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BIOL 1020 2

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BIOL 1020
Anne- Marie Singh

BIOL 1020 - CHAPTER 2 LECTURE NOTES Chapter 2: The chemical context of life You must understand chemistry to understand life! Overview: In many ways, life can be viewed as a complicated chemical reaction. Modern models of how life works at all levels typically have at least some aspect of chemistry as a major component or underpinning. I. Elements andAtoms A. Elements – substances that cannot be further broken down into other substances (at least by ordinary chemical reactions) 1. Every element has a chemical symbol (H for hydrogen, O for oxygen, etc.); this is most familiar from the periodic table 2. There are 92 naturally occurring elements, from hydrogen up to uranium • 4 elements (oxygen, carbon, hydrogen, and nitrogen = O, C, H, N) make about 96% of the mass of most living things • 7 others are consistently present in small amounts in living things (Ca, P, K, S, Na, Mg, Cl) • Several others are typically found only in trace amounts (trace elements); these tend to vary considerably in amount and even presence depending on the type of organism (Cu, Fe, B, Zn, Co, Se, Mo…) B. An atom is the smallest unit of an element that still retains the properties of that element C. Atoms consist of subatomic particles 1. Electron – • Contributes no significant mass to the atom • Carries a (-1) electrical charge 2. Proton – • Contributes a mass of approximately 1 mass unit • Carries a (+1) electrical charge 3. Neutron – • Contributes a mass of approximately 1 mass unit • Carries no net electrical charge 4. Protons and neutrons are found in the nucleus (center) of an atom 5. Elements differ from each other because they contain different numbers of protons (all hydrogen atoms contain 1 proton, all carbon atoms contain 6 protons, all oxygen atoms contain 8 protons) • atomic number = number of protons in the nucleus• the periodic table has elements arranged largely according to atomic number 6. Protons + neutrons determine atomic mass • Each contribute ~1 atomic mass unit (amu, or Dalton) • Atoms that have the same number of protons but have different numbers of neutrons (therefore science (carbon dating, etc.) • Periodic table arranged by number of protons in the nucleus (atomic number) 7. Atomic mass= protons+ neutrons • Each contribute -1 atomic mass unit (amu of Dalton) • Atoms that have the same number of protons but have different numbers of neutrons (therefore different masses) are referred to as isotopes 8. Atomic nuclei can undergo changes (decay) • Some elements are more stable than others; most unstable= radioisotopes • Decay rates are statistical averages, and are used for measuring time passage in many areas of 9. The radiation emitted upon decay (alpha, beta, and/or gamma) can be used as a tool for experiments; can also be used medically; has other uses and dangers (nuclear power, nuclear bombs, radiation poisoning, etc.) 10.Radiation can cause mutations in DNA, can interfere with cell division D. Electrons occupy orbitals surrounding the nucleus and move at the speed of light 1. BecauseATOMS are electrically neutral the number of electrons an atom has always equals the number of protons 2. Electrons can exist at different energy levels, which correspond to orbitals • The further away an orbital carries an electron from the nucleus, the higher the energy level of the electron Different masses) are referred to as isotopes nuclei can undergo changes (deca D.Atomic 1. Some elements are more stable than others 2. Some isotopes are more stable than others (most unstable = radioisotopes) 3. Decay rates are statistical averages, and are used for measuring time passage in many areas of • Electrons with similar energies make up an electron shell 3. The outer electron(s) are known as the valence electron(s); collectively, they occupy the valence shell 4. The chemical properties of an atom are largely determined by the valence electrons II. DescribingAtomic Combinations A. Atoms combine to form molecules and compounds a. Molecule – two or more atoms held together by covalent bonds (defined later) i.May be composed of one or more elements (examples: O2, H2O) ii.Not all substances are molecular (NaCl, table salt, isn’t) iii.If a substance is molecular, then an individual molecule is the smallest unit of the substance that exhibits the properties of the substance iv. Thus, a molecule differs in its physical and chemical properties from the elements that make it up b. Compound - a specific combination of two or more different elements chemically combined in a fixed ratio • compounds have unique physical and chemical properties that differ from those of the elements used to make it • some compounds are held together by covalent bonds and are therefore molecular; some are held together by ionic bonds (defined later) B. Chemists use two types of formulas to describe substances a. Chemical formula – i. Ashorthand formula showing the number of atoms of each element present in a molecule ii. Often called molecular formula if a molecule is involved; examples: H2O, CO2, O2, C6H12O6 • follows simplest ratio for ionic substances (NaCl, etc.) b. Structural formula - shows the arrangement of atoms in a molecule • examples: water H─O─H carbon dioxide O═C═O molecular oxygen O═O C. The number of units of a substance are described using the mole a. Molecular mass is the sum of the atomic masses of the atoms in the molecule b. Since the actual mass of an atom is extremely small, it is convenient in the real world to work with a large number of atoms at the same time c. The amount of a substance that in grams has the same number as the atomic mass is a mole d. Thus, water has molecular mass 1+1+16 =
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