2. Atoms, Chemical Bonds and Carbon

2. Atoms, Chemical Bonds and Carbon - Unit 2a Atoms,...

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Unit 2a Atoms, Chemical Bonds, Carbon atoms - neutrons, protons, electrons - atomic mass, atomic number electrons – orbitals, shells unpaired electrons and reactivity chemical bonds – covalent (polar, non-polar), non-covalent, ionic water - polarity, hydrogen bonding - dissociation, pH scale, acid-base reactions energy - kinetic, potential - exergonic and endergonic reactions 1
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What are cells made of? four types of atoms make up 96% of all matter found in living organisms carbon, hydrogen, oxygen, nitrogen (C, H, O, N) mostly combined in complex ‘macromolecules’ also present in simple forms - CO2, H2O objective of Unit 2a: What features of these atoms allow them to serve as biological building blocks? 2
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Parts of an Atom nucleus – dense core in centre, consists of protons and neutrons electrons – continually orbit the nucleus - # of protons – defining feature of an element = atomic number - # protons + # neutrons = mass of an atom = mass number 3 Hydrogen Carbon nucleus electron proton neutron + + + Fig 2.1a, Freeman
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Atomic Mass mass of a neutron or proton is approximated at one ‘atomic mass unit’ or dalton masses of electrons tiny  ignored 4 He 4 2 Na 23 11 H 1 1 C 12 6 atomic number mass number practice opportunity: How many protons does helium (He) have? How many neutrons does hydrogen have? And helium? Which is the ‘heaviest’ atom of these four?
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Electrons travel around atomic nuclei in ‘orbitals’. orbitals are grouped into layers or shells , based on how far the electrons in that shell travel from the nucleus 5 max 2 e- max 8 e- max 8 e-
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Electrons, shells and valence 6 innermost shells fill first once first shell filled with pair of electrons, next shells fill with 4 singles then subsequent electrons form pairs outermost ‘valence’ shell influences an atom’s reactivity electrons in outermost shell  valence electrons ‘valence’ of an atom refers to number of unpaired electrons in its valence shell C 12 6 carbon: 4 unpaired electrons, valence = 4 Cl 35 17 chlorine: 1 unpaired electrons, valence = 1 Ar 40 18 argon: no unpaired electrons, valence = 0
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Unpaired Valence Electrons and Reactivity 7 Fig 2.3 - completely filled valence shells  non-reactive (stable) eg He, Ne, Ar - closest to filling valence shell  most reactive eg Cl, Fl, O - atoms with same # valence electrons have similar chemical behaviour - elements abundant in organisms have at least one unpaired valence electron blue – abundant in biological systems
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Unpaired Electrons and Biological Reactions biological reactions are driven by tendency of atoms to  fill outer shells  balance positive and negative charges 8 How can atoms achieve full valence shells? sharing electrons - forming chemical bonds
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This note was uploaded on 03/01/2012 for the course BIOL 130 taught by Professor Bols during the Spring '08 term at Waterloo.

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2. Atoms, Chemical Bonds and Carbon - Unit 2a Atoms,...

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