Unit 7: Chemical Bonding and Molecular Geometry
How atoms hold together, from an electron changing owners entirely to two atoms sharing a pair unevenly, and how repulsion alone builds a molecule's shape.
10 topics · 41 questions
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7.1 Ionic Bonding and Lattice Energy
- How it worksHow does an electron transferring completely from a metal to a nonmetal create the electrostatic attraction that is an ionic bond?
- Common mistakeWhy isn't there a single distinct molecule of NaCl the way there's a distinct molecule of water?
- Worked exampleFind which of two ionic compounds, NaCl or MgO, has the higher lattice energy, using ion charge and size.
- Going furtherWhat happens to lattice energy as the ionic radii of the cation and anion both get smaller?
Your turn1 of 2
Sodium and chloride ions are similar in size to magnesium and oxide ions, yet MgO's lattice energy is more than four times higher than NaCl's. What explains this large difference?7.1 The Born-Haber Cycle
- How it worksHow does the Born-Haber cycle break the formation of an ionic solid into steps whose energies you can actually look up?
- Worked exampleFind the lattice energy of NaCl from a Born-Haber cycle built out of sublimation, ionization, dissociation, electron affinity, and formation energies.
- Common mistakeWhy isn't lattice energy something you can measure by directly breaking apart an ionic crystal in one step?
- Ties togetherHow does the Born-Haber cycle's use of energy steps that sum to a total connect back to Hess's law?
Your turn1 of 2
A Born-Haber cycle for forming an ionic solid combines these steps: sublimation +120 kJ, ionization +495 kJ, bond dissociation +240 kJ, and electron affinity -344 kJ. The overall (known) formation enthalpy is -487 kJ. Find the lattice energy that makes these steps sum to the overall formation enthalpy (lattice energy = formation enthalpy minus the sum of the other four steps).7.1 Covalent Bonding and Electronegativity
- Common mistakeWhy isn't there a sharp line separating a polar covalent bond from an ionic one?
- How it worksHow does the electronegativity difference between two bonded atoms decide whether a bond is nonpolar, polar, or ionic?
- Worked exampleFind whether the bond in HF is best described as nonpolar covalent, polar covalent, or ionic using electronegativity values.
- Going furtherWhat happens to a bond's character as the electronegativity difference between its two atoms keeps growing?
Your turn1 of 2
Two bonded atoms have electronegativity values of 3 and 2.84. Find the magnitude of their electronegativity difference.7.2 Drawing Lewis Structures
- How it worksWalk me through the step-by-step method for drawing the Lewis structure of a molecule like CO2.
- How it worksHow do lone pairs and bonding pairs both count toward satisfying the octet rule around an atom?
- Worked exampleFind the Lewis structure of ammonia, NH3, including every lone pair.
- Common mistakeWhy isn't a double bond in a Lewis structure just two single bonds squeezed into the same spot?
Your turn1 of 2
A Lewis structure shows a double bond between two atoms as two shared pairs of electrons. Why isn't a double bond simply two single bonds squeezed into the same location?7.2 Formal Charge and Choosing the Best Lewis Structure
- How it worksHow does formal charge compare the electrons an atom 'owns' in a structure to the electrons it started with as a free atom?
- Common mistakeWhy isn't formal charge the same thing as an atom's actual charge in the real molecule?
- Worked exampleFind the formal charge on each atom in one Lewis structure of the cyanate ion, OCN-.
- Going furtherWhat happens when you compare formal charges to decide which of two competing Lewis structures for a molecule is more stable?
Your turn1 of 2
In one Lewis structure, an atom has 6 valence electrons as a free atom, 4 nonbonding (lone pair) electrons around it, and is part of 4 bonding pairs. Find its formal charge, using formal charge = valence electrons - lone pair electrons - number of bonding pairs.7.2 Resonance Structures
- How it worksHow do curved arrows moving electron pairs turn one resonance structure into another without moving a single atom?
- How it worksWalk me through drawing all the resonance structures of the carbonate ion, CO3 2-.
- Common mistakeWhy isn't a molecule with resonance structures actually flickering back and forth between them?
- Going furtherWhat happens to a molecule's real bond length and bond order once you average across all of its resonance structures?
Your turn1 of 2
Ozone is often drawn with two resonance structures, each showing one double bond and one single bond in different places. Why isn't the real molecule actually flickering back and forth between these two structures over time?7.3 Exceptions to the Octet Rule
- Common mistakeWhy isn't the octet rule a law that every stable molecule has to obey?
- How it worksHow do elements in period 3 and beyond use available d orbitals to hold more than eight electrons around a central atom?
- Worked exampleFind the Lewis structure of sulfur hexafluoride, SF6, and count the electrons surrounding the central sulfur.
- Going furtherWhat happens to a molecule's Lewis structure when the central atom has an odd total number of valence electrons, like in NO?
Your turn1 of 2
A central atom forms 5 single bonds and has 2 lone pairs around it. How many total electrons surround it, counting both bonding and lone pair electrons?7.6 VSEPR: Basic Electron Domain Geometries
- How it worksHow does electron pair repulsion alone predict a molecule's three-dimensional shape with no calculation at all?
- Worked exampleFind the electron domain geometry and molecular geometry of CH4 using VSEPR.
- Worked exampleFind the molecular geometry of BF3 and explain why it differs from CH4's.
- Common mistakeWhy isn't a molecule's molecular geometry always the same as its electron domain geometry?
Your turn1 of 2
A central atom has four bonding pairs of electrons around it. Why do those four pairs arrange themselves into a tetrahedron rather than, say, a flat square?7.6 VSEPR with Lone Pairs: Bent, Pyramidal, and Seesaw Shapes
- How it worksHow does a lone pair take up more space than a bonding pair and squeeze the real bond angles smaller than the ideal geometry predicts?
- Worked exampleFind the molecular geometry and approximate bond angle of ammonia, NH3, using VSEPR.
- Worked exampleFind the molecular geometry of SF4, which has four bonding pairs and one lone pair on sulfur.
- Common mistakeWhy doesn't water's bond angle come out to exactly 109.5 degrees even though it has four electron domains?
- Ties togetherHow does the shrinking bond angle from methane to ammonia to water connect back to lone pair repulsion strength?
Your turn1 of 2
Water has four electron domains around its oxygen atom, the same number methane has around its carbon -- yet water's bond angle is noticeably less than methane's 109.5 degrees. Why doesn't water's bond angle come out to exactly 109.5 degrees?7.6 Molecular Polarity from Bond Dipoles and Geometry
- How it worksHow do you add up individual bond dipoles as vectors to decide whether an entire molecule ends up polar?
- Common mistakeWhy isn't a molecule automatically polar just because it contains polar bonds?
- Worked exampleFind whether CCl4 and CHCl3 are polar or nonpolar by combining their bond dipoles with their molecular geometry.
- Going furtherWhat happens to a molecule's overall polarity when one hydrogen in a symmetric molecule like CH4 is replaced by a more electronegative atom?
Your turn1 of 2
Each carbon-oxygen bond in carbon dioxide is quite polar, yet the overall CO2 molecule is nonpolar. Why?