The Structure of Matter

1. The Structure of Matter Physical Science Chapter 6

2. 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. Physical Science chapter 6

3. Chemical bonds • Forces that hold together the atoms in a compound • When atoms gain, lose, or share electrons they are forming chemical bonds. Physical Science chapter 6

4. Chemical Structure • The way the atoms are bonded in a compound Physical Science chapter 6

5. 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: H2O 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

6. Chemical structure representations • Chemical formula – show how many of each type of atom there is • Water: H2O Methane CH4 • Structural formula – shows how atoms are arranged. Physical Science chapter 6

7. Chemical structure representations • Space filling model – shows relative volumes of the electron clouds. • Ball-and-stick model – shows bond angles Physical Science chapter 6

8. 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

9. 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

10. Discuss • Classify the following as mixtures or compounds • Air • CO • SnF2 • Pure water • Draw a ball-and stick model of a boron trifluoride, BF3, molecule. A boron atom is attached to three fluorine atoms. Each bond angle is 120 degrees and each bond is the same length. • Predict which molecules have a greater attraction for each other: C3H8O molecules in liquid rubbing alcohol or CH4 molecules in methane gas. Physical Science chapter 6

11. 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. Physical Science chapter 6

12. Examples • NaCl • MgF2 Physical Science chapter 6

13. Ionic compounds • Smallest unit is a formula unit. • Generally have high melting points and high boiling points. • Are usually crystalline solids at room temperature. Physical Science chapter 6

14. 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. Physical Science chapter 6

15. Examples • HCl • Cl2 • N2 • O2 Physical Science chapter 6

16. 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. Physical Science chapter 6

17. Discuss • Determine if the following compounds are likely to have ionic or covalent bonds. • Magnesium oxide, MgO • Strontium Chloride, SrCl2 • Ozone, O3 • Methanol, CH3OH • Identify which two of the following substances will conduct electric current, and explain why. • Aluminum foil • Sugar, C12H22O11 dissolved in water • Potassium hydroxide, KOH, dissolved in water Physical Science chapter 6

18. Polar molecules • Atoms in molecules don’t always share their electrons equally. Physical Science chapter 6

19. Examples • HCl • H2O Physical Science chapter 6

20. Polar molecule • Has a positive end and a negative end. • Example: stream of water Physical Science chapter 6

21. Nonpolar molecules • Do not have negative and positive ends. • Example: CO2 • Nonpolar vs. polar Physical Science chapter 6

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

23. 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

24. Compounds with polyatomic ions • They form compounds just like monatomic (one atom) ions do. • Examples • LiOH, lithium hyrdoxide • Mg(NO3)2, magnesium nitrate Physical Science chapter 6

25. Discuss • 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. Physical Science chapter 6

26. 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

27. Examples • CaF2 • Calcium fluoride • Li2O • Lithium oxide • K2S • Potassium sulfide Physical Science chapter 6

28. Writing formulas for ionic compounds • The charge on the compound must add up to zero. • Add subscripts as needed Physical Science chapter 6

29. Examples • Cesium Oxide • Cs2O • Beryllium chloride • BeCl2 • Calcium Phosphide • Ca3P2 Physical Science chapter 6

30. Transition Metals • Groups 3 – 12 • Can have more than one charge when forming compounds • Copper and oxygen can make CuO or Cu2O3 • To name them, we need to specify the charge of the cation using a roman numeral • CuO is copper (II) oxide • Cu2O3 is copper (III) oxide Physical Science chapter 6

31. Examples • Titanium (III) nitride • TiN • Fe2O3 • Iron (III) oxide • Iron (II) oxide • FeO Physical Science chapter 6

32. 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

33. Examples • PF5 • Phosphorus pentafluoride • N2O5 • Dinitrogenpentoxide • OF2 • Oxygen difluoride • Phosphorus trichloride • PCl3 • Dinitrogenpentoxide • N2O5 Physical Science chapter 6

34. 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 C6H12O6, empirical formula is CH2O Physical Science chapter 6

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

36. 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

37. 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? Physical Science chapter 6

38. 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. Physical Science chapter 6