Chapter 2

1. Chapter 2 • Atoms, Molecules, and Life

2. What Are Atoms? • Smallest unit of an element • Atoms are the fundamental structural units of matter and are composed of three types of particles e electron shell e p p n p n atomic nucleus e (a) Hydrogen (H) (b) Helium (He)

3. What Are Atoms? • Elements • An element is a substance that cannot be broken down by ordinary chemical reactions • All atoms belong to one of 92 types of naturally occurring elements

4. What Are Atoms? • The atomic mass of an element is the total mass of its protons, neutrons, and electrons • Atomic number - The number of protons in the nucleus of an atom • is the defining value for an element • All atoms of an element have the same atomic number • For example, carbon has six protons, nitrogen has seven

5. Table 2-1

6. What Are Atoms? • Isotopes • Atoms of an element with different numbers of neutrons • Different mass number • Some isotopes are radioactive (meaning that they spontaneously break apart, forming different atoms and releasing energy) and are used in research • At room temperature, elements may occur as solids, liquids, or gases

7. What Are Atoms? • Electron shells • Electrons are distributed around the nucleus of an atom in electron shells • The first shell, or energy level, holds two electrons • Subsequent shells holds up to eight • Larger atoms can accommodate more electrons than smaller ones can 2e 8e 5e 4e 6e 8e 8e 2e 2e 2e 2e 6p 8p 15p 20p 6n 8n 16n 20n Carbon (C) Oxygen (O) Phosphorus (P) Calcium (Ca) Ca O C P

8. What Are Atoms? • Energy capture and release • Life depends on electrons capturing and releasing energy • Electron shells correspond to energy levels 3 The electron drops back into lower-energy shell, releasing energy as light 2 The energy boosts the electron to a higher-energy shell 1 An electron absorbs energy heat energy  light     

9. How Do Atoms Interact to Form Molecules? • Interaction between atoms • Atoms will not react with other atoms if the outermost shell is completely empty or full (such atoms are considered inert) • Example: Neon, with eight electrons in its outermost shell is full • Atoms will react with other atoms if the outermost shell is partially full (such atoms are considered reactive) • Example: Oxygen, with six electrons in its outermost shell, can hold two more electrons, and so is susceptible to reacting

10. How Do Atoms Interact to Form Molecules? • Chemical bonds hold atoms together in molecules • the force of attraction between neighboring atoms that holds them together within a molecule • Reactive atoms gain stability through electron interactions (chemical reactions) • A chemical reaction is a process by which new chemical bonds are formed or existing bonds are broken, converting one substance into another • Three major types of chemical bonds: ionic, covalent, and hydrogen

11. Chemical Bonds Sodium atom (neutral) Chlorine atom (neutral) – – – – – – – – – – – • Ions and ionic bonds • Atoms that have lost or gained electrons, thereby altering the balance between protons and electrons, are charged, and are called ions • Oppositely charged ions that are attracted to each other are bound into a molecule by ionic bonds • Salt crystals are formed by a repeated, orderly arrangement of sodium (Na+) and chloride (Cl-) ions – – – – – – – 11p 11n 17p 18n – – – – – – – – – – Electron transferred (a) Neutral atoms Sodium ion () Chloride ion (–) – – – – – – – – – – – – – – – – – – 11p 11n 17p 18n – – – – – – – – – – Attraction between opposite charges (b) Ions Na Cl Cl Na Cl Na Na Cl Cl (c) An ionic compound: NaCl

12. Chemical Bonds • Covalent bonds • form between uncharged atoms that share electrons • An atom with a partially full outermost electron shell can become stable • found in H2 (single bond), O2 (double bond), N2 (triple bond), and H2O • stronger than ionic bonds but vary in their stability

13. Chemical Bonds • Nonpolar or polar covalent bonds • Nonpolar covalent bond – both atoms exert the same pulling force on bond electrons (H2) • Polar covalent bonds - molecules where atoms of different elements are involved (H2O), the electrons are not always equally shared • H2O is a polar molecule • Slightly positively charged pole is around each hydrogen • Slightly negatively charged pole is around the oxygen

14. Covalent Bonds Involve Shared Electrons (oxygen: slightly negative) (–) Larger positive charge _ _ _ _ _ Same charge on both nuclei Electrons spend more time near the larger nucleus _ _ _ _ 8p 8n _ _ + + _ + + Smaller positive charge Electrons spend equal time near each nucleus (hydrogens: slightly positive) (hydrogens: uncharged) (+) (+) (a) Nonpolar covalent bonding in hydrogen gas (H2) (b) Polar covalent bonding in water (H2O) Fig. 2-6

15. How Do Atoms Interact to Form Molecules? • Free radicals • Some cellular reactions produce free radicals • A free radical is a molecule in which atoms have one or more unpaired electrons in their outer shells • Free radicals are highly unstable and reactive • Free radicals steal electrons, destroying other molecules • Cell death can occur from free radical attack

16. Chemical Bonds • Hydrogen bonds • are attractive forces between polar molecules • Hydrogen bonds form when partial opposite charges in different molecules attract each other • The partially positive hydrogen atoms of one water molecule are attracted to the partially negative oxygen on another • Polar biological molecules can form hydrogen bonds with water, each other, or even within the same molecule • Hydrogen bonds are comparatively weak but, collectively, can be quite strong

17. Why Is Water So Important to Life? • Water molecules attract one another • Cohesion is the tendency of the molecules of a substance to stick together • Hydrogen bonding between water molecules • Cohesion of water molecules along a surface produces surface tension • tendency for a water surface to resist being broken

18. Why Is Water So Important to Life? • Water interacts with many other molecules • Water is an excellent solvent (completely surrounds and disperses individual atoms) • A wide range of substances dissolvein water to form solutions

19. Why Is Water So Important to Life? • Water interacts with many other molecules • Water-soluble molecules are hydrophilic • Water molecules are attracted to and can surround • Dissolve readily in water • Water-insoluble molecules are hydrophobic • repel and drive together uncharged and nonpolar molecules like fats and oils • The “clumping” of nonpolar molecules is called hydrophobic interaction

20. Why Is Water So Important to Life? • Water moderates the effects of temperature change • The energy required to heat 1 gram of a substance by 1°C is called its specific heat • It takes a lot of energy to heat water • Temperature reflects the speed of molecular motion • It requires 1 calorie of energy to raise the temperature of 1g of water 1°C (the specific heat of water), which is a very slow process

21. Why Is Water So Important to Life? • Water moderates the effects of temperature change • The heat of vaporization is the amount of heat needed to cause a substance such as water to evaporate (to change from a liquid to a vapor) • Evaporating water uses up heat from its surroundings, cooling the nearby environment (as occurs during sweating) • It takes a lot of energy to cause water to evaporate • Because the human body is mostly water, a sunbather can absorb a lot of heat energy without sending her/his body temperature soaring

22. Why Is Water So Important to Life? • Water forms an unusual solid: ice • Most substances become denser when they solidify from a liquid • Ice is unusual because it is less dense than liquid water • Water molecules spread apart slightly during the freezing process

23. Why Is Water So Important to Life? • Water-based solutions can be acidic, basic, or neutral • A small fraction of water molecules are ionized: H2O  OH– + H+ ( ) (  ) H  O O H H H water (H2O) hydrogen ion (H) hydroxide ion (OH)

24. Why Is Water So Important to Life? • Water-based solutions can be acidic, basic, or neutral • Solutions where H+ > OH– are acidic • Substance that releases H+ into solution • For example, hydrochloric acid ionizes in water: HCl H+ + Cl– • Lemon juice and vinegar are naturally occurring acids

25. Why Is Water So Important to Life? • Water-based solutions can be acidic, basic, or neutral • Solutions where OH– > H+ are basic • Substance that removes H+from solution • For example, sodium hydroxide ionizes in water: NaOH Na+ + OH– • Baking soda, chlorine bleach, and ammonia are basic

26. The degree of acidity of a solution is measured using the pH scale household ammonia (11.9) 1 molar hydrochloric acid (HCI) chlorine bleach (12.6) 1 molar sodium hydroxide (NaOH) "acid rain" (2.5–5.5) drain cleaner (14.0) oven cleaner (13.0) vinegar, cola (3.0) washing soda (12) seawater (7.8–8.3) blood, sweat (7.4) baking soda (8.4) black coffee (5.0) stomach acid (2) lemon juice (2.3) normal rain (5.6) pure water (7.0) tomatoes (4.5) orange (3.5) antacid (10) urine (5.7) beer (4.1) milk (6.4) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 pH value (HOH) (H <OH) neutral (H OH) 100 10–1 10–2 10–3 10–4 10–7 10–8 10–9 10–10 10–11 10–12 10–14 10–6 10–13 10–5 increasingly basic increasingly acidic Hconcentration in moles/liter

27. Why Is Water So Important to Life? • A buffer helps maintain a relatively constant pH in a solution • A buffer is a compound that accepts or releases H+ in response to a pH change • If the solution becomes too acidic, a buffer accepts (and absorbs) H+which creates an acidic molecule • If the solution becomes too basic, an acidic molecule liberates hydrogen ions to combine with OH– to form water