Chemical Bonding

Introduction to Bonding

Neutral or “free” atoms are rarely found in nature Most elements exist as part of a compound Compounds are chemically bonded atoms

Chemical bond: strong attractive force that exists between atoms or ions in a compound.

So, why do atoms form bonds?

Atoms form bonds to become more stable.

Bonding involves only valence electrons

Remember… The noble gases are particularly stable because their outer shell is full of electrons (usually 8)

Octet Rule: Atoms tend to gain, lose, or share electrons until they are surrounded by eight valence electrons (noble gas configuration)

Types of Chemical Bonds Ionic Valence electrons are transferred from one atom to another. Usually formed between a metal and a non-metal.

This creates + ions and - ions which are then electrostatically attracted.

2. Covalent Pairs of electrons are shared between atoms. Usually formed between two non-metals.

Two Types of Covalent Bonds A. Polar Covalent Electrons are shared unequally. Usually between two different nonmetals.

B. Non-Polar Covalent Electrons are shared equally. Usually between atoms of the same element.

Metallic attraction between a metallic cation and delocalized electrons. Delocalized electrons – valence electrons not held by any specific atom, but free to move from one atom to another. “Electron Sea” Usually formed between metals.

Bond Properties formula unit (NaCl) molecule (CO2) ionic vs. covalent formula unit (NaCl) molecule (CO2) hard & brittle pliable soluble in H2O PC: soluble in H2O NPC: insoluble in H2O

Bond Properties ionic vs. covalent high melting & low melting & boiling points boiling points electrolytes non-electrolytes electron Lewis structures transfer diagrams

Formula unit ionic compounds are not found as single molecules. the simplest ratio of cations to anions Example: ZnCl2 represents the simplest combination of zinc and chlorine: one Zn 2+ ion and two Cl- ions

WATER Contains polar covalent bonds. Called the “universal solvent” “Like dissolves like”.

Properties of Metals Luster High density Good heat conductor Good electrical conductor High melting/boiling points Malleable & Ductile

Calculating Bond Type

Very few bonds are purely one type. The degree to which bonds are ionic or covalent can be estimated by comparing their electronegativities.

Remember: Electronegativity: the tendency of an atom to attract electrons to itself when chemically bonded to other atoms.

Electronegativity Remember: Increases as you move from left to right across PT Decreases as you move from top to bottom on periodic table

To Calculate Bond Type: Look up values on Table of Electronegativities. Subtract. Locate difference on Bond Type Chart.

Bond Type Chart % ionic character 100% 50% 0.5% 0.0% 4.0 1.7 0.3 0.0 ionic polar nonpolar covalent covalent

Examples Ca and Br

O and O

H and S

IONIC BONDS

Ionization: formation of an ion by the loss or gain of one or more valence electrons.

Cations: positive ions formed by the loss of one or more valence electrons. Metals tend to form cations.

2) Anions: negative ions formed by the gain of one or more valence electrons Non-metals tend to form anions.

Remember: Oxidation Numbers tell you how many valence electrons an atom will lose or gain to become stable.

We use Electron Transfer Diagrams to represent ionic bonds

To write Electron Transfer Diagrams: Use equation format: Left of arrow: show electron dot diagrams   Right of arrow: show ions formed and coefficients to give proper ion ratio

Stop and do the Examples

COVALENT BONDS

Covalent bonds are usually formed between non-metals Remember: Covalent bonds are usually formed between non-metals

We use Lewis Structures to represent covalent bonds

Understanding Lewis Structures Element symbols represent the nuclei and core electrons.

Element symbols represent nuclei and core electrons Dashes between symbols represent shared pairs of electrons Dot pairs around the outside of a symbol represent unshared electrons

Single bond = 1 dash Double bond = 2 dashes Triple bond = 3 dashes

Central atom is the least electronegative atom.

Multiple bonds are most common with Carbon Nitrogen Oxygen

To Write Lewis Structures Count the total number of valence electrons.

Choose central atom (least electronegative)

Connect remaining atoms to central atom with single dash.

Add remaining electrons around each atom until you have drawn proper number of electrons.

Polyatomic ions need to be enclosed in brackets. The charge should be written in the upper right outside the bracket.

Remember Hydrogen can only have two electrons in its valence shell. Never put extra electrons around Hydrogen!

Check for stability. Each atom should be surrounded by exactly eight electrons. If all are stable, drawing is complete. If not, rearrange unshared electrons creating multiple bonds as needed.

If not stable, create multiple bonds.

and do examples

Resonance Structures

Some covalent molecules cannot be represented by a single Lewis structure. The Lewis structure for ozone, O3, can be drawn two ways:

Which one is correct? Neither structure is correct by itself. You will need to draw both. Use a double headed arrow between the structures to show that the actual molecule is an average of the two possible states.

Resonance structure: any one of two or more possible configurations of the same compound that have identical geometry but different arrangements of electrons.

Example Draw the resonance structures for sulfur trioxide.

and do homework