Chapter 2 A Chemical Connection to Biology o Biology is the study of life o Living organisms and their environments are subject to basic laws of physics and chemistry o One example is the use of formic acid by wood ants to protect themselves against predators and microbial parasites Concept 2 1 Matter consists of chemical elements in pure form and in combinations called compounds o Organisms are composed of matter o Matter is anything that makes up space and has mass o Element substance that cannot be broken down into other substances by chemical reactions o 92 naturally occurring elements o Each element has a unique symbol Usually the first one of two letters of the name o Compound substance consisting of two or more elements in a fixed ration o Table salt sodium chloride or NaCl Compound with equal numbers of Cl and Na atoms o Pure sodium is a metal and chlorine is a gas Either alone can be deadly o Their combination forms an edible compound Emergent property Too much or too little NaCl may be detrimental to your health Concept 2 2 An element s properties depend on the structure of its atoms o Each element consists of unique atoms o An atom is the smallest unit of matter that still retains the properties of an element Subatomic Particles o Atoms are composed of subatomic particles o Relevant subatomic particles include o Neutrons no electrical charge o Protons positive charge o Electrons negative charge o Neutrons and protons form the atomic nucleus o Electrons forma c cloud around the nucleus o Neutron mass and proton mass are almost identical and are measured in Daltons Atomic Number o Atomic number number of protons in nucleus o Example 2 He o Unique for each element o Neutral uncharged atom Fewer or more protons DIFFERENT ELEMENT Atomic number number of protons number of electrons Number of protons number of electrons How much does an atom weigh o Neutron mass proton mass 1 7 x 10 24 gram o 1 Dalton or one atomic mass unit 1 amu o Electron mass 1 2000th that of a neutron or proton o Ignore electrons to estimate atom s total mass o Mass number number of protons number of neutrons in nucleus o Number of neutrons mass number atomic number o Atomic weight mass number Isotopes o Isotopes two atoms of the same element that have atomic number same number of protons o Same o Different o Example If the number of protons is different then you have a different element mass number different number of neutrons o In nature an element occurs as a mixture of isotopes 99 of carbon atoms have 6 neutrons 12 C Most of the remaining 1 of carbon atoms have 7 neutrons 13 C Rarest isotope with 8 neutrons is 14 C o Stable isotopes most isotopes are stable and do not tend to loose particles o Both 12 C and 13 C are stable isotopes o Radioactive isotopes nuclei of some isotopes are unstable and decay spontaneously emitting particles and energy o 14 C is a radioactive isotope o When 14 C decays a neutron is converted to a proton and an electron Converts 14 6 C to 14 7 N Same mass number 14 New atomic number 7 new element N 1 more proton 1 less neuron o Relative size if an atom was the size of Doak Campbell Stadium o Nucleus would be the size of a pencil eraser o Electrons would be like gnats buzzing around the stadium o Atoms are mostly empty space o When wo elements interact during a chemical reaction it s the electrons that are involved o Nuclei do not come close enough to interact Energy Levels of an atom s electrons o Energy ability to do work o Electrons of an atom may process different amounts of energy o Potential energy PE energy that matter stores because of its position or location o Ball at the top of steps has more PE o Ball at the bottom of steps has less PE o Electrons have PE because of their position relative to the nucleus o Negatively charged electrons are attracted to the positively charged nucleus o Electrons farther from nucleus have more PE o Electron shells o Different energy levels PE states of an atom s electrons o First shell Closest to the nucleus lowest PE o Outer shell Further from the nucleus electrons have more PE o Electrons can move to another shell only if the absorb or release a quantity of energy that matches the difference in PE between the 2 levels o First electron shell can hold only 2 electrons 2 e of He fill the first shell o Atoms with more than 2 e must place the extra e in higher shells Example Lithium Li 3 total e 2 e in first shell 1 e in second shell o Second electron shell can hold up to 8 e Example Neon Ne 10 total e 2 e in first shell 8 e in the second shell 1st and 2nd shells filled o Chemical behavior of an atom is determined by its electron configuration the distribution of electrons in its electron shells The first 18 elements including those most important in biological processes can be arranged in 8 columns and 3 rows part of periodic table Moving from left to right each element has a sequential addition Elements in the same row use the same shells of electrons and protons Valence o Valence shell an atom s outermost shell o Valence electrons electrons in the valence shell o Chemical behavior of an atom depends mostly on the number of valence electrons shells o Atoms with the same number of valence electrons have similar chemical behavior o Columns of periodic table o An atom with a completed valence shell is inert unreactive o Noble gasses o All other atoms are chemically reactive because they have incomplete valence o Valence bonding capacity of an atom usually the number of unpaired e to fill the valence shell o Examples the valence of o Hydrogen 1 o Oxygen 2 o Nitrogen 3 o Carbon 4 o Exception phosphorus Expect valence of 3 3 unpaired electrons BUT In biological molecules it generally has a valence of 5 forming three single covalent bonds and one double bond Electron Orbitals o An orbital is the three dimensional space where an electron is found 90 of the time o Each electron shell consists of a specific number of orbitals Concept 2 3 The formation and function of molecules depend on chemical bonding between atoms o Atoms with incomplete valence shells can share or transfer valence electrons with o These interactions usually result in atoms staying close together held by attractions certain other atoms called chemical bonds Covalent bonds o A covalent bond is the sharing of a pair of valence electrons by two atoms o In a covalent bond the shared electrons count as apart of each atom s valence shell o Reactivity due to unpaired electrons in
View Full Document
Unlocking...