Essential Chemistry for Biology

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1 Essential Chemistry for Biology
Chapter 2 Essential Chemistry for Biology Why do we need chemistry in a biology class? Well, the biological system is made up of chemicals and these are undergoing a continuous series of chemical reactions. In order to understand the biology, we need to know the chemistry involved in it. Take any biological system apart, and you eventually end up at the chemical level

2 Biology and Society: More Precious than Gold
A drought is a period of abnormally dry weather that changes the environment and one of the most devastating disasters. Droughts can cause severe crop damage, shortages of drinking water, dust storms, famine, habitat loss, and mass migration. Throughout human history, droughts have helped wipe out societies and even whole civilizations. Droughts are catastrophic because life cannot exist without water. © 2013 Pearson Education, Inc. 2

3 Matter: Elements and Compounds
Matter is anything that occupies space and has mass. found on the Earth in three physical states: Solid, Liquid and Gas © 2013 Pearson Education, Inc. Biology, we need to know the chemistry involved in it. Take any biological system apart, and you eventually end up at the chemical level 3

4 Matter: Elements and Compounds
Matter is composed of chemical elements:. substances that cannot be broken down into other substances. There are 92 naturally occurring elements on Earth. All of the elements are listed in the periodic table. Twenty-five are essential to life. Four make up about 96% of the weight of the human body CHON: Carbon, Hydrogen, Oxygen and Nitrogen © 2013 Pearson Education, Inc. Biology, we need to know the chemistry involved in it. Take any biological system apart, and you eventually end up at the chemical level 4

5 Chemical composition of the human body by weight
Carbon (C): 18.5% Oxygen (O): 65.0% Calcium (Ca): 1.5% Phosphorus (P): 1.0% Potassium (K): 0.4% Sulfur (S): 0.3% Sodium (Na): 0.2% Chlorine (Cl): 0.2% Magnesium (Mg): 0.1% Hydrogen (H): 9.5% Trace elements: less than 0.01% Boron (B) Manganese (Mn) Chromium (Cr) Molybdenum (Mo) Nitrogen (N): 3.3% Cobalt (Co) Selenium (Se) Copper (Cu) Silicon (Si) Fluorine (F) Tin (Sn) Iodine (I) Vanadium (V) Iron (Fe) Zinc (Zn) Figure 2.2 Figure 2.2 Chemical composition of the human body by weight 5

6 Minor & Trace elements Minor elements Trace elements Calcium Ca
Building strong bones and teeth Found abundantly in milk & dairy products, sardines, green & leafy vegetables Phosphorus Component of DNA Found in eggs, beans and nuts Trace elements Iodine Essential ingredient of thyroid hormone Found in dairy products Deficiency leads to goiter Fluorine Needed for healthy bones and teeth Figure 2.1a Abbreviated periodic table of the elements.

7 Matter: Elements and Compounds
Trace elements are required in only very small amounts and are essential for life. An iodine deficiency causes goiter. Fluorine is added to dental products and drinking water and helps to maintain healthy bones and teeth. © 2013 Pearson Education, Inc. 7

8 8 Hyper functioning thyroid (goiter)
Figure 2.3 Figure 2.3 The relationship between diet and goiter This is an example why trace element are important 8

9 Matter: Elements, Atoms and Compounds
Each element consists of one kind of atom. An atom is the smallest unit of matter that still retains the properties of an element. Elements can combine to form compounds. Compounds are substances that contain two or more elements in a fixed ratio. Common compounds include NaCl (table salt) and H2O (water). © 2013 Pearson Education, Inc. 9

10 C Periodic table of the elements 10 6 12 Atomic number
(number of protons) 6 C Element symbol Mass number (number of protons plus neutrons) 12 H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba La Hf Ta W Re Os Ir Pt Au Hg TI Pb Bi Po At Rn Fr Ra Ac Rf Db Sg Bh Hs Mt Ds Rg Cn Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr Figure 2.1a Figure 2.1 Abbreviated periodic table of the elements (part 1) 10

11 Periodic table of the elements
The properties of the chemical elements are often summarized using the periodic table that organizes the elements by increasing atomic number into rows ("periods") in which the columns ("groups") share recurring ("periodic") physical and chemical properties

12 Practice Ordinary oxygen has an atomic mass of 16 and an atomic number of 8. How many neutrons does this type of oxygen have? Phosphorous has an atomic number of 15 and an atomic weight of How many neutrons does this type of oxygen have? Silicon has an atomic number of 14 and a mass number of 28. How many neutrons are found in silicon?

13 The Structure of Atoms Atoms are composed of three subatomic particles
A proton is positively charged, mass of 1 dalton An element is defined by its number of protons A neutron is electrically neutral mass of 1 dalton Neutrons can vary between isotopes An electron is negatively charged minimal mass 1/2000th 1 dalton, found outside (orbiting) the nucleus. Most atoms have protons and neutrons packed tightly into the nucleus. The nucleus is the atom’s central core. Electrons orbit the nucleus. Electron shell or Electron energy level © 2013 Pearson Education, Inc. 13

14 Helium atom 14 2 Protons Nucleus 2 Neutrons 2 Electrons Nucleus 2e–
Electron cloud Figure 2.4 Figure 2.4 Two simplified models of a helium atom This model is slightly more realistic, shows the electrons as spherical cloud of negative charge surrounding the nucleus 14

15 Structure of an atom Proton Electron • Positive charge
• Determines element Electron • Negative charge • Participates in chemical reactions • Outer-shell electrons determine chemical behavior Neutron • No charge • Determines isotope Nucleus • Consists of neutrons and protons Atom Figure 2.UN5 Summary: Structure of an atom

16 Atomic and mass number Elements differ in the number of subatomic particles in their atoms. The number of protons, the atomic number, determines which element it is. Mass is a measure of the amount of material in an object. An atom’s mass number is the sum of the number of protons and neutrons in its nucleus. © 2013 Pearson Education, Inc. 16

17 Isotopes Isotopes are alternate mass forms of an element.
Isotopes have the same number of protons and electrons, but they have a different number of neutrons. Table 2.1 Isotopes of Carbon

18 Radioactive Isotope The nucleus of a radioactive isotope decays, giving off particles and energy. Radioactive isotopes have many uses in research and medicine. They can be used to determine the fate of atoms in living organisms. They are used in PET scans to diagnose heart disorders and some cancers. Uncontrolled exposure to radioactive isotopes can harm living organisms by damaging DNA. The 1986 Chernobyl nuclear accident released large amounts of radioactive isotopes. Naturally occurring radon gas may cause lung cancer. Once the cells take up the radioactive isotopes, the location and the concentration of the isotopes can be detected because they emit the radiation

19 Figure 2.5 Figure 2.5 A PET scan 19

20 Electron Arrangement and the Chemical Properties of Atoms
Of the three subatomic particles, only electrons are directly involved in the chemical activity of an atom. Electrons orbit the nucleus of an atom in specific electron shells. The farther an electron is from the nucleus, the greater its energy. The number of electrons in the outermost shell (valance) determines the chemical properties of an atom. Valance electrons determine how an atom behaves when it encounter other atoms © 2013 Pearson Education, Inc. 20

21 Atoms of the four elements most abundant in living matter
How many electrons can each orbital hold? First electron shell (can hold 2 electrons) Outer electron shell (can hold 8 electrons) Electron Hydrogen (H) Atomic number  1 Carbon (C) Atomic number  6 Nitrogen (N) Atomic number  7 Oxygen (O) Atomic number  8 Figure 2.6 The rule really is to fill one level before you move to the next level Figure 2.6 Atoms of the four elements most abundant in living matter 21

22 Practice By definition all atoms of carbon have exactly six ______________ , but the number of ______________ varies from one isotope to another. As two atoms approach each other, which subatomic particles will first come into contacts? By definition all atoms of carbon have exactly six_______________ , but the number of _________________ varies from one isotope to another. As two atoms approach each other, which subatomic particles will first come into contacts?

23 Chemical Bonding and Molecules
In chemical reactivity, a toms tend to - complete a partially filled valence or - empty a partially filled valence Chemical reactions enable atoms to give up or acquire electrons, in order to complete their outer shells. Chemical reactions usually result in atoms staying close together and being held together by attractions called chemical bonds. EIGHT is GREAT. Will steal or share electrons to get eight valance electrons © 2013 Pearson Education, Inc. The structure of the atoms is really the beginning What is really interesting is how they combine 23

24 Chemical bonding between atoms
Covalent bonds are formed when atoms share electrons Non polar covalent bonds = Equal attraction; ie: H2 Polar covalent bonds = Assymetric attraction; ie: HO2 Ionic bonds are attractions between ions of opposite charge steal electrons from other = Unequal attraction Hydrogen bonds form between a positively charged hydrogen atom and any other negatively charged atom van der Waals interactions form between negatively charged and positively charged regions in molecules.

25 Sodium chloride (NaCl)
Ionic Bonds When an atom loses or gains electrons, it becomes electrically charged. Charged atoms are called IONS. Ionic bonds are formed between oppositely charged ions. steal electrons from other = Unequal attraction The outer electron is stripped from sodium and completes the chlorine atom’s outer shell Outer shell has 1 electron Complete outer shells Outer shell has 7 electrons The attraction between the ions—an ionic bond—holds them together Na Sodium ion Cl Chlorine ion Na Sodium atom Cl Chlorine atom Sodium chloride (NaCl) Figure 2.7 Electron transfer and ionic bonding (step 1)

26 Calcium chloride molecule is an ionic compound formed by an ionic bond between calcium ion and two chloride ions.

27 Covalent Bonds A covalent bond forms when two atoms share one or more pairs of outer-shell electrons. Non polar covalent bonds = Equal attraction (H2) Polar covalent bonds = Asymmetric attraction (HO2 Covalent bonds are the strongest of the various bonds. Covalent bonds hold atoms together in a molecule. The number of covalent bonds an atom can form is equal to the number of additional electrons needed to fill its outer shell. © 2013 Pearson Education, Inc. 27

28 Common ways to represent molecules
Electron configuration Structural formula Space-filling model Ball-and-stick model H H Hydrogen gas (H2) O O Double bounds Oxygen gas (O2) H H C H H Overlapping of orbitals/No + or – charges Figure 2.8 Methane (CH4) Figure 2.8 Alternative ways to represent molecules A molecular formula, such as H2O or CH4 tells you the number of each kind of atom in a molecule but not how they are attached together. This figure shows 4 common ways of representing the arrangement of atoms in molecules. Electron configuration: shows how each atom completes its outer shell by sharing e- Structural formula: represents each covalent bond with a line Space-filling model shows the shape of a molecule by symbolizing atoms with color coded balls 28

29 Hydrogen Bonds Water is a compound in which the electrons in its covalent bonds are shared unequally This causes water to be a polar molecule, one with opposite charges on opposite ends (+ poles versus - poles) Oxygen has stronger “attraction” for the electrons than hydrogen and therefore “higher electronegativity” H O slightly  slightly – H O

30 Hydrogen Bonds The polarity of water results in weak electrical attractions between neighboring water molecules. These interactions are called hydrogen bonds. Hydrogen bond

31 Practice What happens when the outermost electron shell of an element such as neon contains the maximum number of eight electrons? It becomes extremely unstable and reactive It becomes stable and inert It forms covalent bonds in an aqueous solution An atom with an unfilled outer shell is extremely unreactive likely to bond with another atom unable to bond with another atom polar

32 Practice The number of protons of an atom of an element determines its
weight isotopic status mass number tendency to form ionic bonds atomic number Fluorine’s atomic number is 9 and its mass number is 19. which of the following is true of fluorine? It contains 9 protons and 10 neutrons It contains 19 protons and 9 neutrons It has no net charge when it has 19 electrons It contains 9 protons and 19 neutrons It is lighter than helium

33 Chemical Reactions Chemical reactions occurs when: One substance combine with another One substance is broken down into another Chemical bonds are broken and other bonds form Cells constantly rearrange molecules by breaking existing chemical bonds and forming new ones. Such changes in the chemical composition of matter are called chemical reactions. Reaction between oxygen gas & hydrogen gas that forms water. Chemical reactions include reactants, the starting materials, and products, the end materials. Chemical reactions can rearrange matter but cannot create or destroy matter. One of these three things has to happen in order for a chemical reaction to occur 33

34 + 2 H2 O2 2 H2O Hydrogen gas Oxygen gas Water Reactants Products 34
Figure 2.UN03 2 H2 + O2 2 H2O Hydrogen gas Oxygen gas Water Reactants Products Figure 2.UN03 In-text figure, chemical reactions, p. 28 34

35 Water: The Molecule That Supports All of Life
Water is the biological medium on Earth - All life occurs in water The abundance of water is the main reason Earth is habitable All living organisms require water more than any other substance Most cells are surrounded by water, and cells themselves are about 70–95% water Water is the only common substance in the natural environment that exists in all three physical states of matter: solid, liquid, and gas. Water is a reactant in many of the chemical reactions of life. Water is the substance that makes life on Earth possible Life is based on water because it is a great solvent It dissolves other substances Life on Earth began in water and evolved there for 3 billion years. Modern life remains tied to water. Your cells are composed of 70–95% water. The abundance of water is a major reason Earth is habitable.

36 Figure 2.10 Figure 2.10 A watery world 36

37 Water’s Life-Supporting Properties
The polarity of water molecules and the hydrogen bonding that results explain most of water’s life-supporting properties. Water molecules stick together forming weak attractions between partially negative -O and partially positive +H of another polar molecule. Water has a strong resistance to change in temperature. Frozen water floats Water is a common solvent for life. © 2013 Pearson Education, Inc. 37

38 The Cohesion of Water Because of small size, bent shape and polarity, Water molecules stick together as a result of hydrogen bonding This tendency of molecules of the same kind to stick together is called Cohesion Cohesion is vital for the transport of water from the roots to the leaves of plants. Evaporation from the leaves Microscopic water- conducting tubes Cohesion due to hydrogen bonds between water molecules Flow of water Colorized SEM Water has other interesting properties Figure 2.11 Cohesion and water transport in plants 38

39 The Cohesion of Water Surface tension is the measure of how difficult it is to stretch or break the surface of a liquid. Hydrogen bonds give water an unusually high surface tension. Figure 2.12 © 2013 Pearson Education, Inc. The cumulative strength of hydrogen bonds between water molecules allows this spider to walk on pond water without breaking the surface. 39

40 2. How Water Moderates Temperature
Because of hydrogen bonds water has a strong resistance to temperature change. Heat and temperature are related, but different. Heat is the amount of energy associated with the movement of the atoms and molecules in a body of matter. Temperature measures the intensity of heat. Water can absorb and store large amounts of heat while only changing a few degrees in temperature. Water can moderate temperatures. Earth’s giant water supply causes temperatures to stay within limits that permit life. Evaporative cooling occurs when a substance evaporates and the surface of the liquid remaining behind cools down removes heat from Earth and from organisms © 2013 Pearson Education, Inc. Water has the ability to moderate temperature Metal pot – water heats up much more slowly than the pot. The swimmer in the gulf of mexico has higher temperature than the water but the bay contains far more heat because of its immense volume. When water is heated, the heat energy is first used to disrupt the hydrogen bond rather than raise the temperature, water absorbs and stores a large amount of heat while warming up only a few degrees. Earth’s giant water supply causes temperatures to stay within limits that permit life by storing a huge amount of heat from the sun during warm period and giving off heat to warm the sir during cold conditions. 40

41 Evaporative cooling When a substance evaporates by changing the physical state from a liquid to gas , the surface of the liquid remaining behind cools down. The molecules with the greatest energy (hottest one) tend to vaporize first Evaporative cooling removes heat from the Earth and from organisms.

42 3. The Biological Significance of Ice Floating
When water molecules get cold enough, they move apart (the movement of its molecules slows), forming ice. - A chunk of ice has fewer molecules than an equal volume of liquid water. Ice floats because it is less dense than the liquid water around it. - the water molecules are spaced out more than in liquid water Oceans & lakes don’t freeze solid b/c surface ice insulates water below allowing life to survive the winter If ice sank, ponds, lakes, & even oceans would freeze solid Life in water could not survive if bodies of water froze solid.

43 43 Hydrogen bond Liquid water Ice Hydrogen bonds constantly break
and re-form. Stable hydrogen bonds hold molecules apart, making ice less dense than water. Figure 2.14 Figure 2.14 Why ice floats 43

44 4. Water as the Solvent of Life
A solution is a liquid consisting of a homogeneous mixture of two or more substances. The dissolving agent is the solvent (medium of for chemical reaction). The dissolved substance is the solute (table salt ). When water is the solvent, the result is an aqueous solution Water dissolve an enormous variety of solutes necessary for life. Hydrophilic substance dissolves in water, have attraction to water (polar molecules) Hydrophobic substance that don’t have an attraction to water (non- polar molecules, oil and butter) Water is the solvent inside all cells, in blood, and in plant sap. © 2013 Pearson Education, Inc. 44

45 Polarity makes H2O a good solvent
polar H2O molecules surround + & - ions Sodium ion in solution Chloride ion in solution Na Cl– Na Cl– Salt crystal Figure 2.15 Figure 2.15 A crystal of table salt (NaCl) dissolving in water 45

46 Practice Surface tension and capillary action occur in water because it has hydrogen bonds has ionic bonds is dense is nonpolar is wet In the reaction glucose + fructose sucrose + water, _____ is a reactant and _________ is a product.

47 Practice Why does ice float on water?
Ice has a higher pH than water Molecules of water are spaced father apart than molecules of ice Ice has a lower pH than water Molecules of ice are spaced father apart than molecules of water Cocoa is mixed with hot water to make hot chocolate. The cocoa is the ______, the hot water is the _____, and the hot chocolate is the _________ . solute ……..…...solvent ……… solution solvent ……..… solution ……..solute solution………... solvent ………...solute solvent ……....… solute……… solution solute……..…... solution ….…....solvent

48 Practice The hydrogen bond between two molecules arises because water is polar hydrophobic a liquid nonpolar Which of the following is not a property of water? polarity a good solvent cohesion nonpolarity store heat well

49 Practice Which of the following pH values indicates a slightly acidic solution? 9 7 3 11 6.5 H2SO4 can ionize to yield two H+ ions and one ion. H2SO4 is molar an acid a base a solution a buffer

50 Practice A basic solution contains
more OH- ions than H+ ions the same number of OH- ions as H+ ions no OH- ions more H+ ions than OH- ions Which of the following has the greatest concentration of hydrogen ions? baking soda at pH 9 Household ammoniac at pH 11 human blood at pH 7 black coffee at pH 5

51 Practice Which of these refers to atoms with the same atomic number but different atomic masses? these atoms are different elements these atoms have different numbers of elements these atoms are isomers these atoms are isotopes these atoms have different numbers of protons An ionic bond involves______. the sharing of a single pair of electrons an attraction between ions of opposite charge no atoms other than sodium and chlorine the unequal sharing of an electron pair water avoidance

52 Practice Which of these refers to atoms with the same atomic number but different atomic masses? these atoms are different elements these atoms have different numbers of elements these atoms are isomers these atoms are isotopes these atoms have different numbers of protons An ionic bond involves______. the sharing of a single pair of electrons an attraction between ions of opposite charge no atoms other than sodium and chlorine the unequal sharing of an electron pair water avoidance

53 Practice What determines the types of chemical reactions that an atom participates in ? its atomic mass the number of electrons in the innermost electron shell the number of electrons in the outermost electron shell the number of protons it contains its atomic number An unchanged atom of boron has an atomic number of 5 and an atomic mass of 11. How many protons does boron have?

54 Practice The unequal sharing of electrons within a water molecule makes the water molecule __________. In a neutral solution the concentration of _________. hydrogen ions is equal to the concentration of hydroxide ions water molecules is less than the concentration of hydrogen ions hydrogen ions is less than the concentration of hydroxide ions water molecules is less than the concentration of hydroxide ions hydrogen ions is greater than the concentration of hydroxide ions

55 Practice Which of the following substance is a substance that resists changes in pH? acid buffer base Solvent solution What is the atomic number of an atom that has 6 protons, 6 neutrons, and 6 electrons?

56 Acids, Bases, and pH 14 Oven cleaner A chemical compound that releases H+ to a solution is an acid. A compound that accepts H+ and removes them from solution is a base. To describe the acidity of a solution, chemists use the pH scale. To describe the relative amount of H+ in solutions, we use the pH scale OH− 13 OH− Household bleach OH− OH− H OH− OH− H 12 Household ammonia Basic solution 11 Lower H concentration Milk of magnesia 10 9 Seawater 8 OH− H Human blood OH− H OH− 7 [H]  [OH−] Pure water H OH− H 6 Urine Neutral solution 5 Black coffee Tomato juice 4 Greater H concentration 3 Grapefruit juice, soft drink H H 2 Lemon juice, stomach acid OH− H H H OH− H Battery acid 1 Acidic solution pH scale Figure 2.17 Figure 2.17 The pH scale 56

57 57 pH 14 Basic Lower H concentration Neutral pH 7 [H]  [OH−]
H concentration equal to OH− concentration Greater H concentration Acidic pH 0 pH scale Figure 2.UN10 Figure 2.UN10 Summary of Key Concepts: Acids, Bases, and pH Most of the water molecules in aqueous solutions within the organisms are intact. However, some molecules disassociates and the balance of these 2 ions is critical for the proper functioning of chemical process within organism. 57

58 Acids, Bases, and pH Biological fluids contain buffers which are substances that resist pH change accept H+ ions when they are in excess and donate H+ ions when they are depleted Buffers & Cellular regulation - pH of cells must be kept at ~ 7 - pH affects shape of molecules - shape of molecules affect function - therefore pH affects cellular function © 2013 Pearson Education, Inc. Even a slight change in pH can be harmful to organisms because the molecules in cells are extremely sensitive to H and OH ocncentration. The CO2 released into the atmosphere is largely absrobed by oceans and when it combines with water, it produces an acid. This acidification caused coral reef collapse and widespread death of marine organisms. 58

59 Increases in global CO2 concentrations may lead to
the acidification of the oceans and - ecological disasters Figure 2.18 Figure 2.18 The release of volcanic CO2, causing undersea bubbling off the coast of Italy 59

60 Can Exercise Boost Your Brain Power?
Observation: Human brains shrink as we age. Question: Can aerobic exercise slow or reverse brain loss? Hypothesis: MRI (magnetic resonance imaging) scans would reveal differences between people who regularly exercised aerobically and those who did not. Prediction: Brains of active people would shrink less than the brains of less active people. Experiment: Twenty-nine people in their 60s and 70s exercised for three one-hour sessions per week. A control group of 29 people engaged in non-aerobic stretching exercises for the same periods. Results: The aerobic group showed significant increases in brain volume compared to the non-aerobic group We have discussed 4 special properties of water. Next we will consider one way that water’s unique structure can reveal important clues about the human brain. Biologist use different imaging techniques to reveal the inner structure of the body. One is MRI that depends on the behavior of H atoms in water molecules. The radio signals released by H are picked up by MRI scanner and projects the image in the computer. Water molecules stick together. Water has a strong resistance to change in temperature. Frozen water floats. Water is a common solvent for life

61 Figure 2.UN04 Figure 2.UN04 In-text figure, aerobic exercise, p. 31 61

62 The colors highlight brain regions that grew in aerobic exercise participants compared to a control group of non-aerobic exercises. Figure 2.16 Figure 2.16 The effect of aerobic exercise on brain size 62

63 Fluoride in the Water Fluoride Fluoride prevents cavities by
Is a common ingredient in Earth’s crust Helps maintain healthy teeth Fluoride prevents cavities by Affecting the metabolism of oral bacteria Promoting the replacement of lost minerals on the tooth surface © 2010 Pearson Education, Inc. Frequent exposure to small amounts of fluoride is a best way to prevent tooth decay The ability to dissolve fluoride in addition to many other substance is another example of water’s solvent property that makes water vital for life on earth.

64 Evolution Connection: The Search for Extraterrestrial Life
If life similar to ours has evolved elsewhere in the universe, then it too would depend upon water. Researchers at NASA missions have found evidence that water was once abundant on Mars. Microbial life may exist below the Martian surface. © 2013 Pearson Education, Inc. 64

65 Attraction between oppositely charged ions holds the ions in
Figure 2.UN06 Attraction between oppositely charged ions holds the ions in an ionic bond. Li F− Lithium ion Fluoride ion Lithium fluoride (ionic compound) Figure 2.UN06 Summary of Key Concepts: Chemical Bonding and Molecules (Ionic Bonds) 65

66 Atoms joined into a molecule
Figure 2.UN07 Electron sharing H O H Atoms joined into a molecule via covalent bonds Figure 2.UN07 Summary of Key Concepts: Chemical Bonding and Molecules (Covalent Bonds) 66

67 67 Hydrogen bond Figure 2.UN08
Figure 2.UN08 Summary of Key Concepts: Chemical Bonding and Molecules (Hydrogen Bonds) 67

68 Liquid water Hydrogen bonds constantly break and re-form. 68
Figure 2.UN09a Liquid water Hydrogen bonds constantly break and re-form. Figure 2.UN09 Summary of Key Concepts: Water’s Life-Supporting Properties (part 1) 68

69 Ice Stable hydrogen bonds hold molecules apart, making ice
Figure 2.UN09b Ice Stable hydrogen bonds hold molecules apart, making ice less dense than water. Figure 2.UN09 Summary of Key Concepts: Water’s Life-Supporting Properties (part 2) 69

70 Figure 2.UN11 Figure 2.UN11 Self-Quiz, question 6 70

71 F K Fluorine atom Potassium atom 71 Figure 2.UN12
Figure 2.UN12 The Process of Science, question 12 71