Chapter 6: The Structure of Matter

Section 1: Compounds and Molecules

Key Ideas

• What holds a compound together?• How can the structure of chemicals

compounds be shown?• What determines the properties of a

compound?

Key Terms

• Chemical bonds• Chemical structure• Bond length• Bond angle

Chemical Bonds

• The forces that hold atoms or ions together in a compound are called chemical bonds

Chemical Structure

• Chemical structure is the way the compound’s atoms are bonded to make compound

• Just as the structure of buildings can be represented by blueprints, the structure of chemical compounds can be shown by various models

Chemical Structure

• Ball-and-stick model– Represents bond lengths and angles– Bond length is the distance between the nuclei of

two bonded atoms– Bond angle is the angle formed by two bonds to

the same atom, tells which way these atoms point.

Chemical Structure• Ball-and-stick model

Chemical Structure

• Space-filling models– This model shows the space that atoms take up

Chemical Structure

• Bonds can bend, stretch, and rotate without breaking

How Does Structure Affect Properties?

• The chemical structure of a compound determines the properties of that compound– Compounds with network structures are strong

solids like quartz, made of silicon dioxide, SiO2

How Does Structure Affect Properties?

• The chemical structure of a compound determines the properties of that compound– Some networks are made of bonded ions like table

salt (sodium chloride), that are found in the form of regularly shaped crystals

How Does Structure Affect Properties?

• The chemical structure of a compound determines the properties of that compound– Some compounds are made of molecules like

sugar (C12H22O11), that is made of carbon, hydrogen, and oxygen atoms that are joined by bonds

How Does Structure Affect Properties?

• The strength of attractions between molecules varies

Compound State (25oC) Melting Point (oC) Boiling Point (oC)

Sugar, C12H22O11 Solid 185-186 -

Water, H2O Liquid 0 100

Dihydrogen Sulfide, H2S

Gas -86 -61

Homework

• Classify the following substances as mixtures or compounds: air, CO, SnF2, pure water.

• Draw a ball-and-stick model of boron trifluoride, BF3, molecule. In this molecule, a boron atom is attached to three fluorine atoms. Each F-B-F bond angle is 120o, and all B-F bonds are the same length.

• Explain why glass, which is made of mostly SiO2, is often used to make cookware.