1. Chapter 2 Notes The Chemical Context of Life

2. Concept 2.1 Organisms are composed of matter: anything that takes up space or has mass Element: a substance that cannot be broken down by chemical reactions Compound: substance consisting of two or more elements in a fixed ratio

3. Concept 2.1 Life requires about 25 elements 4 of those make up 96% of living matter Carbon, Oxygen, Hydrogen, and Nitrogen Trace Elements are those that are required in only minute quantities -ex. Iron, iodine

4. Concept 2.1

5. Concept 2.2 Atom: smallest unit of matter that still retains the properties of an element Subatomic particles: Protons (+), Neutrons (0), Electrons (-) Protons and Neutrons have a mass of 1 dalton. Electrons have no mass

6. Concept 2.2 Atomic number: # of protons Mass number: sum of protons + neutrons Isotopes: different atomic forms of an element. -ex. Carbon-12 (99%), Carbon-13 (1%), Carbon-14 (<1%)

7. Concept 2.2 C-12 and C-13 are stable. C-14 is unstable, and radioactive. It will decay giving off particles and energy. Carbon-14 will decay into Nitrogen

8. Concept 2.2 An atom’s electrons vary in the amount of energy they possess. Energy: the ability to do work. Potential Energy: energy that matter stores because of its position or location Electrons have potential energy because of their position in relation to the nucleus.

9. (a) A ball bouncing down a flight of stairs provides an analogy for energy levels of electrons Third shell (highest energy level) Second shell (higher energy level) Energy absorbed First shell (lowest energy level) Atomic nucleus Energy lost Concept 2.2

10. The different states of potential energy that electrons have in an atom are called energy levels or electron shells. - the first shell has the lowest energy. The second shell has more than the first, etc. Valence electrons: those in the outermost shell

11. Concept 2.2 Hydrogen 1 H Lithium 3 Li Beryllium 4 Be Boron 5 B Carbon 6 C Nitrogen 7 N Oxygen 8 O Fluorine 9 F Neon 10 Ne Helium 2 He Atomic number Element symbol Electron- distribution diagram Atomic mass 2 He 4.00 First shell Second shell Third shell Sodium 11 Na Magnesium 12 Mg Aluminum 13 Al Silicon 14 Si Phosphorus 15 P Sulfur 16 S Chlorine 17 Cl Argon 18 Ar

12. Atoms will bond with others to gain stability Covalent bonds: sharing of a pair of valence e- by two atoms -ex. Hydrogen atoms will share their electrons. They become H-H Concept 2.3

14. Electronegativity: the attraction of an atom for the electrons of a covalent bond Nonpolar: when the electrons are shared equally Polar: when one atom is bonded to a more electronegative atom

15. Concept 2.3  – – ++ ++ H H O H2OH2O

16. Ionic bonds: when two atoms are so unequal in their attraction for electrons that one atom will strip the electrons from its partner Ion: a charged atom; cation has a positive charge, anion has a negative charge (ca+ion; a negative ion) Compounds formed by ionic bonds are salts

17. Concept 2.3 NaCl Na Sodium atom Chlorine atom Cl

18. Concept 2.3 NaCl Na Cl Na Sodium atom Chlorine atom Cl Na + Sodium ion (a cation) Cl – Chloride ion (an anion) Sodium chloride (NaCl)

19. Concept 2.3 Na + Cl –

20. The advantage of weak bonding is that the contact can be brief Hydrogen bonds: H is covalently bonded to an electronegative atom and attracted to another electronegative atom. -ex. Water and ammonia Concept 2.3

21.   ++ ++    ++ ++ ++ Water (H 2 O) Ammonia (NH 3 ) Hydrogen bond

22. Concept 2.4 Chemical reactions: making and breaking of chemical bonds. - starting material is reactants - ending material is products Chemical equilibrium: point at which reactions offset one another.

23. Concept 2.4