Ch09 lecture
- 1. PowerPoint Lectures to accompany Physical Science, 10e Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 9 Chemical Bonds An Introduction to Chemistry
- 2. Core Concept Electron structure will explain how and why atoms join together in certain numbers.
- 3. Compounds and Chemical Change • Atom - smallest elemental unit • Molecule – smallest particle still retaining the characteristic chemical properties of a substance – Examples: • oxygen, hydrogen gas - diatomic molecules • Ozone - triatomic oxygen molecule • Noble gases: helium, neon - “monatomic” molecules
- 4. Chemical Reactions • Formation and/or breaking of chemical bonds to form new molecules (products) from old ones (reactants) • Chemical energy - internal bonding potential energy • Chemical equation - symbolic summary of chemical reaction
- 5. Valence Electrons and Ions • Outer electrons determining the chemical properties of an atom • Octet rule – Atoms attempt to acquire an outer shell of eight electrons – Electrons can be gained/lost/shared in the process • Example: sodium (Na)
- 6. Chemical Bonds • Attractive forces holding atoms together in compounds • Can be described in terms of molecular (delocalized) or atomic (localized) orbitals Three types: • Ionic – Electrons transferred between atoms – Electrostatic force = binding force • Covalent – Octets achieved through sharing electrons – Typically between nonmetallic elements, r.h.s of periodic table • Metallic bonds – Outer electrons move freely throughout metal – “Electron gas” within rigid lattice of metal atoms – Conduct heat and electricity well
- 7. Ionic Bonds • Chemical bond of electrostatic attraction • Form crystalline solids with orderly geometric structure • Example: NaCl • Na loses; Cl gains • No single NaCl molecule, per se
- 8. Energy and Electrons in Ionic Bonding • Reaction energy released = heat of formation • Divided conceptually into half-reactions Electron transfer rules • Electrons lost/gained to form closed octets • Number gained = number lost
- 9. Ionic Compounds and Formulas Formulas • List elements in compound and their proportions • Proportions decided by electron gain/loss Ionic compounds • Characterized by ionic bonds • White, crystalline solids soluble in water • Families IA and IIA lose electrons and form positive ions • Families VIA and VIIA gain electrons to form negative ions
- 10. Covalent Bonds • Chemical bonds formed by sharing pairs of electrons • Electrons shared to form octets, ideally • Overlap of shared electron clouds between nuclei yields net attraction • Atoms within covalent compounds are electrically neutral, or nearly so
- 11. Covalent Compounds and Formulas • Covalent compound - held together by covalent bonds • Electrons shared in covalent bonds • Electron dot representation – Bonding pairs shared – Lone (non-bonding) pairs not shared
- 12. Multiple Bonds • Sharing of more than one electron pair • Examples – Ethylene - double bond – Acetylene - triple bond
- 13. Bond Polarity • Result of unequal sharing of electrons • Electronegativity – Measure of an atom’s ability to attract electrons – Differences: • 1.7 or greater - ionic • 0.5-1.7 - polar covalent • Less than 0.5 - covalent
- 15. Composition of Compounds • Millions of different combinations of over 90 elements • Common names – Often related to historical usage (baking soda, washing soda,…) – Difficult to relate to actual molecular composition • Modern approach - systematic sets of rules – Different for ionic and covalent compounds – One common rule - “-ide” means compound contains only two different elements
- 16. Ionic Compound Names • Name of metal (positive) ion first; then nonmetal (negative) ion • Many elements have variable charges • Historical suffix usage – “-ic” for higher of two; – “-ous” for lower • Modern approach – English name of metal followed by Roman numeral indicating charge
- 17. Ionic Compound Formulas • Two rules – Write symbol for positive ion first followed by negative ion symbol – Assign subscripts to assure compound is electrically neutral • Example: Calcium chloride
- 18. Covalent Compound Names • Molecular - composed of two or more nonmetals • Same elements can combine to form a number of different compounds Two rules • First element in formula named first with number indicated by Greek prefix • Stem name of second element next; Greek prefix for number; ending in “-ide” (for two elements)
- 19. Covalent Compound Formulas • Examples: carbon dioxide, carbon tetrachloride • Valence – Number of covalent bonds an atom can form – Hydrogen valence = 1 – Oxygen = 2; single and double bonds – Nitrogen = 3; single, double and triple bonds – Carbon = 4 - single, double and triple bonds