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Atoms, Molecules, and Life The Importance of Water Chapter 2 Atoms • The basic structural unit of matter. proton neutron electron nucleus electron shell Hydrogen Helium Carbon 2 1 3 4 1 H Hydrogen 1.008 1 - Atomic Number = # of protons 2 - Element 3 - Symbol 4 - Molecular Mass ~ # protons + # neutrons 6 C Carbon 12.01 2 1 3 4

Atoms, Molecules, and Life Atoms - Ventura Collegeacademic.venturacollege.edu/sflagan/protected/Ch2_Water.pdf · Atoms, Molecules, and Life The Importance of Water Chapter 2 Atoms

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Atoms, Molecules, and LifeThe Importance of Water

Chapter 2

Atoms

• The basic structural unit of matter.

proton

neutron

electron

nucleus

electron shell

Hydrogen

Helium

Carbon

2

1 3

4

1H

Hydrogen

1.008

1 - Atomic Number = # of protons2 - Element3 - Symbol4 - Molecular Mass ~ # protons + # neutrons

6 CCarbon

12.012

1 3

4

Properties of Elements• Element: a substance that cannot be broken

down nor converted to another substanceby ordinary chemical means.

• Isotope: atoms of an element that containdifferent numbers of neutrons– 12C has 6 protons and 6 neutrons (normal)– 14C has 6 protons and 8 neutrons (“heavy” carbon)

• Some isotopes are radioactive, they spontaneously break apart, releasing energy and forming new elements.

Electrons: the key to bonding

• Negatively charged• Repel each other• Attracted to the positively-charged nucleus• Can become excited

Excited Electrons:Disobeying Electron Shells

• Electrons orbit through different energy levels– 1st shell: up to 2 electrons (e-)– 2nd shell (and each larger shell): up to 8 e-

• Always fill up the lowest energy level first

• More stable when shells are full

Free Radicals• Atoms with one or more unpaired electron in

their outer shell.• Very unstable.• Capture electrons from neighboring

molecules, creating new free radicals.• Can lead to cell death and a variety of

diseases.• Antioxidants react with free radicals, rendering them harmless to the body.

Interacting Elements• An atom whose outer electron shell is full

cannot interact with other atoms and is calledinert.

• Atoms is reactive when its outer electron shellis only partially full and it can react with otheratoms.

Molecules• Two or more atoms of the same or different

elements held together.• Molecules formed from different elements

are compounds.• Atoms can form three main types of bonds:

– Ionic, Covalent, and Hydrogen bonds

Ionic Bonds

• An electron is transferred, creatingpositive and negative ions that attractone another.

Sodium (11 p+, 11 n) Chlorine (17 p+, 18 n)

Example: MgCl2

• MgCl2:– Total # protons:

• 12 + (17)*2 = 46

– Total # electrons:• 12 + (17)*2 = 46

– Combined MolecularMass:

• 24.31 + (34.45)*2 = 93.21 g/mol

12Mg

Magnesium

24.31

17Cl

Chlorine

34.45

Chlorine: 17 protons 18 neutrons 17 electrons (7 e- in the outer shell)

Magnesium: 12 protons 12 neutrons 12 electrons (2 e- in the outer shell)

Ionic bonding, with one electron from Magnesiumdonated to each Chlorine atom

Covalent Bonds• Electron pairs are shared.

• Nonpolar covalent bonds: equal sharing.– Two hydrogen atoms share electrons to form uncharged H2.– Electrons spend the same amount of time

around each nuclei.

Covalent Bonds, continued

• Polar covalent bonds: unequal sharing.– Electrons spend more time around one atom

than the other.

- -

++

Oxygen is slightly negative

Hydrogen is slightly positive

Water, H2O

Example: NH3,Ammonia

• Polar, covalent bonding, sharingthe electrons of Hydrogen to fillNitrogen’s outer e- shell. • NH3:

– Total # protons:• 7 + (1)*3 = 10

– Total # electrons:• 7 + (1)*3 = 10

– Combined MolecularMass:

• 14.01 + (1.008)*3 = 17.034 g/mol

7N

Nitrogen

14.01

Hydrogen: 1 proton 0 neutrons 1 electrons (1 e- in the outer shell)

Nitrogen: 7 protons 7 neutrons 7 electrons (5 e- in the outer shell)

1H

Hydrogen

1.008

Covalent bonds are the most common bondfound in biologic systems.

Hydrogen Bonds

• The slightly attraction between atomsinvolved in polar covalent bonds fromdifferent molecules.

- -

++

- -

++

- -

++

Properties of Water

• Life likely began underwater.• All living organisms contain 60-90% H2O.• All life depends on water.

– (examples: Photosynthesis and Digestion)

Properties of Water

1. Very good solvent.– It is able to dissolve a wide range of

substances.

Solvent: Water

Solute: Salt

Solution:Salt Water

Properties of Water

• Because of itspolarity, waterattracts bothpositive andnegative ions,breaking ionicbonds.

Properties of Water

2. Hydrophobic and Hydrophilic qualities. •Hydrophobic (water-fearing)

molecules are non-polar anduncharged.•Water cannot dissolve them.

•Lipids, Fats and Oils

•Hydrophilic (water-loving)molecules are polar.•Water dissolves the covalent bonds.

•Sugars, Salts, and Amino Acids

Properties of Water

3. Water molecules tend to stick together.

• Because of their hydrogen bonds, watermolecules have high cohesion(stickiness).

• Cohesion of water molecules at thesurface creates surface tension.• The tendency for the water surface to resist

breaking.

Examples of SurfaceTension and Cohesion

Properties of Water4. Water can be acidic, basic, or neutral.

• Water molecules are easily ionized:

• Pure water has equal concentrations ofhydroxide (OH-) and hydrogen (H+) ions.

Acids• Acidic solutions have a higher concentration of H+

than OH-.• Acids release hydrogen ions when dissolved in

water.• Many acidic solutions taste sour (vinegar, lemon

juice, etc).

Bases• Basic solutions have a higher concentration of OH-.• Bases combine with hydrogen ions, reducing their

number.• Many basic solutions taste bitter, and are very

harmful if consumed (ammonia, bleach, etc).

pH Scale (0-14)

• Measures the ratio of H+ to OH-.• Neutrality (equal concentrations

of H+ and OH-) is set to 7.• Acids have a pH below 7.• Bases have a pH above 7.

Neutral

Basic

Acidic

14

0

7

Buffers• A complex molecule that helps maintain a

solution at a constant pH.• Can combine with H+ to increase pH.• Releases H+ to combine with OH- when needed to decrease pH.

Example: Carbonate Buffering of Blood

CO32- <--> HCO3

- <--> H2CO3

pH ~ 6 pH ~ 9.5

Properties of Water

• H20 heats slowly– Takes a lot of energy to break H-bonds

• H20 moderates high and low temperatures• H20 freezes slowly

– A large amount of energy must be removed toform ice crystals

•Water moderates the effects oftemperature changes.

Water forms an unusual solid:Water forms an unusual solid:IceIce

•• Less dense than liquid water!Less dense than liquid water!

LiquidLiquid SolidSolid

HomeworkChapter 2

The lizard in the beginning of this lectureappears to be running on water.

Aside from the speed at which the lizard runs,what properties of water allow the lizard tomove like this? How does this property work?(i.e. describe the arrangement of molecules that giveswater this property)

Give two other examples where these propertiesof water are used in nature.