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Atoms, Molecules, and Life Atoms The Importance of Water Chapter 2 • The basic structural unit of matter. nucleus proton neutron electron electron shell 1 Hydrogen 2 1 H 3 Hydrogen 1.008 4 1 - Atomic Number = # of protons Helium 2 - Element 3 - Symbol 4 - Molecular Mass ~ # protons + # neutrons Carbon 1 2 6 C Carbon 12.01 3 4 Properties of Elements • Element: a substance that cannot be broken down nor converted to another substance by ordinary chemical means. • Isotope: atoms of an element that contain different numbers of neutrons – – 12C has 6 protons and 6 neutrons (normal) 14C has 6 protons and 8 neutrons (“heavy” carbon) • Some isotopes are radioactive, they spontaneously break apart, releasing energy and forming new elements. Electrons: the key to bonding • Negatively charged • Repel each other • Attracted to the positively-charged nucleus • Can become excited Excited Electrons: Disobeying Electron Shells • Electrons orbit through different energy levels – 1st shell: up to 2 electrons (e-) – 2nd shell (and each larger shell): up to 8 e- • Always fill up the lowest energy level first Free Radicals • Atoms with one or more unpaired electron in their outer shell. • Very unstable. • Capture electrons from neighboring molecules, creating new free radicals. • Can lead to cell death and a variety of diseases. • Antioxidants react with free radicals, rendering them harmless to the body. • More stable when shells are full Interacting Elements • An atom whose outer electron shell is full cannot interact with other atoms and is called inert. • Atoms is reactive when its outer electron shell is only partially full and it can react with other atoms. Molecules • Two or more atoms of the same or different elements held together. • Molecules formed from different elements are compounds. • Atoms can form three main types of bonds: – Ionic, Covalent, and Hydrogen bonds Ionic Bonds Example: MgCl2 Ionic bonding, with one electron from Magnesium donated to each Chlorine atom • An electron is transferred, creating positive and negative ions that attract one another. 12 Mg Magnesium 24.31 12 protons 12 neutrons 12 electrons (2 e- in the outer shell) • MgCl2: – Total # protons: • 12 + (17)*2 = 46 – Total # electrons: Chlorine: 17 Cl Chlorine 34.45 Sodium (11 p+, 11 n) Magnesium: • 12 + (17)*2 = 46 17 protons 18 neutrons 17 electrons (7 e- in the outer shell) – Combined Molecular Mass: • 24.31 + (34.45)*2 = 93.21 g/mol Chlorine (17 p+, 18 n) Covalent Bonds Covalent Bonds, continued • Electron pairs are shared. • Polar covalent bonds: unequal sharing. • Nonpolar covalent bonds: equal sharing. – Electrons spend more time around one atom than the other. – Two hydrogen atoms share electrons to form uncharged H2. – Electrons spend the same amount of time around each nuclei. - - Oxygen is slightly negative Hydrogen is slightly positive + Water, H2O + Example: NH3, Ammonia • Polar, covalent bonding, sharing the electrons of Hydrogen to fill Nitrogen’s outer e- shell. 7 N Nitrogen: 14.01 H – Total # protons: 7 protons 7 neutrons 7 electrons (5 e- in the outer shell) Nitrogen 1 • NH3: • 7 + (1)*3 = 10 – Total # electrons: • 7 + (1)*3 = 10 Hydrogen: – Combined Molecular Mass: 1 proton 0 neutrons 1 electrons (1 e- in the outer shell) Hydrogen 1.008 • 14.01 + (1.008)*3 = 17.034 g/mol Properties of Water Hydrogen Bonds • The slightly attraction between atoms involved in polar covalent bonds from different molecules. + - + + - - + • Life likely began underwater. • All living organisms contain 60-90% H2O. • All life depends on water. – (examples: Photosynthesis and Digestion) + + Covalent bonds are the most common bond found in biologic systems. Properties of Water Properties of Water 1. Very good solvent. – It is able to dissolve a wide range of substances. Solute: Salt Solvent: Water Solution: Salt Water • Because of its polarity, water attracts both positive and negative ions, breaking ionic bonds. Properties of Water Properties of Water 2. Hydrophobic and Hydrophilic qualities. •Hydrophobic (water-fearing) molecules are non-polar and uncharged. 3. Water molecules tend to stick together. Because of their hydrogen bonds, water molecules have high cohesion (stickiness). Cohesion of water molecules at the surface creates surface tension. • •Water cannot dissolve them. •Lipids, Fats and Oils •Hydrophilic (water-loving) molecules are polar. •Water dissolves the covalent bonds. •Sugars, Salts, and Amino Acids • • The tendency for the water surface to resist breaking. Properties of Water 4. Water can be acidic, basic, or neutral. • Water molecules are easily ionized: Examples of Surface Tension and Cohesion Acids • Acidic solutions have a higher concentration of H+ than OH-. • Acids release hydrogen ions when dissolved in water. • Many acidic solutions taste sour (vinegar, lemon juice, etc). Bases • Basic solutions have a higher concentration of OH-. • Bases combine with hydrogen ions, reducing their number. • Many basic solutions taste bitter, and are very harmful if consumed (ammonia, bleach, etc). • Pure water has equal concentrations of hydroxide (OH-) and hydrogen (H+) ions. pH Scale (0-14) • Measures the ratio of H+ to OH-. • Neutrality (equal concentrations of H+ and OH-) is set to 7. • Acids have a pH below 7. • Bases have a pH above 7. 0 Acidic 7 Neutral 14 Basic Buffers • A complex molecule that helps maintain a solution at a constant pH. • Can combine with H+ to increase pH. • Releases H+ to combine with OH- when needed to decrease pH. Example: Carbonate Buffering of Blood CO32- <--> HCO3- <--> H2CO3 pH ~ 6 Properties of Water •Water moderates the effects of temperature changes. pH ~ 9.5 Water forms an unusual solid: Ice • Less dense than liquid water! • H20 heats slowly – Takes a lot of energy to break H-bonds • H20 moderates high and low temperatures • H20 freezes slowly – A large amount of energy must be removed to form ice crystals Liquid Solid Homework Chapter 2 The lizard in the beginning of this lecture appears to be running on water. Aside from the speed at which the lizard runs, what properties of water allow the lizard to move like this? How does this property work? (i.e. describe the arrangement of molecules that gives water this property) Give two other examples where these properties of water are used in nature.