Lesson 7Lesson 7

Chemical BondingChemical Bonding

Anything in black letters = write it in your notes (‘knowts’)

Review…Review…Elements in the same column have similar properties because…

they have the same number of valence electrons

electrons in the highest energy level

variable

Lewis Symbol – show valence electrons as dots around chemical symbol

C Na O Br

Section 1 – Ionic & Covalent Bonding

Lewis Symbols of Some Group A Elements

PeriodGroup

1A 2A 3A 4A 5A 6A 7A 8A

1

2

3

4

Octet Rule –

Atoms will gain or lose electrons to have 8 valence electrons.

metals lose e-; to form ca+ions

nonmetals tend to gain e-; to form anions

Normal Ion Charges

Ionic bonds are formed by electron transfer.

(between metal & nonmetal)

Covalent bonds are formed by electron sharing.

(between 2 nonmetals)

The more electronegative element acquires most of the e- charge

Ionic compounds consist of cations and anions arranged in repeating patterns;

NOT as molecules

A formula unit is the lowest ratio of ions in an ionic compound.

O2H2

double bondsingle bond

Each atom shares e- with another to complete the octet.

Covalent Bond

IONIC COVALENT

transfer of e- (ions!) sharing of e- (COvalent)

formula units molecules

metal & nonmetal 2 nonmetals

high melting points low melting points

(most are gases or liquids at room temp)

conduct e- when molten or dissolved (aqueous)

(solid crystals at room temp)

COMPOUNDS

Collection of water molecules

Molecule of water

Chemical formula H2O

Array of sodium ions and

chloride ions

Formula unit ofsodium chloride

Chemical formulaNaCl

FORMULA UNITS vs. MOLECULES

Covalent bonding involves a sharing of electrons between atoms but this sharing may not be equal…

Section 2 – Polar Covalent Bonding

First, a review of electronegativity….

Noble gases do not have e-neg values

Electronegativity Review…

B<H<C

A nonpolar covalent bond occurs between 2 identical atoms.

Since each atom in a nonpolar bond has the same electronegativity, the electrons are shared equally.

7 nonpolar (diatomic) molecules (H2, N2, O2, F2, Cl2, Br2, I2)

Polar Covalent Bond (polar bond) – covalent bond in which the electrons are shared unequally.

The more electronegative atom attracts electrons more and has a slightly negative charge (δ-)

δ-δ+

partial positive charge

partial negative charge

partial positive charge

partial negative charge

H Clδ-δ+

H Cl

A bond or molecule with +/- charged ends is also called a dipole. “die-pull”

The polar nature of a bond can also be shown by an arrow pointing to the more electronegative atom.

H—ClIdentify the bonds between these elements as ionic, polar covalent or nonpolar covalent.

a)H – Br b) K – Cl

c)C – O d) Li – O

e)Cl – F f) Br – Br

g)H – O h) H – Br

Place a δ- symbol above the more electronegative atom in the bond.

If the polar bonds in a molecule cancel out, the molecule is nonpolar.

When the polar bonds do NOT cancel out, the molecule is polar.

We will not cover molecular geometries and shapes except for that of water…

unshared pairs of e-

H2O

(lone or nonbonding pair)

A molecule of water has a bent shape due to the space needed by the lone pairs of e-

So far, we have talked about bonding between atoms (ionic, covalent)

Now, we will talk about bonding between molecules

The bonds between separate molecules are much weaker than ionic or covalent bonds, but without these forces there would be no liquids or solids.

Section 3 – Bonding Between Molecules

Forces between separate molecules are called Intermolecular Forces (or Van der Waals Forces)

2 types of intermolecular forces that we will discuss,

Hydrogen Bonding

Dispersion Forces (London Forces)

Hydrogen Bond – intermolecular force between H in one molecule and an electronegative atom (N, O or F) in another nearby molecule.

Hydrogen Bonds in H2O

Opposites partial poles attract

hydrogen bond

Dispersion Forces – temporary attractive forces between molecules due to electron dispersion (motion)

F2 melts at 53 K, Boils at 85 K

Without dispersion forces, nonpolar molecules could never be liquids or solids.

Molecule Melt Pt Boil Pt

F2 53 K 85 K

Cl2 171.6 K 239.1 K

Br2 265.8 K 332.0 K

I2 386.8 K 457.4 K

Dispersion Force

Cohesion – attraction to same substance.

Adhesion – attraction to different substance

butane (C4H10) 58 amu

acetone (C3H6O) 58 amu

b.p. = -0.5°C gas @ room temp

b.p. = 56.1°C liquid @ room temp

A tale of two molecules…

1. Why does acetone have a higher boiling point?

2. What would cause butane molecules to stick to each other to become a liquid

Why does acetone have a higher boiling point?

the opposite partial charges on each molecule hold the molecules together.

δ+

δ-

δ+

etc…

etc…etc…

etc… butane is a nonpolar molecule and does not have this type of intermolecular attraction

2. What would cause butane molecules to stick to each other to become a liquid?

butane, like all molecules, has electrons that are randomly moving. This produces temporary poles within the molecule.

The valence electrons in metals are loosely held and are free to move.

The properties of metals can be explained by the ‘sea of electrons’ model.

Section 4 – Metallic Bonding

Alloy – mixture of metals.Steel –

Stainless Steel –

Bronze –

Solder –

Brass –

Sterling Silver –

Amalgam –

Nichrome –

Alloy – mixture of metals.Steel – Fe & C

Stainless Steel – steel w/ Cr, Ni, or Mn

Bronze – Cu & Sn

Solder – Sn & Pb

Brass – Cu & Zn

Sterling Silver – Ag & usually Cu

Amalgam – Hg w/ other metals (Ag, Sn, Cu)

Nichrome – Ni & Cr