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Chapter 2The Chemical Context of Lifeand Properties of Water

AP Biology

Why are we studying chemistry?Chemistry is the foundation of Biology

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Basics You Should KnowMatterElementsCompoundsAtomsSubatomic ParticlesAtomic Number Atomic Mass IsotopesElectron

Shells/Energy LevelsElectron Orbitals

AP BiologyProton Neutron Electron

Hydrogen1 proton1 electron

Oxygen8 protons8 neutrons8 electrons

+ 0 –

Everything is made of matter Matter is made of atoms

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The World of Elements

C

Different kinds of atoms = elementsDifferent kinds of atoms = elements

H

ON

P SNa

K

Mg

Ca

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Life requires ~25 chemical elements

About 25 elements are essential for life Four elements make up 96% of living

matter:

• carbon (C) • hydrogen (H)

• oxygen (O) • nitrogen (N) Four elements make up most of

remaining 4%:

• phosphorus (P) • calcium (Ca)

• sulfur (S) • potassium (K)

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Deficiencies

(a) Nitrogen deficiency(b) Iodine deficiency

(Goiter)

If there is a deficiency of an essential element, disease results

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Trace Elements

Trace elements Are required by an organism in only minute quantities

Minerals such as Fe and Zn are trace elements

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Bonding properties

Effect of electrons electrons determine

chemical behavior of atom

depends on numberof electrons in atom’s outermost shell valence shell

How does this atom behave?How does this atom behave?

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Bonding propertiesWhat’s themagic number?

How does this atom behave?How does this atom behave? How does this atom behave?How does this atom behave?

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Elements & their valence shells

Elements in the same row have the same number of shellsElements in the same row have the same number of shells

Moving from left to right, each element has a sequential addition of electrons (& protons)Moving from left to right, each element has a sequential addition of electrons (& protons)

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Elements & their valence shells

Elements in the same column have the same valence & similar chemical properties

Elements in the same column have the same valence & similar chemical properties

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–

–

–

Chemical reactivity Atoms tend to

complete a partially filled valence shell

or empty a partially filled valence shell

This tendency drives chemical reactions…This tendency drives chemical reactions…

and creates bondsand creates bonds

Ionic bonding

High electronegativity difference strips valence electrons away from another atom

Electron transfer creates ions (charged atoms)

Cation (positive ion); anion (negative ion)

Ex: Salts (sodium chloride)

Covalent Bonding

Sharing pair of valence electrons

Number of electrons required to complete an atom’s valence shell determines how many bonds will form

Ex: Hydrogen & oxygen bonding in water; methane

Nonpolar covalent bond

Pair of electrons shared equally by 2 atoms example: hydrocarbons = CxHx

methane (CH4 )

balanced, stable,good building block

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H

H

Oxygen

Polar covalent bonds Pair of electrons shared

unequally by 2 atoms example: water = H2O

oxygen has higher electronegativity

oxygen has stronger “attraction” for the electrons than hydrogen

water is a polar molecule + vs – poles leads to many interesting

properties of water…

+

+

––

––

Hydrogen bonds

Hydrogen atom covalently bonded to one electronegative atom is also attracted to another electronegative atom (oxygen or nitrogen)

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Properties of Water

Earth is ¾ WaterSupports All of Life

Most animals and plants contain more than 60% water by volume.

All living organisms require water more than any other substance

Properties of Water

1.Cohesion/Adhesion

2.Surface tension

3.Temperature Moderation

4.High specific heat

5.Evaporative cooling

6.Universal solvent

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Water can dissociate Into hydronium ions (H+ or H3O+) and hydroxide (OH-) ions

Changes in the concentration of these ions Can have a great affect on pH in living organisms

H

Hydroniumion (H3O+)

H

Hydroxideion (OH–)

H

H

H

H

H

H

+ –

+

Figure on p. 53 of water dissociating

Effects of Changes in pH

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Acids and BasesAn acid

Is any substance that increases the hydrogen ion concentration of a solution

A base Is any substance that reduces the hydrogen

ion concentration of a solution (more OH- ions)

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Buffers

The internal pH of most living cells Must remain close to pH 7

Buffers Are substances that minimize changes in

the concentrations of hydrogen and hydroxide ions in a solution