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Chemistry of Life Chapter 2 (2.1 and 2.2) Atomic Structure • Diagram of an Atom: He 2 4 Atomic number the number of protons in an atom Atomic mass the number of protons and neutrons in an atom In a neutral atom the number of electrons = number of protons SUMMARY 1. The Atomic Number of an atom = number of protons in the nucleus. 2. The Atomic Mass of an atom = number of Protons + Neutrons in the nucleus. 3. The number of Protons = Number of Electrons. 4. 5. Electrons orbit the nucleus in shells. Each shell can only carry a set number of electrons. Elements • An element is a substance that consists of only one type of atom. • Examples: Isotopes: Isotope- An atom of an element that has a different number of neutrons than the other atoms of that element. Compounds • composed of 2 or more elements in a fixed ratio • properties differ from those of individual elements • Examples: Ionic Bonding • Ionic bonds form when atoms give and take valence electrons. • Diagram: Covalent Bonding • Covalent bonds form when atoms share electrons to form a full number of valence electrons. • Diagram: Molecules • Molecules are formed when atoms are joined by covalent bonds. – Smallest unit of most compounds Polar and Non-Polar Covalent Bonds • Non-polar covalent bonds – When the electrons are not shared equally – One of the atoms has a greater attraction for the electrons • Diagram: Van der Waals Forces • When atoms or molecules have attractive or repelling forces that are not due to ionic or covalent bonds. Ted Talk - Gecko • http://www.ted.com/talks/robert_full_learni ng_from_the_gecko_s_tail?language=en Properties of Water • Water is a polar molecule. • There is a uneven distribution of electrons Properties of Water • Water forms hydrogen bonds. • Hydrogen bonds form when the positive H atom is attracted to a negative atom on another molecule. Properties of Water • Water is cohesive. – The molecules stick to each other • How does this explain surface tension? http://spaceflightsystems.grc.nasa.gov/WaterBalloo n/#AIRPLANE Properties of Water • Water is adhesive. – attraction between different molecules • Water’s cohesive property and adhesive property allow for capillary action. Why is it important? Properties of Water • Draw a diagram of a meniscus. Label where cohesion and adhesion occur. Properties of Water • Water is a great solvent. • How does the salt and water become a solution? • Why is this property important? Properties of Water • Water retains its temperature. • Why is this important? Properties of Water • When water freezes- it floats. – The density of water is lower in its solid form • Why is this important? Acids and Bases • Water can break apart into H+ and OH• Water is neutral because the number of H+ and OH- ions are equal. Acids and Bases • If a substance has more H+ ions than OHions it is said to be acidic. • If a substance has more OH- ions than H+ ions it is said to be basic. pH Scale • Acidic solutions – Higher concentration of H+ than pure water – pH is below pH7 • Basic solutions – Lower concentration of H+ than pure water – pH is above 7 pH Scale • pH of common items • Each step on the pH scale represents a factor of 10. • Example: Buffers • Buffers – weak acids or bases that react with strong acids or bases to prevent sudden or drastic pH changes. • Why is this important to living organisms?