Chapter 2:

The Chemical Context of Life

AP Biology

Why are we studying chemistry?

Chemistry is the foundation of Biology

Chemical Context of Life

Matter (space & mass)

Elements & Compounds

The Atom

Atomic Number (# of protons);

Mass Number (protons + neutrons)

Isotopes (different # of neutrons);

radioactive isotopes (nuclear decay)

Energy (ability to do work); energy

levels (electron states of potential

energy)

AP BiologyProton Neutron Electron

Hydrogen1 proton

1 electron

Oxygen8 protons

8 neutrons

8 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 = 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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Bonding properties

Effect of electrons

electrons determine

chemical behavior of

atom

depends on number

of electrons in atom’s

outermost shell

valence shell

How does this

atom behave?

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Bonding properties

Effect of electrons

chemical behavior of an atom depends on

number of electrons in its valence shell

What’s themagic number?

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 shells

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

Remembersome food chains

are built on reducing O to H2O

& some on reducing S to H2S

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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…

and creates bonds

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 )

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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 stronger

“attraction” for the

electrons than hydrogen

oxygen has higher

electronegativity

water is a polar molecule

+ vs – poles

leads to many interesting

properties of water…

+

+

––

––

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Electronegativity

F O Cl N Br I C H

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)

Hydrogen bonds

Hydrogen atom covalently

bonded to one

electronegative atom is

also attracted to another

electronegative atom

(oxygen or nitrogen)

Hydrogen bonding

Polar water creates molecular attractions

attraction between positive H in one H2O

molecule to negative O in another H2O

also can occur wherever

an -OH exists in a larger

molecule

Weak bond

H

O

H

van der Waals interactions

Weak interactions brought about by

localized change fluctuations

Because electrons are constantly in

motion and at any given instant, ever-

changing “hot spots” of negative or

positive charge may develop

Strength of Bonds

Covalent

Ionic

Hydrogen

van der Waals

Weak Bonds

1. Numbers

2. Shape

3. Duration

4. Fluidity