both achieve a noble-gas electron configuration.
How can you predict the charge of a main-group ion?
Group 1, 2, 13 metals lose electrons to form +1, +2, +3 cations. Group 15, 16, 17 non-metals gain electrons to form -3, -2, -1 anions.
EXAM TRAP: Which electrons are lost first when transition metals form ions?
The 4s electrons are ALWAYS lost before the 3d electrons, even though 4s fills first during the Aufbau process.
Why do transition metals exhibit variable oxidation states?
The 4s and 3d subshells are very close in energy, so successive ionization energies show only a gradual increase. This allows different numbers of 3d electrons to be removed.
What is the electron configuration of Fe3+?
Neutral Fe is [Ar] 3d6 4s2. Remove 2 x 4s then 1 x 3d to give [Ar] 3d5. This is particularly stable due to the half-filled d-subshell.
How does ion size compare to atom size?
Cations are SMALLER than the parent atom (lost outer shell, more protons than electrons). Anions are LARGER (gained electrons, increased electron-electron repulsion).
EXAM TRAP: What is the precise definition of an ionic bond?
The non-directional electrostatic attraction between oppositely charged ions. Saying it is the transfer of electrons describes ion FORMATION, not the bond itself.
What is the IUPAC rule for naming binary ionic compounds?
The cation (metal) is named first, followed by the anion (non-metal) with its suffix changed to -ide. E.g. Sodium + chlorine = sodium chloride.
When must Stock notation (Roman numerals) be used?
For transition metals with variable oxidation states, to indicate the specific charge. E.g. Iron(II) chloride (FeCl2) vs Iron(III) chloride (FeCl3).
What is a polyatomic ion?
A group of covalently bonded atoms that carries a net charge and acts as a single unit. Key ones: NH4+, OH-, NO3-, HCO3-, CO3 2-, SO4 2-, PO4 3-.
How do you deduce the formula of aluminium sulfate?
Al3+ and SO4 2-. Cross charges: 2 x Al3+ = +6 and 3 x SO4 2- = -6. Formula: Al2(SO4)3. Brackets around SO4 are mandatory!
Do ionic compounds exist as molecules?
NO. They form giant 3D lattice structures. The formula (e.g. NaCl) is an empirical formula showing the simplest ratio of ions, not a molecular formula. Never say molecule for ionic compounds.
Why do ionic compounds have high melting points?
Large amounts of thermal energy are needed to overcome the strong electrostatic attractions between oppositely charged ions throughout the giant lattice.
EXAM TRAP: Why do molten/aqueous ionic compounds conduct?
The IONS are free to move and carry charge. NEVER say electrons are free to move - this gives zero marks. The charge carriers are mobile ions, not electrons.
Why are ionic compounds brittle?
When mechanical stress displaces a layer, like charges align directly opposite each other. The resulting electrostatic repulsion shatters the crystal along the plane.
Why do many ionic compounds dissolve in water more readily than in hexane, and why do some remain sparingly soluble?
Water is polar and forms ion-dipole attractions that release hydration enthalpy; dissolution occurs if hydration and entropy compensate for lattice enthalpy. Non-polar solvents provide little stabilisation of separated ions, and many salts with very high lattice enthalpies remain sparingly soluble or insoluble in water.
What determines the formula of an ionic compound?
Electrical neutrality: total positive charge must equal total negative charge. Use the criss-cross method - swap charges as subscripts and simplify.
What is the coordination number?
The number of nearest-neighbour ions of opposite charge surrounding a given ion. In NaCl, each Na⁺ is surrounded by 6 Cl^- ions (and vice versa).
(HL) Define lattice enthalpy.
The enthalpy change to completely separate one mole of a solid ionic compound into its constituent gaseous ions. It is always endothermic (positive delta H).
(HL) What two factors determine lattice enthalpy magnitude?
Ionic charge (higher charge = stronger attraction) and ionic radius (smaller ions = closer together = stronger attraction). Both follow from Coulombs Law: F is proportional to q+q-/r2.
(HL) Why does MgO have a much higher lattice enthalpy than NaCl?
Mg²⁺/O^2- have higher charges (+2/-2 vs +1/-1) AND smaller ionic radii than Na⁺/Cl^-. Both factors increase electrostatic attraction dramatically.
(HL) What is a Born-Haber cycle?
An application of Hess Law that calculates lattice enthalpy by breaking ionic compound formation into steps: atomisation, ionisation energy, electron affinity, and formation enthalpy.
(HL) What do state symbols mean in Born-Haber cycles?
They are ESSENTIAL. The lattice must be (s) and the separated ions must be (g). Omitting state symbols loses marks in every step of the cycle.
(HL) How does comparing experimental vs theoretical lattice enthalpy reveal covalent character?
If experimental (Born-Haber) closely matches theoretical (Coulombs Law), the bond is predominantly ionic. If experimental is significantly HIGHER, the bond has covalent character from ion polarisation.
(HL) What is the formal charge equation?
FC = V - L - B/2, where V = valence electrons, L = lone pair electrons, B = bonding electrons. The preferred Lewis formula minimises formal charges to zero.

2.2 The Covalent Model: read all 30 questions and answers

How do you calculate total valence electrons for a polyatomic anion?
Sum the valence electrons for all atoms and ADD one extra electron for each negative charge on the ion. For cations, subtract electrons.
EXAM TRAP: Can hydrogen or fluorine be the central atom in a Lewis formula?
No. Hydrogen forms only one bond and fluorine is highly electronegative with one bond. Both are always placed as terminal (outer) atoms, never as the central atom.
How does increasing bond multiplicity affect bond length and strength?
As multiplicity increases (single to double to triple): bond strength INCREASES and bond length DECREASES. More shared electrons create stronger electrostatic attraction, pulling nuclei closer.
What defines a coordination bond (historically coordinate or dative covalent bond)?
A covalent bond where BOTH shared electrons originate from the same donor atom. The acceptor has an empty orbital. Shown with an arrow from donor to acceptor.
Is a coordination bond different from a regular covalent bond once formed?
NO. Once formed, it is completely identical in length, strength, and reactivity. The term coordination only refers to how it was formed, not its final properties.
What is the core principle of VSEPR theory?
Electron domains (bonds and lone pairs) repel each other and arrange to maximise distance, minimising electrostatic repulsion. This determines molecular shape.
EXAM TRAP: Do bonding pairs and lone pairs exert the same repulsion?
NO. The repulsion hierarchy is LP-LP > LP-BP > BP-BP. Lone pairs spread more broadly (attracted to only one nucleus) and compress bond angles below ideal values.
What causes a covalent bond to be polar?
A difference in electronegativity between the two bonded atoms. The shared pair is drawn toward the more electronegative atom, creating partial charges.
How are bond dipoles represented in diagrams?
Using partial charges or dipole vector arrows pointing toward the more electronegative atom. Use data booklet Pauling values to justify.
Can a molecule with polar bonds be non-polar overall?
YES. If the geometry is highly symmetrical (e.g. CO2, CCl4, BF3), the individual bond dipoles cancel out perfectly, giving a net zero dipole moment.
EXAM TRAP: What is the most common mistake about molecular polarity?
Assuming polar bonds always make a polar molecule. Symmetrical molecules (no lone pairs, identical terminal atoms) have bond dipoles that cancel. Do not confuse bond polarity with molecular polarity.
Name three carbon allotropes that are giant covalent structures.
Diamond (3D tetrahedral, sp3), graphite (2D layers, sp2), and graphene (single graphite layer). Note: fullerene C60 is MOLECULAR, not giant covalent.
Why do covalent network structures have very high melting points?
Large numbers of atoms are linked by strong covalent bonds in continuous 3D networks. Breaking requires massive energy to overcome these strong intramolecular bonds.
What three types of IMF fall under van der Waals forces in IB?
London (dispersion) forces, dipole-dipole attractions, and dipole-induced dipole forces. Hydrogen bonding is NOT classified under van der Waals in the IB syllabus.
What conditions are required for hydrogen bonding?
Hydrogen must be DIRECTLY covalently bonded to F, O, or N. The H then interacts with a lone pair on F/O/N of a neighbouring molecule. H-Cl does NOT form H-bonds.
Why do simple molecular substances have low melting/boiling points?
They are held together by weak intermolecular forces (not covalent bonds). Little energy is needed to overcome these weak IMFs during changes of state.
EXAM TRAP: What happens to covalent bonds when a molecular substance boils?
NOTHING. Boiling only overcomes intermolecular forces BETWEEN molecules. The covalent bonds WITHIN each molecule remain completely intact. Never say covalent bonds break during boiling.
How does polarity affect solubility?
Like dissolves like. Polar/ionic solutes dissolve in polar solvents (water) via ion-dipole or dipole-dipole interactions. Non-polar solutes dissolve in non-polar solvents via London forces.
What determines separation in paper chromatography?
The differing intermolecular forces (affinities) each component has for the stationary phase vs the mobile phase. More affinity for mobile phase = higher Rf value = travels further.
(HL) What is a resonance hybrid?
When multiple valid Lewis formulas exist, the real molecule is a composite average of all resonance forms. Use double-headed arrows between structures. The molecule does NOT flip between forms.
(HL) How does resonance affect bond properties in NO3-?
Delocalised electrons make all three N-O bonds equivalent, with bond lengths and strengths intermediate between single and double bonds. Bond order = 1 and one-third.
(HL) What evidence supports delocalised bonding in benzene?
All six C-C bonds are identical at 140 pm (between single 154 pm and double 134 pm). Hydrogenation enthalpy is less exothermic than predicted. Benzene favours substitution over addition.
(HL) Which elements can form expanded octets?
Elements in Period 3 and beyond (e.g. P, S, Xe) because their larger atomic radius allows 5 or 6 electron domains (10 or 12 valence electrons in Lewis counting) around the central atom, enabled by multicentre bonding rather than d-expansion.
(HL) What is the geometry of 5 electron domains (e.g. PCl5)?
Trigonal bipyramidal with 3 equatorial positions (120 degrees apart) and 2 axial positions (90 degrees from equatorial). Lone pairs preferentially occupy equatorial positions.
(HL) What is the formal charge formula?
FC = V - L - B/2 where V = valence electrons, L = lone pair electrons, B = bonding electrons. The best Lewis formula minimises formal charges toward zero.
(HL) When comparing Lewis formulas using formal charge, which is most stable?
The structure where formal charges are closest to zero AND any negative formal charge sits on the most electronegative atom. Avoid adjacent same-sign charges.
(HL) How does sigma bonding differ from pi bonding?
Sigma bonds form by HEAD-ON orbital overlap with electron density along the bond axis. Pi bonds form by LATERAL (sideways) overlap of p-orbitals with density above and below the axis.
(HL) What is the sigma/pi composition of a double and triple bond?
Double bond = 1 sigma + 1 pi. Triple bond = 1 sigma + 2 pi. Every single covalent bond is a sigma bond. Free rotation occurs around sigma bonds but NOT around pi bonds.
(HL) What is orbital hybridisation?
The mixing of different atomic orbitals (s and p) on the same atom to form new equivalent hybrid orbitals. Determines geometry: sp3 = tetrahedral, sp2 = trigonal planar, sp = linear.
(HL) How do you quickly determine hybridisation?
Count electron domains (sigma bonds + lone pairs) around the central atom: 4 domains = sp3, 3 domains = sp2, 2 domains = sp. H2O and NH3 are both sp3 (lone pairs count!).

2.3 The Metallic Model: read all 15 questions and answers

Define metallic bonding using the exact IB wording.
The electrostatic attraction between a lattice of positive metal cations and a sea of delocalised electrons. You must say CATIONS (not atoms) and DELOCALISED (not free).
Why are metals good electrical conductors?
Delocalised electrons are free to move through the structure when a potential difference is applied, carrying charge. Cation vibrations also transfer kinetic energy (thermal).
Why are metals malleable and ductile?
Layers of cations can slide over each other without breaking bonds because the electron sea is non-directional and redistributes around the new positions.
EXAM TRAP: What is the key vocabulary mistake in metallic bonding?
Saying free electrons instead of DELOCALISED electrons, or saying atoms instead of CATIONS. Also never say metals have more metallic bonds - say stronger electrostatic attraction.
What two factors increase metallic bond strength?
(1) Higher cation charge = more delocalised electrons = stronger attraction. (2) Smaller ionic radius = electrons closer to nucleus = stronger attraction. E.g. Mg > Na.
Compare Na and Mg melting points and explain why.
Mg (650 C) >> Na (98 C). Mg²⁺ has higher charge (+2 vs +1), more delocalised electrons (2 vs 1), and smaller ionic radius (72 pm vs 102 pm) giving stronger metallic bonding.
What is an alloy?
A MIXTURE of a metal with one or more other elements. NOT a compound. The different-sized atoms disrupt the regular lattice, preventing layers from sliding, making it harder.
What is a substitutional alloy?
An alloy where the added atoms are SIMILAR in size to the host metal and REPLACE some host atoms in the lattice positions. Examples: bronze (Cu + Sn), brass (Cu + Zn).
What is an interstitial alloy?
An alloy where the added atoms are MUCH SMALLER and fit into the GAPS (interstices) between the host metal atoms. Example: steel (Fe + C).
EXAM TRAP: Why are alloys harder than pure metals?
Different-sized atoms prevent the LAYERS OF CATIONS FROM SLIDING over each other. Simply saying it disrupts the lattice is NOT enough for full marks.
How does alloying affect electrical conductivity?
Alloying typically DECREASES conductivity because the irregular lattice scatters the flow of delocalised electrons.
Why do metals have lustre (shine)?
Delocalised electrons absorb photons of visible light and re-emit them at many frequencies, producing a shiny reflective surface.
(HL) Define transition element.
By common exam convention: a d-block element that forms at least one stable ion with an incomplete (partially filled) d-sublevel. Note: IUPAC also includes neutral atoms with incomplete d-subshells (qualifying Sc as [Ar] 3d1 4s2, while Zn is excluded under both conventions).
(HL) Why do transition metals have higher melting points than s-block metals?
They delocalise both s AND d electrons into the electron sea. More delocalised electrons + higher cation charge + smaller radius = much stronger metallic bonding. E.g. Fe (1538 C) vs Na (98 C).
(HL) Why are Zn and Sc often excluded from transition metals despite being d-block?
Under the stable-ion convention, Zn forms only Zn²⁺ (3d¹⁰, completely full) and Sc forms only Sc³⁺ (3d⁰, completely empty). However, under the IUPAC neutral-atom definition, Sc ([Ar] 3d1 4s2) has an incomplete d-subshell and qualifies, whereas Zn is excluded under both conventions.

2.4 From Models to Materials: read all 18 questions and answers

What two values must you calculate to place a compound on the bonding triangle?
(1) Average electronegativity of the bonded atoms (x-axis) and (2) Difference in electronegativity (delta EN) between them (y-axis). You must show BOTH calculations for full marks.
Where do ionic, covalent, and metallic compounds sit on the van Arkel-Ketelaar triangle?
Top vertex = Ionic (high delta EN, medium avg EN, e.g. NaCl). Bottom-right = Covalent (low delta EN, high avg EN, e.g. Cl₂). Bottom-left = Metallic (low delta EN, low avg EN, e.g. Na).
EXAM TRAP: Why is it wrong to say a bond is purely ionic or purely covalent?
Bonding is a continuum, not a simple classification. Most bonds exhibit a mixture of character. E.g. AlCl₃ is classified as ionic but behaves as a covalent dimer when vaporised. Always discuss bonding as a continuum.
Describe the bonding transition across Period 3 oxides.
Na₂O, MgO = giant ionic (high mp, basic). Al₂O₃ = transitional (significant covalent character due to small Al³⁺). SiO₂ = giant covalent (very high mp). P₄O₁₀, SO₃, Cl₂O = simple molecular (low mp, acidic solutions).
Name three common alloys and their components.
Bronze = Cu + Sn (tools, bells). Brass = Cu + Zn (plumbing, musical instruments). Stainless steel = Fe + Cr + Ni + C (surgical instruments, cookware - Cr forms self-healing oxide layer).
EXAM TRAP: Why is calling an alloy a compound wrong?
An alloy is a MIXTURE of a metal with other metals or non-metals. Using the word compound will lose you the definition mark. The components are not chemically bonded in fixed ratios.
What is graphene and why is it special?
A single layer of sp2-bonded carbon atoms (one atom thick). Strongest known material, excellent electrical conductor (delocalised pi electrons), transparent, and flexible.
Compare sp2 vs sp3 carbon allotropes.
sp3 (diamond): 4 single C-C bonds per carbon, tetrahedral (109.5 deg), no delocalised electrons = electrical insulator, extremely hard 3D giant covalent network. sp2 network allotropes (graphite, graphene): 3 sigma bonds + 1 delocalised pi electron per carbon = electrical conductor along 2D planar sheets. (In contrast, carbon nanotubes are cylindrical rolls and C60 fullerene is a discrete molecular cage with limited conductivity).
What are fullerenes (C60) and carbon nanotubes?
C60 = spherical cage molecule (truncated icosahedron), soluble in non-polar solvents, used in drug delivery. CNTs = graphene rolled into cylinders, extraordinary tensile strength and thermal conductivity.
What is the key principle of addition polymerisation?
Monomers with a C=C double bond undergo polymerisation. The pi bond breaks and freed electrons form new C-C sigma bonds. No atoms are lost - everything in the monomer appears in the polymer.
Name four common addition polymers and their monomers.
Ethene → Polyethene (PE, bags). Chloroethene → PVC (pipes). Tetrafluoroethene → PTFE/Teflon (non-stick). Phenylethene → Polystyrene (PS, insulation).
How do you draw the repeat unit of an addition polymer?
1) Break the C=C into C-C single bond. 2) Add extension (trailing) bonds at each end. 3) Enclose in square brackets with subscript n. Full marks require BOTH trailing bonds outside brackets AND subscript n.
Why are addition polymers non-biodegradable?
The polymer backbone consists of strong, saturated C-C single bonds that are chemically inert. There are no functional groups for enzymes or water to attack, so they persist in the environment.
[HL] What is the key principle of condensation polymerisation?
Monomers with TWO functional groups (bifunctional) react, releasing a small molecule (usually H₂O) at each linkage. Unlike addition, atoms ARE lost.
[HL] Compare polyesters and polyamides.
Polyester: diol (-OH) + dicarboxylic acid (-COOH) = ester linkage (-COO-), e.g. PET. Polyamide: diamine (-NH2) + dicarboxylic acid (-COOH) = amide linkage (-CONH-), e.g. Nylon-6,6. Both lose H₂O.
[HL] List three key differences between addition and condensation polymers.
Addition: unsaturated C=C monomers, no small molecules eliminated (100% atom economy), non-biodegradable hydrocarbon backbone. Condensation: monomers with two reactive functional groups, small molecule (e.g. H₂O, HCl) eliminated per linkage, hydrolysable backbone = potentially biodegradable.
[HL] Why must condensation monomers be bifunctional?
Each monomer needs a functional group at BOTH ends to react and continue the chain. If a monomer only has one functional group, the chain terminates immediately - no polymer can form.
[HL] Is the small molecule lost in condensation always water?
Usually H₂O, but can also be HCl (e.g. When acyl chlorides are used as monomers). Always identify the specific small molecule in your answer - do not assume water.

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