Chapter 2: Chemistry & Life

Transcription

1 Chapter 2: Chemistry & Life 1. Atoms 2. Molecules 3. Water

2 1. Atoms

3 A Generic Atom

4 All Matter is made of Atoms Atoms are the smallest indivisible unit of matter 92 different types of atoms: ELEMENTS Atoms are made of: protons (positively charged) neutrons (no charge) nucleus electrons (negatively charged) orbit the nucleus in orbitals within electron shells **# of protons determines identity of atom** **in a neutral atom, # protons = # electrons** (neutrons help stabilize the nucleus)

5 The Periodic Table of Elements atomic number = # of protons atomic mass = # of neutrons & protons

6 Hydrogen & Helium Atoms Atoms of the same element having different numbers of neutrons are different isotopes of that element, some of which are unstable and thus radioactive.

7 Electrons & Chemical Behavior Electrons (e - ) occupy orbitals within e - shells by the following rules: 1) up to 2 e - /orbital, up to 4 orbitals/shell 1 st e - shell holds 1 orbital, up to 2 e - 2 nd, 3 rd, 4 th, e - shells hold 4 orbitals, up to 8 e - 2) an atom s electrons fill shells in order i.e., 1 st shell before 2 nd before 3 rd ea orbital must have 1 e - before any get a 2 nd e - Outermost shell determines chemical behavior (based on the # of e - in outermost shell) **atoms are happiest when shells are full (or empty)**

8 e - Shells of some Larger Elements Unpaired e - (in outermost shell) are the key to chemical behavior: atoms share unpaired e - to fill their orbitals this is the nature of chemical bond formation **elements w/same # of outer shell e - have similar behavior**

9 Some Atoms form Ions gain or lose electrons have a net + or - charge atoms w/shells that are almost full or empty tend to lose or gain e - to avoid having partially filled e - shells

10 Sodium and Chlorine Ions ionic bond sodium easily forms a + ion (cation) by giving up 1 e - chlorine easily forms a - ion (anion) by receiving an e - Now they re both happy! **any charged atom (or molecule) is an ion**

11 2. Molecules

12 So what s a Molecule? 2 or more atoms connected by a chemical bond(s) constitute a molecule. Chemical bonds involve the sharing of a pair of e - 1 unpaired e - from each atom = 1 bond sharing of e - fills shared orbital between the atoms If the pair of e - is shared, the bond is covalent equal sharing = non-polar covalent bond (non-polar mol.) unequal sharing = polar covalent bond (polar molecule) If the pair of e - is not shared at all (one gives, one receives), the chemical bond is ionic e.g., sodium chloride (NaCl)

13 Some Polar & Non-polar Molecules NON-POLAR POLAR hydrogen gas (H 2 ) oxygen gas (O 2 ) water (H 2 O) different elements vary in their attractiveness for e - (electronegativity) **different electronegativities = polar covalent bond**

14 Chemical Bonds involve unpaired e - Atoms of a given element tend to form as many bonds as unpaired e - in outermost shell more than one (up to 3) covalent bond can form between any 2 atoms (i.e., double, triple bonds)

15 3. Water

16 Water is a Polar Molecule Covalent bonds between O & H are polar. Water s polarity gives it unique properties: is a great solvent for polar, charged particles high heat capacity liquid in a broad range of temperature

17 Water forms Hydrogen Bonds water s polar nature makes it sticky due to attraction between partial + & - charges such interactions are called hydrogen bonds

18 Due to interactions between opposing charges in water and other polar or charged molecules water does not dissolve non-polar (hydrophobic) mol. Water is a Good Solvent SOLVENT = liquid in which a substance is dissolved SOLUTE = a substance dissolved in liquid

19 Water can form ions acid acidic [H + ] > [OH - ] base basic [OH - ] > [H + ]

20 The ph of Water is very Important ph & acidic vs basic : ph depends on the balance of hydrogen ions (H + ) & hydroxide ions (OH - ) H + = a hydrogen atom that has lost an e- OH - = a hydroxide group that s gained an e - ph value reflects amount of H + [H + ] > [OH - ] : acidic (ph 1-6) [H + ] = [OH - ] : neutral (ph 7) [H + ] < [OH - ] : basic (ph 8-14) alkaline

21 Acids and Bases Pure water is always neutral: [H + ] = [OH - ],ph 7 When other substances (molecules) are dissolved in water the ph can change: ACID = substance that releases hydrogen ions (H + ) into an aqueous solution increase [H + ], lower the ph value BASE = substance that combines with H + in an aqueous solution reduce [H + ], raise the ph value

22 ph scale is logarithmic ea unit reflects a 10-fold change in [H + ] higher ph value = lower [H + ] ph 3 = 10-3 = [H + ] ph 9 = 10-9 = [H + ] **chemical buffers resist changes in ph**

23 Key Terms for Chapter 2 protons, neutrons, electrons orbitals, electron shells ions, covalent bonds, ionic bonds polar, non-polar, hydrogen bond electronegativity solvent, solute, ph, buffer acid vs base, acidic vs basic Relevant Review Questions: 1, 3-8, 12-16, 18-23