6.2 Covalent Bonding6.2 Covalent Bonding

Key ConceptsKey Concepts

• How are atoms held together in a covalent How are atoms held together in a covalent bond?bond?

• What happens when atoms don’t share What happens when atoms don’t share electrons equally?electrons equally?

• What factors determine whether a What factors determine whether a molecule is polar?molecule is polar?

• How do attractions between polar How do attractions between polar molecules compare to attractions between molecules compare to attractions between nonpolar molecules?nonpolar molecules?

VocabularyVocabulary

• Covalent bondCovalent bond

• MoleculeMolecule

• Polar covalent bondPolar covalent bond

Covalent BondsCovalent Bonds

• When dealing with nonmetals, electrons When dealing with nonmetals, electrons typically don’t transfertypically don’t transfer

• Consider: a treasure hunt with a prizeConsider: a treasure hunt with a prize• Instead, nonmetal atoms tend to share their Instead, nonmetal atoms tend to share their

electrons to achieve stable electron electrons to achieve stable electron configurationsconfigurations

• A chemical bond in which two atoms share a A chemical bond in which two atoms share a pair of valence electrons is a pair of valence electrons is a covalent bondcovalent bond

• Atoms can share more than one pair of Atoms can share more than one pair of electronselectrons

Molecules of ElementsMolecules of Elements

• A A moleculemolecule is a group of atoms that are joined is a group of atoms that are joined together by one or more covalent bondstogether by one or more covalent bonds

• The attractions between the shared electrons and The attractions between the shared electrons and the protons in each nucleus hold the atoms the protons in each nucleus hold the atoms together in a covalent bond.together in a covalent bond.

• Chemical formulas can be used to describe the Chemical formulas can be used to describe the molecules of an element as well as a compound.molecules of an element as well as a compound.

• Many nonmetal elements exist as diatomic Many nonmetal elements exist as diatomic molecules.molecules.

• There are many different ways to represent There are many different ways to represent covalent bonds.covalent bonds.

Electron Dot and Structural Electron Dot and Structural FormulaFormula

http://www.wikinfo.org/upload/5/5d/Electron_dot.jpg

Space Filling / Electron Cloud Space Filling / Electron Cloud ModelsModels

Unequal Sharing of Unequal Sharing of ElectronsElectrons• Generally trends on the periodic table:Generally trends on the periodic table:

– elements towards the right of the periodic elements towards the right of the periodic table tend to have greater attractions for table tend to have greater attractions for electrons than elements on the leftelectrons than elements on the left

– elements at the top of a group have a greater elements at the top of a group have a greater attraction for electrons than elements at the attraction for electrons than elements at the bottom of a groupbottom of a group

• These trends typically cause covalently These trends typically cause covalently bonded atoms to share electrons bonded atoms to share electrons unequallyunequally

Polar Covalent BondsPolar Covalent Bonds

• In molecules of elements, both atoms attract In molecules of elements, both atoms attract electrons equallyelectrons equally

• In compounds, this isn’t necessarily the caseIn compounds, this isn’t necessarily the case• A covalent bond in which electrons are not shared A covalent bond in which electrons are not shared

equally is called a equally is called a polar covalent bondpolar covalent bond• Example: WaterExample: Water• When atoms form a polar covalent bond, the When atoms form a polar covalent bond, the

atom with the greater attraction for electrons has atom with the greater attraction for electrons has a partial negative charge. The other atom has a a partial negative charge. The other atom has a partial positive charge.partial positive charge.

• This is represented by either This is represented by either δδ+ or + or δδ--

Polar and Nonpolar Polar and Nonpolar MoleculesMolecules• If a molecule has only two atoms, it If a molecule has only two atoms, it

will be polarwill be polar• When atoms have more than two When atoms have more than two

atoms, they might not be polaratoms, they might not be polar• The type of atoms in a molecule and The type of atoms in a molecule and

its shape are factors that determine its shape are factors that determine whether a molecule is polar or whether a molecule is polar or nonpolar.nonpolar.

• Compare: Carbon Dioxide and WaterCompare: Carbon Dioxide and Water

Comparing COComparing CO22 and H and H22OO

Carbon Dioxide Water

Attraction Between Attraction Between MoleculesMolecules• In a molecular compound, there are forces In a molecular compound, there are forces

of attraction between moleculesof attraction between molecules• These attractions are not as strong as These attractions are not as strong as

ionic or covalent bonds, but strong enough ionic or covalent bonds, but strong enough to hold together molecules in a liquid or to hold together molecules in a liquid or solidsolid

• Attractions between polar molecules are Attractions between polar molecules are stronger than attractions between stronger than attractions between nonpolar moleculesnonpolar molecules

• Example: WaterExample: Water

Exit TicketExit Ticket

1)1) What attractions hold atoms together in What attractions hold atoms together in a covalent bond?a covalent bond?

2)2) What happens to the charge on atoms What happens to the charge on atoms when they form a polar covalent bond?when they form a polar covalent bond?

3)3) Name the two factors that determine Name the two factors that determine whether a molecule is polar.whether a molecule is polar.

4)4) Compare the strength of attraction Compare the strength of attraction between polar molecules to the strength between polar molecules to the strength of attraction between nonpolar of attraction between nonpolar molecules.molecules.