The Structure of MatterSection 4.1: Compounds and Molecules

Objectives

Distinguish between compounds and mixtures.Relate the chemical formula of a compound to the relative numbers of atoms or ions present in the compound.Use models to visualize a compound’s chemical structure.Describe how the chemical structure of a compound affects its properties.

What Are Compounds?

When elements combine to form a compound, the compound has properties very different from those of the elements that make it.Example:

Sodium and Chlorine combine to form Sodium Chloride, NaCl, or table salt.

The silvery metal sodium combines with

Poisonous, yellowish green chlorine gas

To form white granules of table salt that you can eat.

Placing a lit candle under a balloon containing hydrogen gas and oxygen gas causes the balloon to melt, releasing the mixed gases.

The mixed gases are ignited by the candle flame, and water is produced.

Chemical Bonds

Chemical bonds – the attractive forces that hold different atoms or ions together in compounds.In a chemical reaction, chemical bonds are broken, and atoms are rearranged. New bonds are formed and a compound with properties different from the original substances is formed.

Chemical Formulas

A chemical formula shows the types and numbers of atoms or ions making up the simplest unit of the compound.Compounds are always made of the same elements in the same proportions.

Chemical Structure

Chemical structure shows how the atoms or ions in a compound are connected.The atoms in a compound and the way the atoms are arranged determine many of the compound’s properties.Bond length – gives the distance between the nuclei of two bonded atoms.Bond angle – tells how the atoms are oriented when a compound has three or more atoms.

Models of Compounds

Ball-and-stick model – atoms are represented by balls and bonds are represented by sticks. Although bonds between atoms are not really as rigid as sticks, this model makes it easy to see the bonds and angles they form in a compound.

Models of Compounds

Structural formulas – also show the structures of compounds.Only chemical symbols are used to represent atoms.

Models of Compounds

Space-filling models – shows the space that is occupied by each atom in the compound.Harder to see the bond lengths and bond angles.

Network Structures

Atoms in the compound are held together in a regular, repeating arrangement.Very strong, rigid structureTakes a lot of energy to break these strong bonds.Melting and boiling points are very high.

Some Compounds With Network StructuresCompound State

(25°C)Melting point (°C)

Boiling point (°C)

Silicon dioxide, SiO2

(quartz, sand)

Solid 1700 2230

Magnesium fluoride, MgF2

Solid 1261 2239

Sodium chloride, NaCl

(table salt)

Solid 801 1413

Networks of Bonded Ions

Example: sodium chlorideRepeating network connected by strong bondsTightly packed positively charged sodium ions and negatively charged chlorine ions.Strong attractions between oppositely charged ions cause high melting and boiling points.

Compounds Made of Molecules

Example: sugar, C12H22O11Molecule made of carbon, hydrogen, and oxygen atoms joined by bonds.Molecules are attracted to each other to form crystals.These attractions between molecules are much weaker than the attractions that hold the atoms together to make the sugar molecule

Compounds Made of Molecules

Example: oxygen gas, O2Within each molecule, the atoms are so strongly attracted to each other that they are bonded.Molecules have very little attraction for each other.Because they are not very attracted to one another, the molecules spread out as much as they can and take up a lot of space.

Comparing Compounds Made of MoleculesCompound State

(25°C)Melting point (°C)

Boiling point (°C)

Sugar, C12H22O11

Solid 185 – 186 --------

Water, H2O Liquid 0 100

Dihydrogen sulfide, H2S

Gas -85 -60

Attractions Between Molecules

Sugar is a solid. That means that sugar molecules have strong attractions for each other.Water is a liquid. Weaker attractions between molecules.Dihydrogen sulfide is a gas. Attractions between these molecules are very weak.

Attractions Between Molecules

Higher melting and boiling points of water suggest that water molecules attract each other more than dihydrogen sulfide molecules do.The oxygen atom of one molecule is attracted to a hydrogen atom of a neighboring water molecule.This is often referred to as hydrogen bonding.The attractions between water molecules is not as strong as the bonds holding oxygen and hydrogen atoms together within a water molecule.