AP Chemistry Unit 2 Unit 2 - Molecular and Ionic Compound Structure and Properties 2.1 Types of Chemical Bonds 2.2 Intramolecular Force and Potential Energy 2.3 Structure of Ionic Solids 2.4 Structure of Metals and Alloys 2.5 Lewis Diagrams 2.6 Resonance and Formal Charge 2.7 VSEPR and Bond Hybridisation ISPS Chemistry Aug 2020 page 1 Molecular and Ionic Compound Structure and Properties AP Chemistry Unit 2 This logo shows it is a Topic Question - it should only require knowledge included in this Topic and it should be giving practice in the Science Practice associated with this Topic. ISPS Chemistry Aug 2020 page 2 Molecular and Ionic Compound Structure and Properties AP Chemistry Unit 2 2.1 Types of Chemical Bonds ISPS Chemistry Aug 2020 page 3 Molecular and Ionic Compound Structure and Properties AP Chemistry Unit 2 Electronegativity Values The key to understanding why different types of bonds exist is an understanding of the importance of electronegativity - the ability to attract shared electrons. In essence, our 4 different labels for bonding (metallic, ionic, polar covalent & pure covalent) are really just different ways in which electrons can be shared. ISPS Chemistry Aug 2020 page 4 Molecular and Ionic Compound Structure and Properties AP Chemistry Unit 2 4 Main Types of Bonding All bonding is based on the attractions (and repulsions) that exist between charged particles. The strength of bonding is governed by Coulombs Law: F q1 q2 r2 ∝ Metallic Bonding - minimal (but equal) sharing - atoms with similar low electronegativities (metals) - leads to delocalised electrons - Fattraction results from multiple attractions between positive cores (temporary cations) and delocalised electrons Pure Covalent Bonding - strong equal sharing - atoms with very similar high electronegativities (non-metals) - electrons localised midway between both atoms - Fattraction results from mutual attractions between two positive nuclei and the equally shared pair of electrons Polar Covalent Bonding - unequal sharing - atoms with different high electronegativities (non-metals) - electrons localised closer to atom of higher electronegativity - Fattraction results from uneven attractions between two positive nuclei and the unequally shared pair of electrons Ionic Bonding - very unequal sharing - atoms with very different electronegativities (metal/non-metal) - electrons transferred to atom of higher electronegativity - Fattraction results from multiple attractions between positive cations and negative anions Metallic bond: electrons shared equally but delocalised The crossover between highly polar cvalent and ionic can be diffcult: if non-metal—non-metal then best to assume highy polar covalent and if metal—non-metal then asssume ionic. ISPS Chemistry Aug 2020 page 5 Molecular and Ionic Compound Structure and Properties AP Chemistry Unit 2 Properties Related to Bonding The properties of a substance will help determine the type of bonding but care is needed as different structures (e.g. molecular or network) and the presence of other attractions (particlarly between molecules) will also have a mojor effect on properties. In general, Metallic Bonding - delocalised electrons mean they are good conductors of both heat and electricity and can reflect light (shiny). - multiple attractions mean that they often have high MPts & high BPts - most are solids at room temperature - delocalised electrons allow bonds to break and reform easily so metals are ductile and malleable Covalent Bonding - localised electrons mean they are poor conductors of both heat and electricity and tend to absorb light (matt) - multiple attractions in networks result in very high MPts & BPts - solids at room temperature - localised attractions in molecules result in low MPts & BPts - solids, liquids or gases at room temperature - localised electrons make it difficult for bonds to reform easily so tend to be soft or brittle Ionic Bonding - localised electrons mean they are poor conductors of both heat and electricity when solid - but when ions are free to move (when molten or in solution), then they become good conductors - multiple attractions in networks result in very high MPts & BPts - solids at room temperature - directional nature make it difficult for ionic bonds to reform easily so tend to be brittle During Unit 3, you will take a more detailed look at the connection between properties and types of bonding - in particular, the wide variety of properies found in covalent substances due to the existence of attractions between molecules (intermolecular) which can be more important than the attractions found within molecules (intramolecular). ISPS Chemistry Aug 2020 page 6 Molecular and Ionic Compound Structure and Properties AP Chemistry Unit 2 2.1 Practice Problems 1. On the basis of the information opposite, which of the following arranges the binary compounds in order of increasing bond polarity? A CH < SiCl < SF B CH < SF < SiCl O 4 4 4 4 4 4 C SF4 < CH4 < SiCl4 D SiCl4 < SF4 < CH4 2. Which of the following has the bonds arranged in order of decreasing polarity? OA H−F > N−F > F−F B H−I > H−Br > H−F C O−N > O−S > O−Te D Sb−I > Sb−Te > Sb−Cl 3. Which of the following scientific claims about the bond in the molecular compound is most likely to be true? A There is a partial negative charge on the H atom. B Electrons are shared equally between the and atoms. C The bond is extremely weak OD The bond is highly polar 4. Which of the following compounds contains both ionic and covalent bonds? A SO B C H OH C MgF D H S E NH Cl 3 2 5 2 2 O 4 5. Two pure elements react to form a compound. One element is an alkali metal, X, and the other element is a halogen, Z. Which of the following is the most valid scientific claim that can be made about the compound? A It has the formula XZ2. B It does not dissolve in water. OC It contains ionic bonds. D It contains covalent bonds. ISPS Chemistry Aug 2020 page 7 Molecular and Ionic Compound Structure and Properties AP Chemistry Unit 2 6. The elements C and Se have the same electronegativity value, 2.55. Which of the following claims about the compound that forms from C and Se is most likely to be true? A The carbon-to-selenium bond is unstable. OB The carbon-to-selenium bond is nonpolar covalent. C The compound has the empirical formula CSe . D A molecule of the compound will have a partial negative charge on the carbon atom. 7. Of the following compounds, which is the most ionic? A SiCl B BrCl C PCl D Cl O E CaCl 4 3 2 O 2 8. Of the following single bonds, which is the LEAST polar? A N—H B H—F OC O—F D I—F E O—H ISPS Chemistry Aug 2020 page 8 Molecular and Ionic Compound Structure and Properties AP Chemistry Unit 2 2.2 Intramolecular Force & Potential Energy ISPS Chemistry Aug 2020 page 9 Molecular and Ionic Compound Structure and Properties AP Chemistry Unit 2 Covalent Bonding & Potential Energy All bonding is based on the attractions (and repulsions) that exist between charged particles. The strength of bonding is governed by Coulombs Law: F q1 q2 r2 ∝ For simple covalent molecules such as H2, the balance between attractive and repulsive forces is at an optimum when the internuculear distance, r, is 74 pm (74 x 10-12 m). This is equivalent to:- covalent radius = 37 pm H—H bond length = 74 pm The stronger the Force of attraction, F , the more energy will be released (exothermic) when the H—H bond forms (or more energy will be needed (endothermic) to break the H—H bond). This is called Bond Energy and, for H2 : Bond Energy = 432 kJ mol-1 For atoms that have similar values of q1 and q2 , the size of the atoms will be the main factor that determines the Force of attraction, F, and hence the bond energy. The Halogens in Group 17 all have the same effective nuclear charge of 7+ due to the screening effect of their inner shells of electrons. Their outer shell each contain 7 electrons, 7- Cl2 - 3 shells, covalent radius = 100 pm - bond length = 199 pm ( r = 199) -1 - bond energy = 243 kJ mol Br2 - 4 shells, covalent radius = 114 pm - bond length = 228 pm ( r = 228) -1 - bond energy = 193 kJ mol I2 - 5 shells, covalent radius = 133 pm - bond length = 266 pm ( r = 266) - bond energy = 151 kJ mol-1 F ⬃ as r ⬀ ISPS Chemistry Aug 2020 page 10 Molecular and Ionic Compound Structure and Properties AP Chemistry Unit 2 The Hydrogenhalides will also have similar values of q1 and q2 . All the halogens have the same effective nuclear charge of 7+ due to the screening effect of their inner shells of electrons. Each hydrogen has an effective charge of 1+. F ⬃ as r ⬀ Whilst it could be tempting to argue the other way round - that increasing attraction results in a decrease in bond length - Colombs Law only provides us with the ability to describe the Force of attraction in terms of charge and distance. Similarly with our next example - multiple Bonds. Carbon has the ability to form single bonds, C—C in which one pair of electrons are shared. For example, ethane Carbon can also form double bonds, C = C in which two pairs of electrons are shared. For example, ethene Carbon can also form triple bonds, C C in which three pairs of electrons are shared. ≡ For example, ethyne Again, the shorter the bond the higher the bond energy. F ⬃ as r ⬀ ISPS Chemistry Aug 2020 page 11 Molecular and Ionic Compound Structure and Properties AP Chemistry Unit 2 Ionic Bonding & Potential Energy All bonding is based on the attractions (and repulsions) that exist between charged particles.