the chemical context of life. matter consists of chemical elements in pure form and in combinations...

The Chemical The Chemical Context of LifeContext of Life

Matter consists of chemical Matter consists of chemical elements in pure form and in elements in pure form and in combinations called compoundscombinations called compounds

• Organisms are composed of Organisms are composed of mattermatter• Matter is anything that takes up space and Matter is anything that takes up space and

has masshas mass

Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

Elements and CompoundsElements and Compounds

An An elementelement is a substance that cannot be is a substance that cannot be broken down to other substances by chemical broken down to other substances by chemical reactions (pure)reactions (pure) CarbonCarbon OxygenOxygen

A A compoundcompound is a substance consisting of two is a substance consisting of two or more elements in a fixed ratioor more elements in a fixed ratio Sodium chlorideSodium chloride


Fig. 2-3Fig. 2-3

Sodium Chlorine Sodiumchloride

Essential Elements of LifeEssential Elements of Life About 25 of the 92 elements are essential to lifeAbout 25 of the 92 elements are essential to life Carbon, hydrogen, oxygen, and nitrogen make up Carbon, hydrogen, oxygen, and nitrogen make up

96% of living matter96% of living matter Most of the remaining 4% consists of calcium, Most of the remaining 4% consists of calcium,

phosphorus, potassium, and sulfurphosphorus, potassium, and sulfur Trace elements Trace elements are those required by an are those required by an

organism in minute quantitiesorganism in minute quantities


Table 2-1Table 2-1

An element’s propertiesAn element’s propertiesdepend on the structure of its depend on the structure of its atomsatoms

Each element consists of unique Each element consists of unique atomsatoms An atom is the smallest unit of matter that An atom is the smallest unit of matter that

still retains the properties of an elementstill retains the properties of an element


Subatomic ParticlesSubatomic Particles

Atoms are composed of Atoms are composed of subatomic particlessubatomic particles Relevant subatomic particles include:Relevant subatomic particles include:

NeutronsNeutrons (no electrical charge) (no electrical charge) ProtonsProtons (positive charge) (positive charge) ElectronsElectrons (negative charge) (negative charge)

• # protons = # protons = atomic numberatomic number• # protons + # neutrons# protons + # neutrons = mass number = mass number• # protons = # electrons# protons = # electrons• Electrons form a cloud around the nucleusElectrons form a cloud around the nucleus


Cloud of negativecharge (2 electrons)

Fig. 2-5Fig. 2-5

Nucleus

Electrons

(b)(a)

IsotopesIsotopes

All atoms of an element have the same number of protons All atoms of an element have the same number of protons but may differ in number of neutronsbut may differ in number of neutrons

Isotopes Isotopes are two atoms of an element that differ in number are two atoms of an element that differ in number of neutronsof neutrons Carbon 12, Carbon 13 and Carbon 14Carbon 12, Carbon 13 and Carbon 14


The Energy Levels of The Energy Levels of ElectronsElectrons• An electron’s state of potential energy is An electron’s state of potential energy is

called its energy level, or called its energy level, or electron shellelectron shell Valence electrons Valence electrons are those in the are those in the

outermost shell, or outermost shell, or valence shellvalence shell The chemical behavior of an atom is mostly The chemical behavior of an atom is mostly

determined by the valence electronsdetermined by the valence electrons


Electron OrbitalsElectron OrbitalsA shorthand notation is the electron configuration:1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6

1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p

s = 2p = 6d = 10f = 14

The formation and function of The formation and function of molecules depend on molecules depend on chemical bonding between chemical bonding between atomsatoms• Atoms with incomplete valence shells can

share or transfer valence electrons with certain other atoms

• These interactions usually result in atoms staying close together, held by attractions called chemical bonds


Covalent BondsCovalent Bonds

A A covalent bond covalent bond is is the sharing of a pair the sharing of a pair of valence electrons of valence electrons by two atomsby two atoms

In a covalent bond, In a covalent bond, the shared electrons the shared electrons count as part of each count as part of each atom’s valence shellatom’s valence shell


(a) Hydrogen (H2)

(b) Oxygen (O2)

(c) Water (H2O)

(d) Methane (CH4)

Name andMolecularFormula

Electron-distribution

Diagram

Lewis DotStructure & Structural Formula

Space-fillingModel

A A molecule molecule consists of two or more atoms consists of two or more atoms held together by covalent bondsheld together by covalent bonds

A single covalent bond, or A single covalent bond, or single bondsingle bond, is , is the sharing of one pair of valence electronsthe sharing of one pair of valence electrons

A double covalent bond, or A double covalent bond, or double bonddouble bond, is , is the sharing of two pairs of valence electronsthe sharing of two pairs of valence electrons


Singlecovalent bond

Doublecovalent bond

Fig. 2-13Fig. 2-13

–

+ +H H

O

H2O

In a In a nonpolar covalent bondnonpolar covalent bond, the atoms , the atoms share the electron equallyshare the electron equally

In a In a polar covalent bondpolar covalent bond, one atom is more , one atom is more electronegative, and the atoms do not share the electron electronegative, and the atoms do not share the electron equally (casuing partial charges)equally (casuing partial charges)

Ionic BondsIonic Bonds

Atoms sometimes strip electrons from Atoms sometimes strip electrons from their bonding partnerstheir bonding partners

An example is the transfer of an An example is the transfer of an electron from sodium to chlorineelectron from sodium to chlorine

After the transfer of an electron, both After the transfer of an electron, both atoms have chargesatoms have charges

A charged atom (or molecule) is called A charged atom (or molecule) is called an an ionion

saltssalts


Na Cl Na Cl

NaSodium atom Chlorine atom

Cl Na+

Sodium ion(a cation)

Cl–Chloride ion

(an anion)

Sodium chloride (NaCl)

- A - A cation cation is a positively charged ion (giving up)is a positively charged ion (giving up)- An - An anion anion is a negatively charged ion (receiving)is a negatively charged ion (receiving)

Animation: Ionic/Covalent Bonds

Weak Chemical Weak Chemical BondsBonds Most of the strongest bonds in Most of the strongest bonds in

organisms are covalent bonds that form organisms are covalent bonds that form a cell’s moleculesa cell’s molecules

Weak chemical bonds, such as ionic Weak chemical bonds, such as ionic bonds and hydrogen bonds, are also bonds and hydrogen bonds, are also importantimportant

Weak chemical bonds reinforce shapes Weak chemical bonds reinforce shapes of large molecules and help molecules of large molecules and help molecules adhere to each otheradhere to each other


Hydrogen BondsHydrogen Bonds

A A hydrogen bond hydrogen bond forms when a forms when a hydrogen atom covalently bonded to hydrogen atom covalently bonded to one electronegative atom is also one electronegative atom is also attracted to another electronegative attracted to another electronegative atomatom

In living cells, the electronegative In living cells, the electronegative partners are usually oxygen or partners are usually oxygen or nitrogen atomsnitrogen atoms


Fig. 2-16Fig. 2-16

+

+

+

+

+

Water (H2O)

Ammonia (NH3)

Hydrogen bond

Van der Waals Van der Waals InteractionsInteractions If electrons are distributed asymmetrically in If electrons are distributed asymmetrically in

molecules or atoms, they can result in “hot molecules or atoms, they can result in “hot spots” of positive or negative chargespots” of positive or negative charge

Van der Waals interactions Van der Waals interactions are attractions are attractions between molecules that are close together between molecules that are close together as a result of these chargesas a result of these charges


Chemical reactions make Chemical reactions make and break chemical bondsand break chemical bonds Chemical reactions Chemical reactions are the making and breaking of are the making and breaking of

chemical bondschemical bonds The starting molecules of a chemical reaction are The starting molecules of a chemical reaction are

called called reactantsreactants The final molecules of a chemical reaction are called The final molecules of a chemical reaction are called

productsproducts

2 H2 O2 2 H2OReactants Reaction Products

Properties of WaterProperties of Water

Cohesion:Cohesion: binding together of like binding together of like molecules, often by hydrogen bonds.molecules, often by hydrogen bonds.

Adhesion:Adhesion: the attraction between the attraction between different kinds of moleculesdifferent kinds of molecules

Surface tension:Surface tension: related to cohesion, a related to cohesion, a measure of how difficult it is to stretch or measure of how difficult it is to stretch or break the surface of a liquid.break the surface of a liquid.

Solvent vs. SoluteSolvent vs. Solute

Solvent: dissolving agent (doing the Solvent: dissolving agent (doing the dissolving)dissolving)

Solute: substance that is being dissolvedSolute: substance that is being dissolved

Aqueous solution: water is the solventAqueous solution: water is the solvent

Types of solutions:Types of solutions:

Hydrophilic: an affinity for water. “water Hydrophilic: an affinity for water. “water loving”.loving”. Colloid: a large molecule that remains Colloid: a large molecule that remains

suspended and does not dissolve suspended and does not dissolve

Hydrophobic: repel water, “water fearing”Hydrophobic: repel water, “water fearing”

Acids and BasesAcids and Bases

Base: substance that reduces the hydrogen ion Base: substance that reduces the hydrogen ion concentration.concentration. Solution with more OH- than H+Solution with more OH- than H+ pH between 9-14pH between 9-14

Acid: increases the hydrogen ion concentration Acid: increases the hydrogen ion concentration of a solution.of a solution. Solution will have more H+ than OH-Solution will have more H+ than OH- pH between 1-5pH between 1-5

Neutral = H+ = OH-Neutral = H+ = OH-

Buffers: minimize changes in the Buffers: minimize changes in the concentration of H+ and OH- in a concentration of H+ and OH- in a solution.solution. Body internally maintains your pH at a Body internally maintains your pH at a

neutral level.neutral level.

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