Chemical Bond: the Force That Holds Two Atoms Together

Chemistry - Mendoza

Ch 9 Covalent Bonding

9.1 The Covalent Bond

Vocabulary

Review

chemical bond: the force that holds two atoms together

New

covalent bond pi bond

molecule endothermic reaction

Lewis structure exothermic reaction

sigma bond

Main Idea- Atoms gain stability when they share electrons and form covalent bonds.

Why do atoms bond?

•  Lower energy states make an atom more stable.

o  ______electrons makes atoms more stable by forming ions with noble-gas electron configurations. (ionic bonds)

o  ______with other atoms also results in noble-gas electron configurations.

•  Atoms in ______compounds ______electrons.

•  The chemical bond that results from sharing electrons is a ______.

•  A ______ is formed when two or more atoms bond covalently.

•  ______(H2, F2 for example) exist because two-atom molecules are more stable than single atoms.

•  The most stable arrangement of atoms

exists at the point of maximum net attraction,

where the atoms bond covalently and form a molecule.

Single Covalent Bonds

•  When only ______of electrons is shared, the result is a ______.

•  The figure shows two hydrogen atoms forming a hydrogen molecule with a single covalent bond, resulting in an electron configuration like helium.

Single Covalent Bonds (cont)

•  In a ______ dots or a line are used to symbolize a single covalent bond.

•  The ______—the group 17 elements—have 7 valence electrons and ______with atoms of other non-metals.

•  Atoms in group 16 can ______electrons and form ______.

•  Water is formed from one oxygen with two hydrogen atoms covalently bonded to it .

•  Atoms in group 15 form ______, such as in ammonia.

•  Atoms of group 14 elements ______, such as in methane.

•  ______ are single covalent bonds.

•  Sigma bonds occur when the pair of shared electrons is in an area centered between the two atoms.

•  Practice Problems – Lewis Structures

1. PH3

1. CCl4

1. SiH4

Multiples Covalent Bonds

•  ______form when ______of electrons are ______between two atoms.

•  ______form when ______of electrons are ______between two atoms.

•  A ______consists

of one sigma bond and at least one pi bond.

§  The ______ is formed when

parallel orbitals overlap and share electrons.

Practice - Predicting Bonds

•  Draw the electron dot diagrams for the elements sulfur, carbon, bromine, oxygen, and hydrogen. Using Lewis structures, predict the number of covalent bonds formed when

1. one atom of sulfur bonds with two atoms of hydrogen

b. one atom of carbon bonds with two atoms of sulfur

c. two atoms of bromine bond with one atom of sulfur

d.  one atom of carbon bonds with four atoms of bromine

e. one atom of sulfur bonds with two atoms of oxygen

The Strength of Covalent Bonds

•  The strength depends on the distance between the two nuclei, or bond length.

•  As length increases, strength decreases.

•  The amount of energy required to break a bond is called the bond dissociation energy.

•  The shorter the bond length, the greater the energy required to break it.

•  An ______ is one where a greater amount of energy is required to break a bond in reactants than is released when the new bonds form in the products.

•  An ______ is one where more energy is released than is required to break the bonds in the initial reactants.

Ionic Vs Covalent Summary

Ionic Bonds

metal + nonmetal

lose e- gain e-

cation anion

Ionic compound properties

•  Most are crystalline solids

•  Held together by electrostatic force (attraction between + and – ions)

•  Conduct electric current when dissolved

•  High B.P. and M.P.

•  Brittle

•  Generally Soluble in water

Covalent Bonds

•  Atoms share electrons

•  Nonmetal + nonmetal

•  Covalent compound properties

•  Low boiling point

•  Solubility in water varies (some high- some low)

•  Do not conduct electricity

•  Variety of states

•  Held together by overlap of orbitals

Bonding - Compare and Contrast (see handout)

9.2 Naming Molecules (Covalent Nomenclature)

Vocabulary

Review

•  Ionic Compound

•  Covalent Compound

•  Acidic Aqueous Solution

•  Oxyanion

New

•  Oxyacid

Main Idea - Specific rules are used when naming binary molecular compounds, binary acids, and oxyacids.

Naming Binary Molecular (Covalent) Compounds

•  The name of a molecular compound reveals its composition and is important in communicating the nature of the compound.

•  You must ______, if it isn’t you would use the ionic nomenclature rules

______

No charges involved – Use prefixes

NAMING:

•  Name the 1st element

•  Name the 2nd element using the root and suffix –ide

•  Prefix used to tell how many of each element.

•  ______- is ______on the ______ element.

•  Prefixes For Numbers of Atoms

1. ______
2. ______
3. ______
4. ______
5. ______
6. ______
7. ______
8. ______
9. ______
10. ______

Examples

•  N2O ______

•  SO2 ______

•  CO ______

WRITING:

1)  Write the correct symbol for the element names.

2)  Use prefixes for your subscripts.

3)  DO NOT crisscross numbers.

Examples

•  Triphosphorus hexafluoride ______

•  Nitrogen pentachloride ______

•  Diphosphorus monoxide ______

Common Names

•  Many compounds were discovered and given common names long before the present naming system was developed

Examples

•  water H2O – ______

•  ammonia NH3 – ______

•  hydrazine N2H4 – ______

•  nitrous oxide (laughing gas) N2O – ______

•  nitric oxide NO – ______

Naming Acids
(This should really be in Ch 9 – Ionic Compounds)

•  An ______ is an acid that contains both a hydrogen atom and an oxyanion.

•  Identify the oxyanion present.

•  The first word is the root of the oxyanion and the prefix per- or hypo- if it is part of the name, plus the suffix -ic if the anion ends in -ate or -ous if the oxyanion ends in -ite.

•  The second word is always acid.

•  An acid, whether a binary acid or an oxyacid,

can have a common name

in addition to its compound name.

9

9.3 Molecular Structures

Vocabulary

Review

•  ionic bond: the electrostatic force that holds oppositely charged particles together in an ionic compound

New

structural formula

resonance

coordinate covalent bond

Main Idea - Structural formulas show the relative positions of atoms within a molecule.

Structural Formulas

•  A ______ uses letter symbols and bonds to show ______of atoms.

•  Drawing Lewis Structures

1.  Predict the location of certain atoms.

2.  Determine the number of electrons available for bonding.

3.  Determine the number of bonding pairs.

4.  Place the bonding pairs.

5.  Determine the number of bonding pairs remaining.

6.  Determine whether the central atom satisfies the octet rule.

Single Bond Example

Draw the Lewis Structure for ammonia (NH3)

Multiple Bond Example

Draw the Lewis Structure for carbon dioxide (CO2)

Polyatomic Ion Example

Draw the Lewis structure for the phosphate ion (PO43-).

v  Atoms within a polyatomic ion are covalently bonded.

Resonance Structures

•  ______ is a condition that occurs when more than one valid Lewis structure can be written for a molecule or ion.

•  This figure shows three correct ways

to draw the structure for (NO3)1.

•  Two or more correct Lewis structures that represent a single ion or molecule are ______.

•  The molecule behaves as though it has only one structure.

•  The bond lengths are identical to
each other and intermediate
between single and double
covalent bonds.

Exceptions to the Octet Rule

•  Some molecules do not obey the octet rule, but we will not go into the details now.

9.4 Molecular Shapes

•  Just FYI, molecules have different shapes, here are some examples.

9.5 Electronegativity and Polarity

Bond Polarity

When two different atoms bond covalently, there is an ______sharing

•  the more ______atom will have a stronger attraction, and will acquire a ______negative charge

•  called a ______ covalent bond, or simply polar bond.

Table of Electronegativities

.

·  Polar covalent bonds form when atoms pull on electrons in a molecule unequally.

·  Electrons spend more time around one atom than another resulting in ______at the ends of the bond

·  Consider HCl (Refer to Table )

o  Only partial charges, much less than a true 1+ or 1- as in ionic bond

o  Written as:

H Cl

δ means “______”

δ+ - partial positive

δ- - partial negative

o  Can also be shown:

2.1 3.0

o  the arrow points to the more electronegative atom.

o  Difference of 0.9 so……

Polar molecules

·  The effect of polar bonds on the ______of the entire molecule depends on the molecule______.

•  carbon dioxide has two polar bonds,

and is ______= ______molecule!

•  Compare water and CCl4.

•  Both bonds are polar, but only water is a polar molecule because of the shape of the molecule.

Bond Character

•  Unequal sharing of electrons results in a polar covalent bond.

•  Bonding is often ______.

•  This graph summarizes the

range of chemical bonds

between two atoms.

Importance of Polar Covalent Bonds

•  ______is the property of a substance’s ability to dissolve in another substance.

o  Polar molecules and ionic solutes are usually soluble in ______solvents.

o  Non-polar molecules dissolve only in ______solvents.

Molecule Polarity Summary

•  A molecule is polar if….

•  Different elements surround the central atom

•  Lone pairs on the central atom

•  A molecule is non-polar if…

•  Same elements surround the central atom

•  No lone pairs on the central atom

•  Symmetrical