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The Chemical Basis of Life

Chapter 2

• Objectives• Identify the four elements that make up 96% of living

matter.

• Distinguish between the following pairs of terms: neutron and proton, atomic number and mass number, atomic weight and mass number

• Distinguish between and discuss the biological importance of the following: nonpolar covalent bonds, polar covalent bonds, ionic bonds, hydrogen bonds, and van der Waals interactions

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Organization of Matter

• Matter-anything that occupies space and has mass– Includes solids, liquids and gases– Made up of 92 naturally-occurring elements– Atom-smallest unique unit of element– Molecule-bonded unit of two or more atoms– Compound-substance in which relative

percentages of two or more elements never vary

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• Essential elements– include carbon, hydrogen, oxygen, and nitrogen– make up 96% of living matter

• a few other elements make up the remaining 4% of living matter

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Structure of Atoms

• Each element consists of a certain kind of atom that is different from those of other elements– An atom is the smallest unit of matter that still

retains the properties of an element

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• Atoms of each element are composed of three even smaller parts called subatomic particles– Protons-part of nucleus

• positive charge

• number of protons=atomic number

– Neutrons-part of nucleus• no charge• protons+neutrons=atomic mass

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– Electrons-orbit nucleus• negative charge• number of electrons=number of protons

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• Atoms of a given element may occur in different forms– Isotopes of a given element differ in the number

of neutrons in the atomic nucleus• have the same number of protons

– Radioactive isotopes spontaneously give off particles and energy

• can be used in biology

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The Energy Levels of Electrons

• An atom’s electrons vary in the amount of energy they possess– electron’s have potential energy

• due to position in relation to nucleus (electrostatic)

• represented by energy levels or electron shells

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Electron Configuration and Chemical Properties

• The chemical behavior of an atom is defined by its electron configuration and distribution– the periodic table of the elements shows the

electron distribution for all the elements

• Outermost shell of an atom called the valence shell– electrons in the valence shell called valence

electrons• number of valence electrons determines the chemical

properties of atoms

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Electron Orbitals

• Each energy shell is divided into one or more 3d spaces called orbitals– each orbital can contain only two electrons– the maximum number of electrons an energy shell

can hold is determined by the number of orbitals the energy shell is divided into

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Bonds Between Atoms

• Chemical bond-atoms gain, lose or share electrons– two types of bonds formed between atoms

• covalent bond

• ionic bond

– type of bond formed determined by two factors • electron configuration of the outer or valence shell of

electrons

• electronegativity of the atom– electronegativity is a measure of the attraction of an atom

for electrons

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Covalent Bonds

• Covalent bond– atoms share one or more pairs of electrons

• nonpolar-share electrons equally

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• Polar covalent bonds– share electrons unequally

• more electronegative atom in the covalent bond pulls the electrons toward itself

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Ionic Bonds

• Ionic bond– one atom gains electron, one atom loses electron– + and - charges mutually attractive

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Weak Chemical Bonds

• Several types of weak chemical bonds are important in living systems– Hydrogen Bonding

• atom of molecule interacts with hydrogen already in polar bond

• stabilizes nucleic acids and proteins

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• Van der Waals interactions occur when transiently positive and negative regions of molecules attract each other

• Weak chemical bonds– Reinforce the shapes of large molecules– Help molecules adhere to each other

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Atoms and Molecules

• A molecule’s biological function is related to its shape– molecules have characteristic sizes and shapes

symbolized by ball-and-stick and space-filling models

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• The shape of the molecule is the basis for one molecule recognizing another molecule when they interact

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Chemical Reactions

• Chemical reactions break or form chemical bonds to change reactants into products– matter is conserved during reaction

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• Most chemical reactions are reversible– chemical equilibrium is reached when the forward

and backward reaction rates are equal

• Living cells carry out thousands of chemical reactions that rearrange matter in significant ways

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