Ionic Model
An atom has the electron configuration 2.8.6. State the formula of the ion it forms.
What is S²⁻? (gains 2 electrons to complete its octet)
State what is meant by a covalent bond.
What is the electrostatic attraction between a shared pair of electrons and the positively charged nuclei of the two bonded atoms?
What does VSEPR stand for, and what is its main assumption?
What is Valence Shell Electron Pair Repulsion: electron domains around the central atom repel each other and arrange as far apart as possible?
Using electronegativity values (C 2.6, Cl 3.2), state which atom carries the partial negative charge in a C–Cl bond.
What is chlorine (δ−), because it is more electronegative; carbon is δ+?
Define a metallic bond.
What is the electrostatic attraction between a lattice of positive metal cations and delocalized electrons?
Give the formula of calcium nitrate.
What is Ca(NO₃)₂? (Ca²⁺ needs two NO₃⁻ ions)
Deduce the Lewis formula of CO₂. How many bonding pairs and how many lone pairs are in the whole molecule?
What is 4 bonding pairs (two C=O double bonds) and 4 lone pairs (2 on each O)?
State the molecular geometry and the bond angle of CH₄.
What is tetrahedral, 109.5°?
CO₂ contains polar C=O bonds, yet the molecule is non-polar. Explain.
What is: the molecule is linear and symmetrical, so the two bond dipoles are equal and opposite and cancel, giving no net dipole moment?
Explain why metals are malleable.
What is: layers of cations can slide over each other while the delocalized electrons keep attracting the cations, so the metallic bond is maintained and the metal does not fracture?
Solid NaCl does not conduct electricity, but molten NaCl does. Explain.
What is: in the solid the ions are fixed in the lattice so there are no mobile charge carriers; when molten the lattice breaks down and the ions are free to move and carry charge?
NH₄⁺ contains a coordination (dative) bond. Define this type of bond and say how it forms from NH₃ and H⁺.
What is a covalent bond in which both shared electrons come from the same atom: the lone pair on N in NH₃ is donated to H⁺, which has an empty orbital?
Deduce the molecular geometry and approximate bond angle of NH₃, and explain why the angle is less than 109.5°.
What is trigonal pyramidal, about 107°; the lone pair repels the bonding pairs more strongly than bonding pairs repel each other?
Name the strongest intermolecular force in each: (a) CH₄ (b) HCl (c) CH₃OH.
What is (a) London (dispersion) forces, (b) dipole–dipole forces, (c) hydrogen bonding?
Explain why magnesium (mp 650 °C) has a higher melting point than sodium (mp 98 °C).
What is: Mg²⁺ has a greater charge and a smaller radius than Na⁺, and Mg donates 2 delocalized electrons per atom, so the electrostatic attraction in the lattice is stronger?
Explain why sodium chloride has a high melting point and is not volatile.
What is: strong electrostatic attractions between oppositely charged ions act throughout the 3D lattice, so a large amount of energy is needed to separate the ions?
BF₃ does not obey the octet rule. How many electrons surround boron, and what type of bond forms when BF₃ reacts with NH₃?
What is 6 electrons (electron deficient, three bonding pairs); NH₃ donates its lone pair to B, forming a coordination bond?
Deduce the electron domain geometry and the molecular geometry of H₂O, and state its approximate bond angle.
What is tetrahedral electron domain geometry, bent (V-shaped) molecular geometry, about 104.5° (two lone pairs squeeze the angle)?
Explain why water (bp 100 °C) boils at a much higher temperature than H₂S (bp −60 °C).
What is: water molecules form hydrogen bonds (H bonded to highly electronegative O), which are stronger than the dipole–dipole and London forces in H₂S, so more energy is needed to separate the molecules?
Explain, in terms of structure and bonding, why graphite conducts electricity but diamond does not.
What is: in graphite each C bonds to 3 others in layers, leaving one delocalized electron per C that can move along the layers; in diamond each C forms 4 covalent bonds, so there are no delocalized electrons?
Explain, in terms of ions, (a) why an ionic crystal shatters when struck and (b) why NaCl dissolves in water.
What is: (a) a force shifts a layer so like charges line up and repel, so the crystal cleaves (brittle). (b) polar water molecules attract the ions (ion-dipole), overcoming the lattice and hydrating the ions?
Arrange C–C, C=C and C≡C in order of increasing bond length, and state and explain the trend in bond strength.
What is C≡C < C=C < C–C in length; strength increases C–C < C=C < C≡C because more shared electron pairs give a stronger attraction between the nuclei and the shared electrons, pulling the nuclei closer?
Deduce the molecular geometry and approximate bond angle around each carbon atom in ethanal, CH₃CHO.
What is tetrahedral (about 109.5°) around the CH₃ carbon, and trigonal planar (about 120°) around the C=O carbon: it has three electron domains, because a double bond counts as one domain?
Explain why ethanol, C₂H₅OH, is soluble in water but hexane, C₆H₁₄, is not.
What is: ethanol has an O–H group that forms hydrogen bonds with water; hexane is non-polar with only London forces, so it cannot form attractions with water strong enough to replace the hydrogen bonds between water molecules?
SiO₂ melts at about 1700 °C, but CO₂ is a gas at room temperature. Explain, in terms of structure and bonding.
What is: SiO₂ is a giant covalent network, so many strong covalent bonds must be broken to melt it; CO₂ is simple molecular, so only weak London forces between molecules are overcome?