Chemical Bonding: What and Why

F. Schifano, Department of ScienceBayonne High School

Bayonne, NJ 07002

•Five Days of Instruction and Practice

•Two Labs: Covalent vs. Ionic and Molecular Modeling (total 5 points)

•One Day of In-class Review: 16 March 2011

•Afterschool Session: 16 March 2011 3-5 pm

•Full-Unit Test: 17 March 2011 (13.5 points)

This Unit Will Be Nine Days Long

NJCCCS 5.2.12.B.1. (...chemical bonds are the interactions between atoms that hold them together in molecules or between oppositely charged ions.)

This Unit Addresses Key Standards

Explain how and why chemical bonds form

Distinguish between the types of chemical bond

Relate bond types to observable chemical properties

Indicate the three-dimensional structure of molecules

Key Skills

A chemical bond is a relationship between two or more atoms that involves the transfer or sharing of electrons.

What is a chemical bond?

Chemical reactions are the observable consequences of making and breaking bonds.

The formation or breaking of a bond results in a chemical change– an energy change and the production of a new product substance.

How are bonds related to chemical reactions?

Enthalpy is the potential energy stored in chemical bonds.

Chemical reactions release or absorb heat because the total enthalpy in the reactants is different than the total enthalpy of the products.

Enthalpy (H)

Mixtures are combinations of atoms that involve only physical jumbling rather than chemical bonding. ◦ Formed and separated by physical changes

What isn’t a chemical bond?

Intermolecular forces– weak, transient interactions between molecules control some physical, but not chemical properties.◦ Next unit we will focus on these.

What isn’t a chemical bond?

Atoms form bonds because in doing so they can rearrange their electrons in a way that makes them more stable.

Bonds are a way for atoms to achieve valence = 8. ◦ Hydrogen is stable at VAL=0 or VAL =2◦ Boron is stable at VAL = 6

Why do atoms form bonds?

Showing Bonds: 3D Models

Gilbert Lewis (ENG-1916) invented the structures to show on paper how molecules were bonded.

Lewis Structures

Lines represent covalent bonds. Each line counts as two electrons.

Dots represent unshared electrons. Unshared pairs are called lone pairs.

Reading Lewis Structures

Atoms have different needs to achieve stability so they will form different numbers and kinds of bonds.

Writing Lewis Structures

Elements Valence Number of Bonds

Number of Lone Pairs

Satisfied at this valence:

H 1 1 0 1

F, Cl, Br, I 7 1 3 8

O, S, Se 6 2 2 8

N, P 5 3 1 8

C, Si 4 4 0 8

B 3 3 0 6

Covalent Bond Behaviors

You should copy this slide to your formula sheet.

Write the highest bond order atom

in center

Link together all other high bond-order elements

Resolve all remaining bonds

so all VAL are fulfilled

Writing Lewis Structures

Write the highest bond order element in the center.

Attach all other high-bond order elements to this central atom by single bonds.

Show all missing bonds as empty lines. Use any available 1-bond order elements to

fill up empty bonds. If there are any empty bonds on adjacent

molecules, use multiple bonds to fill them.

Step-By-Step Lewis Structures

Write the Lewis structures of each of the following substances.

Practice

C H CH4 H3CCl

CO2 BF3 HCl NH3

CH2O NH2CH3 H2O H2S

CH3CH2OH PCl3 N2 H2C2

C H CH4 H3CCl

Practice

CO2 BF3 HCl NH3

Practice

CH2O NH2CH3 H2O H2S

Practice

CH3CH2OH

PCl3 H2N2 CH2F2

Practice