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1/28/12 Chapter 2: The Chemical Context of Life - Mattero Anything that takes up space and has masso Elements Elements are substances that cannot be broken down to other substances by chemical reactions Essential elements are those that an organism needs to live a healthy life+reproduce Trace elements are required by an organism in only minute quantitieso Compounds Compound is a substance consisting of two or more different elements combined in a fixed ratio- Atomso Smallest unit of matter that still retains the properties of an elemento Subatomic particles Protons and neutrons are in the nucleus with electrons forming a cloud of negative charge around the nucleus Dalton(atomic mass unit or amu) is the unit of measurement for the mass of atoms and subatomic particles(neutrons and protons have a mass of about 1 dalton)o Atomic number Number of protons and the number of electrons Written as a subscript to the left of the symbol for the elemento Mass number Protons+neutrons Approximation of the total mass of an atom(atomic mass) Written as a superscript to the left of the symbol for the elemento Isotopes Different number of neutrons Radioactive isotope is one that is unstable and decays leading to lost particles(could become a different element)o Energy levels of electrons Potential energy is the energy that matter possesses because of its location or structure and matter has a natural tendency to move to the lowest possible state of potential energy Electron shells are where electrons are found and these each have a specific distance and energy level(lowest amount of potential energy is in the first shell) When an electron absorbs energy, it moves to s shell further from the nucleus and when an electron releases energy it moves closer to the nucleuso Chemical behavior of an atom is determined by the distribution of electrons in the atom’s electron shells(mostly depends on the number in the outermost shell) Outermost electrons are the valence electrons and the outermost shell is the valence shell Atoms with the same number of electrons in the valence shell act similarly, and atoms with full valence shells are unreactiveo Electron orbitals Three-dimensional space where an electron is found 90% of the time is called an orbital Each orbital holds no more than 2 electrons- Chemical bondso Attraction between two atoms that results in both atoms having a complete valence shello Covalent bonds Sharing of a pair of valence electrons by two atoms A molecule is two or more atoms held together by covalent bonds Single bond is a pair of shared electrons Double bond is two pairs of shared electrons Valence is an atom’s bonding capacity that is equal to the number of unpaired electronsrequired to complete the atom’s valence shell Electronegativity- The attraction of a particular atom for the electrons of a covalent bond- More electronegative an atom is, the more strongly it pulls shared electrons toward itself- Nonpolar covalent bondo Atoms have the same electronegativityo Occurs between two atoms of the same element- Polar covalent bondo One atom is bonded to a more electronegative atom, electrons are not shared equallyo Higher polarity if difference in electronegativities is highero Ionic bonds Charged atom is an ion(cation when charge is positive, and anion when charge is negative) Any two ions of opposite charge which can occur from the transfer of an electron Compounds formed by ionic bonds are ionic compounds/salts and the formula is not the number of molecules, it is the ratioo Weak chemical bonds Hydrogen bonds- Partial positive charge on a hydrogen atom that is covalently bonded to an electronegative atom allows the hydrogen to be attracted to a different electronegative atom nearby- Occurs between water(H20) and ammonia(NH3) Van der waals interactions- Ever changing regions of partial positive and negative charge that enable all atoms and molecules to stick to one another- Many together have a strong effecto Molecular shape is important for the function of the living cell Shapes are determined by the positions of the atoms’ orbitals Biological molecules often bind temporarily to each other by forming weak bonds, but this can only happen if their shapes are complementary- Chemical reactionso Making and breaking of chemical bonds leading to changes in the composition of mattero Starting materials are reactantso Ending materials are productso All atoms of the reactants must be accounted for in the products because matter is conserved and reactions cannot create or destroy matter but can only reaarange ito Chemical equilibrium Point at which the reactions offset one another exactly(forward and reverse reactions occur at the same rate) Concentrations have stabilized at a particular ratio(reactants and products are NOT equal in


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UMD BSCI 105 - Chapter 2: The Chemical Context of Life

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