Chapter 2 The Chemical Context of Life 1 28 12 Matter o Anything that takes up space and has mass o Elements Elements are substances that cannot be broken down to other substances by chemical Essential elements are those that an organism needs to live a healthy life reproduce Trace elements are required by an organism in only minute quantities Compound is a substance consisting of two or more different elements combined in a reactions o Compounds Atoms fixed ratio o Smallest unit of matter that still retains the properties of an element o 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 o Mass number o Isotopes Number of protons and the number of electrons Written as a subscript to the left of the symbol for the element Protons neutrons Approximation of the total mass of an atom atomic mass Written as a superscript to the left of the symbol for the element 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 nucleus o 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 Atoms with the same number of electrons in the valence shell act similarly and atoms with full valence shells are unreactive o Electron orbitals Three dimensional space where an electron is found 90 of the time is called an shell orbital Each orbital holds no more than 2 electrons Chemical bonds o Attraction between two atoms that results in both atoms having a complete valence shell o 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 electrons required 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 o Atoms have the same electronegativity o Occurs between two atoms of the same element toward itself Nonpolar covalent bond Polar covalent bond shared equally o One atom is bonded to a more electronegative atom electrons are not o Higher polarity if difference in electronegativities is higher o Ionic bonds negative Charged atom is an ion cation when charge is positive and anion when charge is 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 ratio o 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 effect o 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 reactions o Making and breaking of chemical bonds leading to changes in the composition of matter o Starting materials are reactants o Ending materials are products o 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 it o Chemical equilibrium occur at the same rate equal in concentration Point at which the reactions offset one another exactly forward and reverse reactions Concentrations have stabilized at a particular ratio reactants and products are NOT
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