Post on 22-Jul-2015
PowerPoint Lectures to accompany
Physical Science, 10e
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Chapter 9Chemical Bonds
An Introduction to Chemistry
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
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
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)
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
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
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
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
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
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
Multiple Bonds
• Sharing of more than one electron pair
• Examples– Ethylene - double
bond
– Acetylene - triple bond
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
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
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
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
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)