The Structure of Matter

Physical Science Chapter 6

Review

 Compound: atoms of two or more elements that are chemically combined   Most of the matter around us is a compound or a mixture of compounds Compounds have properties unlike those of their elements  During a chemical change, a new substance is produced.

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Chemical bonds

 Forces that hold together the atoms in a compound  When atoms gain, lose, or share electrons they are forming chemical bonds.

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Chemical Structure

 The way the atoms are bonded in a compound Physical Science chapter 6 4

Chemical formulas

  Used as shorthand for writing compounds.

 NaCl is sodium chloride Subscript – means written below     Tells us how many atoms of an element are in a compound If there is no subscript, then there is one.

Example: H 2 O has 2 atoms of hydrogen and one atom of oxygen The ratio of hydrogen atoms to oxygen atoms is 2 to 1 Physical Science chapter 6 5

Chemical structure representations

  Chemical formula – show how many of each type of atom there is  Water: H 2 O Methane CH 4 Structural formula – shows how atoms are arranged.

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Chemical structure representations

 Space filling model – shows relative volumes of the electron clouds.

 Ball-and-stick model – shows bond angles Physical Science chapter 6 7

Effects of chemical bonds

 Compounds with strong chemical bonds   Are rigid and difficult to break Have high melting and boiling points  Compounds made of molecules   Have strong bonds within each molecule Have weak attractions between molecules  Molecules are easy to separate  Lower melting and boiling points Physical Science chapter 6 8

Attractions between molecules

 Some molecules have stronger attractions between them  Example: water   Has hydrogen bonding between molecules Why water has a relatively high boiling point for a molecular compound Physical Science chapter 6 9

1.

2.

Discuss

a.

Classify the following as mixtures or compounds Air three fluorine atoms. Each bond angle is 120 degrees and each bond is the same length.

b.

CO c.

d.

SnF 2 Pure water 3.

Predict which molecules have a Draw a ball-and stick model of a boron trifluoride, BF 3 , molecule. A boron greater attraction for each other: C 3 H 8 O molecules in liquid rubbing alcohol or atom is attached to CH 4 Physical Science chapter 6 molecules in methane gas.

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Ionic compounds

 Ionic compound – a compound made up of two or more ions  Form networks of ions, not individual units.

 Ionic bond – the force that holds the ions in an ionic compound together.

 Ionic compounds have a

net charge

of zero, so the compound is electrically neutral.

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Examples

 NaCl  MgF 2 Physical Science chapter 6 12

Ionic compounds

 Smallest unit is a formula unit.

 Generally have high melting points and high boiling points.

 Are usually crystalline solids at room temperature.

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Covalent compounds

 Covalent compounds are composed of molecules that are created when atoms share electrons  Covalent bonds – the bonds between atoms in a molecule.

 Molecules are also neutral.

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Examples

 HCl  N 2  Cl 2  O 2 Physical Science chapter 6 15

Covalent compounds

 Smallest unit is a molecule.

 Generally have low melting points and boiling points.

 Are usually liquid or gaseous at room temperature, but not always.

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Discuss

1.

a.

b.

c.

d.

Determine if the following compounds are 2.

likely to have ionic or covalent bonds.

Magnesium oxide, MgO Strontium Chloride, SrCl 2 Ozone, O 3 Methanol, CH 3 OH a.

b.

c.

Identify which two of the following substances will conduct electric current, and explain why.

Aluminum foil Sugar, C 12 H 22 O 11 dissolved in water Potassium hydroxide, KOH, dissolved in water Physical Science chapter 6 17

Polar molecules

 Atoms in molecules don’t always share their electrons equally.

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Examples

 HCl  H 2 O Physical Science chapter 6 19

Polar molecule

 Has a positive end and a negative end.

 Example: stream of water Physical Science chapter 6 20

Nonpolar molecules

 Do not have negative and positive ends.

 Example: CO 2  21

Metallic Bonds

 Occur between metal atoms  Atoms are closely packed together  Electron clouds overlap  Electrons move freely between atoms Physical Science chapter 6 22

Polyatomic ions in compounds

 Poly means many  Polyatomic ions have more than one atom in them.

 See Figure 10 on page 190 Physical Science chapter 6 23

Compounds with polyatomic ions

 They form compounds just like monatomic (one atom) ions do.

 Examples  LiOH, lithium hyrdoxide  Mg(NO 3 ) 2 , magnesium nitrate Physical Science chapter 6 24

Discuss

1.

2.

3.

4.

5.

Compare bonds Compare bonds What is the difference between polar molecules and nonpolar molecules?

What are polyatomic ions?

Identify which of the bonds in calcium hydroxide, Ca(OH) 2 are ionic and which are covalent.

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Naming Ionic Compounds

 List the (positive) cation first  Name is usually the same as the element  List the (negative) anion second   Change ending to –ide See figure 2 on page 192 Physical Science chapter 6 26

Examples

 CaF 2  Calcium fluoride  Li 2 O  Lithium oxide  K 2 S  Potassium sulfide Physical Science chapter 6 27

Writing formulas for ionic compounds

 The charge on the compound must add up to zero.

 Add subscripts as needed Physical Science chapter 6 28

Examples

   Cesium Oxide  Cs 2 O Beryllium chloride  BeCl 2 Calcium Phosphide  Ca 3 P 2 Physical Science chapter 6 29

Transition Metals

 Groups 3 – 12   Can have more than one charge when forming compounds  Copper and oxygen can make CuO or Cu 2 O 3 To name them, we need to specify the charge of the cation using a roman numeral   CuO is copper (II) oxide Cu 2 O 3 is copper (III) oxide Physical Science chapter 6 30

Examples

 Titanium (III) nitride  TiN  Fe 2 O 3  Iron (III) oxide  Iron (II) oxide  FeO Physical Science chapter 6 31

Naming Covalent Compounds

     Different rules Use numerical prefixes (figure 5 on page 194) If there is only one atom of the first element, the prefix mono- is omitted.

Change the ending of the second element to -ide If the element starts with a vowel, drop the a or o at the end of the prefix  Example: tetroxide, not tetraoxide Physical Science chapter 6 32

Examples

     PF 5  Phosphorus pentafluoride N 2 O 5  Dinitrogen pentoxide OF 2  Oxygen difluoride Phosphorus trichloride  PCl 3 Dinitrogen pentoxide  N 2 O 5 Physical Science chapter 6 33

Empirical Formulas

 Shows the smallest whole-number ratio of atoms that are in a compound  Ionic compounds  Almost always the same as the chemical formula  Covalent compounds   Not always the same Example: glucose chemical formula is C 6 H 12 O 6 , empirical formula is CH 2 O Physical Science chapter 6 34

Molecular formula

 Shows the actual numbers of atoms of each type in one molecule  The same as the chemical formula Physical Science chapter 6 35

Determining empirical formula

 Convert the mass of each element to moles.

 Find the molar ratio, which gives the empirical formula   The ratio must be whole numbers, because the subscripts in the formula must be whole numbers.

If the ratio isn’t whole numbers, multiply it by a whole number to get rid of the fractions.

 Example: if the ratio is 1.5:1, multiply by 2 to get 3:2 Physical Science chapter 6 36

Examples

 A sample of an unknown compound has 36.04 g of carbon and 6.04 g of hydrogen. What is the compound’s empirical formula?

 A sample of a compound contains 3.6 g of boron and 1.0 g of hydrogen. What is the compound’s empirical formula?

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You try

   A certain compound sample contains 207.2 g of lead and 32.00 g of oxygen. What is its empirical formula?

A compound is analyzed and found to contain 36.70g potassium, 33.27g chlorine, and 30.03g oxygen. What is the empirical formula of the compound?

Find the empirical formula of a compound that contains 53.70g iron and 46.30g sulfur.

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