PowerPoint Lectures to accompany

Physical Science, 10e

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Chapter 9Chemical Bonds

An Introduction to Chemistry

Core Concept

Electron structure will explain how and why atoms join together in certain numbers.

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

Electronegativities

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)

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