The Chemistry of Everything Kimberley Waldron. Chapter 2 Topics

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1 The Chemistry of Everything Kimberley Waldron Chapter 2 Dirt How atoms interact with one another: an introduction to chemical bonds and simple reactions Richard Jarman, College of DuPage 2007 Pearson Prentice Hall 1 Chapter 2 Topics Survey of the periodic table, the diversity of elements, covalent bonding. Ionic bonding, metallic bonding, salts, anions, cations, electronegativity, bond polarity. Mass number, neutrons, isotopes. Scientific notation, moles, Avogadro s number, molar mass. An introduction to oxidation and reduction reactions. Magnetism. Control experiments Pearson Prentice Hall 2 1

2 What is Dirt? Dirt is a mixture of compounds. Compounds are made from elements. The types of elements found in soil are shown below. Even trace elements (like manganese or chromium can have significant effects on the properties Pearson Prentice Hall 3 From the Dust of the Ground Soil is two parts: Organic (carbon) Minerals (inorganic) Humans contain the same elements, but in different amounts Pearson Prentice Hall 4 2

3 Classifying Elements Elements are either: Metals (pink boxes below) Non-metals (blue boxes below) Metals are on the left; nonmetals are on the right Pearson Prentice Hall 5 Group Thinking Elements are characterized by group behavior Group 1 alkali metals (light blue) Group 2 alkaline earth metals (yellow) Group 17 halogens (pale yellow) Group 18 noble gases (green) 2007 Pearson Prentice Hall 6 3

4 Metals, Soils and Humans Trace elements in soils are influenced by human activity. Lead is increased by use of leaded gasoline. Zinc is increased by fossil fuel and tires. Concentrations have been measured in southern states Pearson Prentice Hall 7 Impacts of Environmental Legislation Zinc levels are constant with time. Lead levels increase and then decrease. Lead has been removed from petrol since the mid-70 s Pearson Prentice Hall 8 4

5 Elemental Diversity Each element has a different number of protons. Just one proton (and one electron) makes a world of difference. Lead (82 protons) is highly toxic. Bismuth (83 protons) is a major component of Pepto Bismol Pearson Prentice Hall 9 The Ties That Bind The bonds between atoms determine the properties of substances. Why do atoms form bonds at all? It s all in the number of electrons. The only elements that don t form bonds (except in a few cases) are the noble gases (group 18). He (2), Ne (10), Ar (18), Kr (36), Xe (54) 2007 Pearson Prentice Hall 10 5

6 A Noble Gesture The noble gases have desirable stable electron configurations. The other elements want to be a noble gas electronically. Atoms gain, lose or share electrons to achieve the noble gas configuration. Sulfur, group 16, gains two electrons. Magnesium, group 2, loses two electrons Pearson Prentice Hall 11 Valence Electrons Valence electrons are in the outer shell of the atom. All the other electrons are in the core. Only valence electrons participate in bonds. Counting valence electrons: Start at left of table and count the number of groups to the group the element is in. F (group 17) count 7 groups from left of table 7 valence electrons. O (group 16) count 6 groups from left of table 6 valence electrons Pearson Prentice Hall 12 6

7 Covalent Bonds F is in group 17. Has 7 valence electrons. Needs one to be like neon. Two F atoms share electrons in the molecule F 2. Covalent bonds form between different elements. O S O 2007 Pearson Prentice Hall 13 Unequally Yoked: Polar Bonds Covalent bonds formed between different elements are polar. Electrons are shared unequally. In H 2 O the O atoms attract the electrons more. O H bonds are polar H 2 O molecules are polar 2007 Pearson Prentice Hall 14 7

8 Electron Transfer and Ionic Bonds Cations: Elements on the left side of the table lose electrons and form positive ions (Ca 2+ ) Anions: Elements on the right side of the table gain electrons and form positive ions (Cl - ) Ca 2+ - Cl - ionic bond 2007 Pearson Prentice Hall 15 Predictability of Ionic Compounds Group number predicts the ion charge Na +, Ca 2+, O 2-, F - Ions combine to achieve neutrality: Total of positive charges = total of negative charges NaCl, Li 2 O, CaF 2 etc Pearson Prentice Hall 16 8

9 The Ionic Lattice Ionic compound contains arrays of billions of ions. There are no molecules. Represent by simplest formula and name: Sodium chloride NaCl Potassium oxide K 2 O 2007 Pearson Prentice Hall 17 Electronegativity Electronegativity measures the tendency of an atom to attract electrons in a bond with another atom: Metals have low electronegativity. Nonmetals have high electronegativity Pearson Prentice Hall 18 9

10 Ionic, Polar or Non-polar? Ionic bonds occur when electronegativity difference is > 2.0. Polar covalent electronegativity difference < 2.0. Nonpolar covalent electronegativity difference ~ Pearson Prentice Hall 19 Why do metals conduct electricity? Like ionic compounds metals contain ions. Metal ions swim in a sea of electrons. Unlike covalent bonds, the electrons in metals are fluid and can move easily. Bonding in Metals 2007 Pearson Prentice Hall 20 10

11 Why Neutrons Matter Atoms of the same element contain the same number of protons. Isotopes contain the same number of protons but different number of neutrons. 48 Ti and 50 Ti are isotopes of titanium 48 Ti contains 22 protons and 26 neutrons 50 Ti contains 22 protons and 28 neutrons Isotopes have the same chemical properties Pearson Prentice Hall 21 Isotopes: A Window Into History Ratios of isotopes can reveal the past. Carbon-14 is used in dating. Carbon-14 levels in living organisms is constant. Once dead, the carbon-14 decays Pearson Prentice Hall 22 11

12 Analyzing the Parthenon Marble is calcium carbonate CaCO 3. The isotopes of oxygen give clues about the origins of marble. Oxygen-18 analysis indicated the source of marble was Mount Pendelikon Pearson Prentice Hall 23 Making Sense of Numbers Chemistry contains large numbers. Scientific notation provides a way to describe them. 7,900,000,000,000,00 0,000,000,000. Or 7.9 x Or 1.24 x Pearson Prentice Hall 24 12

13 Counting Atoms: The Mole One mole is the number of atoms in exactly 12 g of carbon-12: Avogadro s number = 6.02 x The tennis racket contains 40 moles of carbon 2.4 x atoms. The mole is the chemist s dozen one mole of anything is always the same number. Substances, atoms and molecules are measured in moles Pearson Prentice Hall 25 Moles and Molar Mass Weighing a mole. Every element has an atomic weight. The figure in the periodic table is the weighted average of the isotope masses. Molar mass of chlorine is g/mol. Molar mass of NH 3 = g(n) + 3 x 1.01 g(h) = g/mol Pearson Prentice Hall 26 13

14 Meteoroids and Redox Chemistry Some meteoroids are mainly iron. Others are mainly magnatite (Fe 3 O 4 ). Oxidation involves loss of electrons. Reduction involves gain of electrons. The iron loses electrons when reacting with oxygen to form Fe 3 O 4. 4 Fe + 3O 2 = 2 Fe 3 O Pearson Prentice Hall 27 Redox Reactions Involve Transfer of Electrons Oxidation half reaction: 4 Fe = 4 Fe electrons. Reduction half reaction: 3O electrons = 6O 2-. The oxidizing agent (O 2 ) is itself reduced. The reducing agent (Fe) is itself oxidized Pearson Prentice Hall 28 14

15 Electrons and Half Reactions Electrons are never shown in complete chemical equations. Half reactions reveal the electron pathway: 3O electrons = 6O 2-4 Fe = 4 Fe electrons Adding: 3O electrons + 4 Fe = 4 Fe O electrons 3O 2 + 4Fe = 6O Fe 3+ Electrons cancel out Pearson Prentice Hall 29 Electrons and Magnetism Electrons are little magnets. In most substances the electrons are paired up and the magnetism cancels out. Magnetite is magnetic not all electrons are paired Pearson Prentice Hall 30 15

16 Magnetite and Navigation Magnetite in living organisms is associated with powers of navigation. Magnetotactic bacteria contain tiny rows of magnetite Pearson Prentice Hall 31 Geological magnetite crystals tend to be irregular and defective. Biological magnetite crystals are tiny, regular and perfect. Martian meteorites contain magnetite crystals like those in bacteria. Life on Mars 2007 Pearson Prentice Hall 32 16

17 Colors in Nature Minerals like corundum (Al 2 O 3 ) are colorless. Small impurities from transition metals give intense colors. Cr 3+ - red, Fe 3+ - yellow, Ti + + Fe 2+ - green Beryl + Cr 3+ (green) 2007 Pearson Prentice Hall 33 Fingerprint File Dirt is specific to a region. The make-up of its minerals can be used to trace its origins. Soil analysis is used extensively in forensic science Pearson Prentice Hall 34 17

18 Control Samples Reliable analysis requires the use of control samples. Identification of the soil samples from a car used in the murder of Albert Coors required over 400 control samples from three locations of interest Pearson Prentice Hall 35 18