Chemical Bonding

Ionic, Metallic, and Covalent

Chemical Bonding

A chemical bond is a force that holds two or more atoms together.

What is a chemical bond?

The ability of an atom to attract electrons in a chemical bondA table of electronegativities appears on p. 263 of the textbook.

Differences in electronegativity cause molecules to have different types of bonds

Electronegativity

Polarity occurs when the distribution of electron is NOT uniform throughout a molecule.

The electrons spend more time around one nucleus than the other

Polarity

Nonpolar covalent bond – electrons are shared equally

Polar covalent bond – electrons are shared unequally

Ionic bond – electrons are completely transferred and ions are held together by electrical forces

Types of Bonds

How to determine bond type1. Look up the electronegativities (p. 263)2. Find the difference between the

electronegativities.3. Use the table to classify the bond type.

Electronegativity Difference Bond Type

0 Nonpolar covalent

0.1 – 1.7 Polar covalent

> 1.7 Ionic

Note: There is some disagreement on the “cut-off” number. Some sources use 2.0 instead of 1.7.

Ion – an atom that has an electrical charge due to the gain or loss of electrons

Monatomic – contains one (1) atomPolyatomic – contains 2 or more atoms

Oxidation number – the charge on a monatomic atom

Vocabulary - Ionic Bonds

Positive ions form when electrons are lost

Groups 1A, 2A, 3A, and the transition metals form positive ions.

Oxidation numbers: Group 1A is 1+ Group 2A is 2+ Group 3A is 3+

Positive Ions

Negative ions form when electrons are gained.

Groups 5A, 6A, and 7A form negative ions.

Oxidation numbers: Group 5A is 3- Group 6A is 2- Group 7A is 1-

Negative Ions

Ionic Bond – The electrostatic force that holds oppositely charged particles together.

Ionic Compound – a chemical compound formed by an ionic bond.

Salt – a name for an ionic compound.

More vocabulary

Properties of Salts

1. very hard –

2. high melting points –

3. brittle –

each ion is bondedto several oppositely-charged ions

many bonds must be broken

with sufficient force,like atoms arebrought next toeach other and repel

calcite

1st ion is the positive ionUse the element name

2nd ion is the negative ionTake the element name, but

change the ending to –ide.Examples:

chlorine becomes ____________sulfur becomes _____________

Naming Binary Ionic Compounds

NaF

MgCl2

Al2O3

Naming Examples

1. Write the symbols for the ions. Include the oxidation numbers (charges).

2. Criss-cross the charges. The charge of one ion will become the subscript on the opposite ion.

3. Reduce the subscripts to a lowest-terms ratio.

Note: the formula for an ionic compound gives the number of atoms in one formula unit.

Writing the Formula for a Binary Ionic Compound

aluminum chloride

magnesium oxide

Formula Examples

Polyatomic ions are charged particles that have 2 or more atoms covalently bonded.

A covalent bond is a chemical bond formed by the sharing of electrons.

Polyatomic ions should be treated as a single unit.

Example: SO42- is the sulfate

ion.

Polyatomic Ions

Cation – positive ionAnion – negative ionThe cation:

If it is a polyatomic ion, use the name of the ion as the first word.

If it is a monatomic ion, use the element name.

Naming Ionic Compounds containing Polyatomic Ions

The anion:If polyatomic, use the ion name as the second

word.If monatomic, use the element name modified

with the ending –ide.Examples:

NH4ClCaSO4

Use the criss-cross technique that is used for the binary compounds.

If the polyatomic ion needs a subscript, then place parentheses around the polyatomic ion. The subscript goes outside of the parentheses.

Writing Formulas for Ionic Compounds with Polyatomic Ions

sodium sulfate

ammonium phosphate

Iron (III) nitrate

Examples

In insulators (like wood),the v.e– are attachedto particular atoms.

Metallic Bonds

In metals, valence shells of atoms overlap, so v.e– are free to travel between atoms through material.

Not so in metals.

All due to free-moving

v.e–.

Properties of Metals

ductile conduct heat and electricity

malleable

A form of chemical bonding where electrons are shared so that each atom has a complete valence shell

One particle of a covalent compound is called a molecule.

Covalent Bonding

Single bond – 2 electrons (1 pair) are shared.Double bond – 4 electrons (2 pairs) are

shared.Triple bond – 6 electrons (3 pairs) are shared.

Types of Covalent Bonds

A single bond is a sigma bond.Double and triple bonds contain pi bonds.Double = 1 sigma, 1 piTriple = 1 sigma, 2 pi

Other Names for Bond Types

Fluorine has seven valence electronsA second F atom also has sevenBy sharing electrons both end up with full

orbitals (stable octets)

Covalent bonding

F F8 Valence electrons

8 Valence electrons

Covalent Bonding

-

-

-

-

n = 1

O[He]2s22p4

n = 2

+

-

----

-

-

O2

Sharing of electrons to achieve a stable octet (8 electrons in valence shell).

--

-

-

-

-

-

-

--

-

-

-

-

-

-

-

O[He]2s22p4

Lewis StructuresLewis structure:

a model of a covalent molecule thatshows all of the valence electrons

1. Two shared electrons make a single covalent bond,

four make a double bond, etc.

2. unshared pairs: pairs of un-bonded valence electrons

3. Each atom needs a full outer shell, i.e., 8 electrons. Exception: H needs 2 electrons

Lewis Structures

1) Count up the total number of valence electrons.2) Connect all atoms with single bonds. - “multiple” atoms usually on outside - “single” atoms usually in center;

C always in the center,H always on the outside.

Gilbert Lewis

3) Complete octets on exterior atoms(not H, though)

4) Check - valence electrons match with Step 1 - all atoms (except H) have an octet;

if not, try multiple bonds - any extra electrons?

Put on central atom

- no unpaired electrons

Example Lewis Diagram: CF4

Example Lewis Diagram: CH4

Example Lewis Diagram: PF3

Identification = Use this system with nonmetal-nonmetal pairs

If the compound contains a metal, it is likely an ionic compound – use the other set of rules for those compounds.

Naming RuleFirst word = prefix + 1st element nameSecond word = prefix + 2nd element name

with –ide ending

Naming Binary Covalent Compounds

Prefixes

1 mono-2 di-3 tri-4 tetra-5 penta-

6 hexa-7 hepta-8 octa-9 nona-10 deca-

Exception: Use mono on the second word only, never the first word.

N2O

SiF4

N2O5

NH3

Examples – Writing Names

sulfur trioxide

dihydrogen monoxide

Examples – Writing Formulas

Valence Shell Electron Pair Repulsion

This is a model to determine the shape of a molecule.

The model is based on an arrangement (of the atoms) that minimizes the repulsion of shared and unshared pairs.

VSEPR

Shared pair = two electrons that are involved in a covalent bond.

Lone pair = two electrons that are not involved in any bond. These are sometimes called unshared pairs.

Bond angle = the angle formed by the two terminal atoms (on the end) and the central atom.

Terms-VSEPR

Bonding and Shape of Molecules

Number of Bonds

Number of Unshared Pairs Shape Examples

2

3

4

3

2

0

0

0

1

2

Linear

Trigonal planar

Tetrahedral

Pyramidal

Bent

BeCl2

BF3

CH4, SiCl4

NH3, PCl3

H2O, H2S, SCl2

-Be-

B

C

N

:

O

:

:

CovalentStructure

......

The VSEPR Model

O OC

Linear

The Shapes of Some Simple ABn Molecules

O OS

BentO O

S

O

Trigonalplanar

FF

F

N

Trigonalpyramidal

F

F

FP

F

F

Trigonalbipyramidal

Octahedral

FF

F

S

F

F

F

AB6

SO2

Brown, LeMay, Bursten, Chemistry The Central Science, 2000, page 305

Tetrahedral