CHAPTER 2 ATOMS AND MOLECULES: The Chemical basis of life. CHEMICALS Non-living building blocks of living things. Provide communication – pheromones, hormones, neurotransmitters. Provide energy. Compose our atmosphere. Love! SUMMARY Elements and the Periodic Table Common elements and functions Atoms and isotopes Special function of electrons Compounds and chemical bonds Moles and molarity Chemical reactions General Oxidation/reduction reactions Water pH, buffers and salts ELEMENTS A substance that cannot be broken down into simpler substances by normal chemical reactions. Each element is unique. Organized in the periodic table by atomic number. Each element is represented by an element key that shows its symbol, average atomic mass and atomic number. THE PERIODIC TABLE ABUNDANT ELEMENTS IN LIVING THINGS Carbon backbone of organic molecules Hydrogen & Oxygen components of water Nitrogen component of proteins and nucleic acids ELEMENTS ARE COMPOSED OF ATOMS Atom Structure Nucleus contains 2 types of subatomic particles: Protons Neutrons Electron “cloud” surrounds the nucleus and contains 1 type of subatomic particle: Electrons ATOM FACTS Each proton and neutron has a mass of 1 AMU or Dalton. Each electron has a mass of 1/1800 AMU or Dalton. Atomic number = Atomic mass = QUESTION If you have the atomic mass and atomic number for an element how do you find out how many Protons the element has? Electrons the element has? Neutrons the element has? ATOMS AND ISOTOPES All elements have atoms with different atomic masses. The atom in the element key is the most abundant. The other atoms are called isotopes. The atomic masses and abundance of each are “averaged” for use in the element key. If all atoms of one element have the same atomic number or number of protons, what causes isotopes to have a different atomic mass? ATOMS AND ISOTOPES Stable isotopes are happy but unstable isotopes are not and try to reduce their mass by releasing energy and radioactive particles. Each isotope “decays” at a certain rate. This is expressed as half-life. Half-life = ELECTRON CLOUD Really orbits that electrons move in around the nucleus. Also called shells. Electron shells represent different energy levels. Electrons in shells close to the nucleus have less energy than those in outer shells. ELECTRON CLOUD The electrons in the outermost shell are called valence electrons and determine the chemical properties of the atom. i.e. if they play with other atoms or not. VALENCE ELECTRONS Atoms like to have 8 electrons in their outer shell – Octet Rule or Rule of Eights. Exception: atoms with only 1 shell are happy with 2 – He and H. If atoms have 8 valence electrons they don’t interact with other atoms. If they have less than eight they will either donate their extra electrons or try to steal electrons from another atom. This also means that they interact with one another. VALENCE ELECTRONS When an atom loses or gains an electron it becomes an ion with an electrical charge. What kind of charge? HINT: Two atoms are walking down the street. Says one atom to the other, “Hey! I think I lost an electron!” The other atom says “are you sure?” The first atom says…. ATOMS AND IONS Atoms that gain an electron become Atoms that lose an electron become Significance: this is the basis for joining atoms together to form compounds and molecules and for harnessing the energy of the sun in photosynthesis. ATOMS AND IONS When two atoms interact, they donate, steal or share valence electrons. When electrons are transferred from one atom to another, one atom becomes a cation and the other becomes an anion. Opposite charges cause an attraction and an ionic bond is formed. When electrons are shared by two or more atoms the bond formed is a covalent bond. IONIC BONDS COVALENT BONDS Some atoms share equally Some atoms share unequally Polar compounds like other polar compounds and non-polar compounds like other non-polar compounds. Example oil and water HYDROGEN BONDS Relatively weak bonds that form between a H atom and usually an O or N atom. MOLECULES AND COMPOUNDS Molecules – Compounds – Chemical formula – shows each atom and the numbers of each atom in the molecule or compound. Ex – CaCl2 NaHCO3 N3 MOLES Used to measure quantities of a substance. For any substance a mole = 6.02 X 1023 atoms, molecules or ions. **Avogadro’s Number. Molecular weight = mass of 1 mole of a substance in grams. Add the atomic masses of each atom in the substance. Ex. What is the molecular weight of NaCl? MOLARITY Used to make solutions with certain quantities of a solute (substance) in 1.0 liter of a solvent, like water. A 1.0 M solution has 1 mole of the substance dissolved in 1 liter of a solvent. CHEMICAL REACTION Involves making and/or breaking chemical bonds. General form: Reactants → Products. 2 basic types: CHEMICAL REACTIONS Important chemical reactions for harvesting energy from substances are called Redox or oxidation-reduction reactions. Electrons (energy) are transferred from a reducing agent to an oxidizing agent. Oxidation – an atom, ion or molecule loses electrons (energy). Reduction – an atom, ion or molecule gains electrons (energy). “Oil Rig” WATER Water is a polar molecule formed by covalent bonds between the 2 H atoms and O. One end is slightly positive and the other end is slightly negative because of unequal sharing of electrons. Because it is polar, water is an excellent solvent for ionic or polar solutes. Oxygen part Hydrogen Partial parts negative charge at Partial oxygen end positive of molecule charge at Hydrogen (H) Oxygen (O) Hydrogen (H) hydrogen end of Water molecule (H2O) molecule Fig. 2-7, p. 34 WATER Water molecules exhibit cohesion (stick to one another) because they form hydrogen bonds with one another. Water molecules exhibit adhesion (stick to other substances) by hydrogen bonding to substances with ionic or polar regions. WATER IS A GOOD SOLVENT BECAUSE OF ADHESION WATER Hydrogen bonds between water molecules expand and make ice less dense than liquid water. Becauseice floats, the aquatic environment is less extreme. Three Phases of Water WATER Water has high specific heat. Hydrogen bonds must break to raise water temperature. Specific heat of water helps: WATER Waterhas a high heat of vaporization. pH Means “parts hydrogen”. An application of molarity. Pure water (H2O) has some free ions H+ and OH-. The concentration of H+ (and OH- ) is 10-7M or 0.0000001M. pH converts the concentration to a whole number. pH = -log of [H+] so… pH H2O = -log of [10-7] The log of 10x = x so… pH H2O = - (-7) or 7 pH Pure water is considered neutral. Buffers Important for living organisms – pH homeostasis. Buffering system - based on a weak acid or a weak base. Buffers - resist changes in pH of a solution when acids or bases are added. Salts Salt- a compound in which the hydrogen atom of an acid is replaced by some other cation. Ex. Na replaces H in HCl to form NaCl. provide many mineral ions Salts essential for life functions.
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