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Chapter 2 Notes The Chemical Context of Life 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 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 Concept 2.1 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 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%) 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 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. Concept 2.2 (a) A ball bouncing down a flight of stairs provides an analogy for energy levels of electrons Third shell (highest energy level) Energy absorbed Second shell (higher energy level) First shell (lowest energy level) Atomic nucleus Energy lost Concept 2.2 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 Concept 2.2 Hydrogen 1H Atomic mass First shell 2 He 4.00 Atomic number Helium 2He Element symbol Electrondistribution diagram Lithium 3Li Beryllium 4Be Boron 5B Carbon 6C Nitrogen 7N Oxygen 8O Fluorine 9F Neon 10Ne Silicon 14Si Phosphorus 15P Sulfur 16S Chlorine 17Cl Argon 18Ar Second shell Sodium Magnesium Aluminum 12Mg 11Na 13Al Third shell Concept 2.3 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 Concept 2.3 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 Concept 2.3 – O H H + H2O + Concept 2.3 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 Concept 2.3 Na Cl Na Sodium atom Cl Chlorine atom Concept 2.3 Na Cl Na Cl Na Sodium atom Cl Chlorine atom Na+ Sodium ion (a cation) Cl– Chloride ion (an anion) Sodium chloride (NaCl) Concept 2.3 Na+ Cl– Concept 2.3 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 + Water (H2O) + Hydrogen bond Ammonia (NH3) + + + 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. Concept 2.4