Chapter 2

Copyright © 2011 Pearson Education Inc. Biology: Life on Earth, 9e

Chapter 2

Atoms, Molecules, and Life


What Are Atoms? Smallest unit of an element Atoms are the fundamental structural units of

matter and are composed of three types of particles

(a) Hydrogen (H)

electronshell

atomicnucleus

e

p

(b) Helium (He)

nn

e

p

p

e


What Are Atoms?

Elements– An element is a substance that cannot be broken down

by ordinary chemical reactions– All atoms belong to one of 92 types of naturally occurring

elements


The atomic mass of an element is the total mass of its protons, neutrons, and electrons

Atomic number - The number of protons in the nucleus of an atom – is the defining value for an element– All atoms of an element have the same atomic number

– For example, carbon has six protons, nitrogen has seven

What Are Atoms?


Table 2-1


Isotopes– Atoms of an element with different numbers of neutrons

– Different mass number– Some isotopes are radioactive (meaning that they

spontaneously break apart, forming different atoms and releasing energy) and are used in research

– At room temperature, elements may occur as solids, liquids, or gases

What Are Atoms?


What Are Atoms? Electron shells

– Electrons are distributed around the nucleus of an atom in electron shells– The first shell, or energy level, holds two electrons– Subsequent shells holds up to eight

– Larger atoms can accommodate more electrons than smaller ones can

Carbon (C) Oxygen (O) Phosphorus (P) Calcium (Ca)

CaOC P

4e 6e5e

2e

8e

8e

8e

2e 2e

2e

2e

6p 8p 15p20p

6n 8n 16n 20n


What Are Atoms? Energy capture and release

– Life depends on electrons capturing and releasing energy– Electron shells correspond to energy levels

heat energy

light

An electronabsorbs energy

The energy booststhe electron to ahigher-energy shell

The electron dropsback into lower-energyshell, releasing energyas light

12

3


How Do Atoms Interact to Form Molecules?

Interaction between atoms– Atoms will not react with other atoms if the outermost

shell is completely empty or full (such atoms are considered inert) – Example: Neon, with eight electrons in its outermost

shell is full– Atoms will react with other atoms if the outermost shell is

partially full (such atoms are considered reactive)– Example: Oxygen, with six electrons in its outermost

shell, can hold two more electrons, and so is susceptible to reacting



Chemical bonds hold atoms together in molecules– the force of attraction between neighboring atoms that

holds them together within a molecule– Reactive atoms gain stability through electron

interactions (chemical reactions)– A chemical reaction is a process by which new chemical

bonds are formed or existing bonds are broken, converting one substance into another

– Three major types of chemical bonds: ionic, covalent, and hydrogen


Chemical Bonds Ions and ionic bonds

– Atoms that have lost or gained electrons, thereby altering the balance between protons and electrons, are charged, and are called ions

– Oppositely charged ions that are attracted to each other are bound into a molecule by ionic bonds

– Salt crystals are formed by a repeated, orderly arrangement of sodium (Na+) and chloride (Cl-) ions

Electron transferred

Na

Cl

Cl

Na

Cl

Cl

Na

Na

Cl

(b) Ions

(c) An ionic compound: NaCl

Attraction betweenopposite charges

– –

–– –

–

–––

–

11p

11n

–

––

– –

––

– ––

––

––

––

––

17p

18n

Sodium ion () Chloride ion (–)

(a) Neutral atoms

Sodium atom (neutral) Chlorine atom (neutral)

– –

–– –

–

––

–

––

11p

11n

– –

––

– –

––

– ––

––

––

––

17p

18n


Covalent bonds – form between uncharged atoms that share electrons– An atom with a partially full outermost electron shell can

become stable – found in H2 (single bond), O2 (double bond), N2 (triple

bond), and H2O

– stronger than ionic bonds but vary in their stability

Chemical Bonds


Nonpolar or polar covalent bonds– Nonpolar covalent bond – both atoms exert the same

pulling force on bond electrons (H2)

– Polar covalent bonds - molecules where atoms of different elements are involved (H2O), the electrons are not always equally shared

– H2O is a polar molecule

– Slightly positively charged pole is around each hydrogen

– Slightly negatively charged pole is around the oxygen

Chemical Bonds


Covalent Bonds Involve Shared Electrons

Fig. 2-6

(b) Polar covalent bonding in water (H2O)

+

(+) (+)

(oxygen: slightly negative)(–)

(hydrogens:slightly positive)

8p

8n

+

__ _

__

_

_ ___

__

(a) Nonpolar covalent bonding in hydrogengas (H2)

(hydrogens: uncharged)

+ +

Electrons spendequal time neareach nucleus

Same charge onboth nuclei

Larger positivecharge

Electrons spendmore time nearthe larger nucleus

Smaller positivecharge



Free radicals– Some cellular reactions produce free radicals

– A free radical is a molecule in which atoms have one or more unpaired electrons in their outer shells

– Free radicals are highly unstable and reactive– Free radicals steal electrons, destroying other

molecules– Cell death can occur from free radical attack


Chemical Bonds

Hydrogen bonds – are attractive forces between polar molecules– Hydrogen bonds form when partial opposite

charges in different molecules attract each other– The partially positive hydrogen atoms of one

water molecule are attracted to the partially negative oxygen on another

– Polar biological molecules can form hydrogen bonds with water, each other, or even within the same molecule

– Hydrogen bonds are comparatively weak but, collectively, can be quite strong


Why Is Water So Important to Life?

Water molecules attract one another– Cohesion is the tendency of the molecules

of a substance to stick together– Hydrogen bonding between water molecules

– Cohesion of water molecules along a surface produces surface tension– tendency for a water surface to resist being broken



Water interacts with many other molecules– Water is an excellent solvent (completely surrounds and

disperses individual atoms)– A wide range of substances dissolve in water to form

solutions



Water interacts with many other molecules– Water-soluble molecules are hydrophilic

– Water molecules are attracted to and can surround– Dissolve readily in water

– Water-insoluble molecules are hydrophobic– repel and drive together uncharged and nonpolar

molecules like fats and oils – The “clumping” of nonpolar molecules is called

hydrophobic interaction



Water moderates the effects of temperature change– The energy required to heat 1 gram of a substance by

1°C is called its specific heat– It takes a lot of energy to heat water

– Temperature reflects the speed of molecular motion – It requires 1 calorie of energy to raise the temperature

of 1g of water 1°C (the specific heat of water), which is a very slow process



Water moderates the effects of temperature change– The heat of vaporization is the amount of heat

needed to cause a substance such as water to evaporate (to change from a liquid to a vapor)

– Evaporating water uses up heat from its surroundings, cooling the nearby environment (as occurs during sweating)

– It takes a lot of energy to cause water to evaporate– Because the human body is mostly water, a sunbather

can absorb a lot of heat energy without sending her/his body temperature soaring



Water forms an unusual solid: ice– Most substances become denser when they solidify from

a liquid– Ice is unusual because it is less dense than liquid water– Water molecules spread apart slightly during the freezing

process



Water-based solutions can be acidic, basic, or neutral– A small fraction of water molecules are ionized:

H2O OH– + H+

hydrogen ion(H)

hydroxide ion(OH)

water(H2O)

( )( )

O

HH

O H

H



Water-based solutions can be acidic, basic, or neutral– Solutions where H+ > OH– are acidic

– Substance that releases H+ into solution– For example, hydrochloric acid ionizes in water:

HCl H+ + Cl–

– Lemon juice and vinegar are naturally occurring acids



Water-based solutions can be acidic, basic, or neutral– Solutions where OH– > H+ are basic

– Substance that removes H+ from solution– For example, sodium hydroxide ionizes in water:

NaOH Na+ + OH–

– Baking soda, chlorine bleach, and ammonia are basic


The degree of acidity of a solution is measured using the pH scale

1 m

ola

r h

ydro

chlo

ric

acid

(H

CI)

sto

mac

h a

cid

(2)

lem

on

ju

ice

(2.3

)

"aci

d r

ain

" (2

.5–5

.5)

bee

r (4

.1)

tom

ato

es (

4.5)

bla

ck

co

ffe

e (5

.0)

no

rmal

ra

in (

5.6)

mil

k (6

.4)

pu

re w

ater

(7.

0)

sea

wat

er (

7.8–

8.3)

bak

ing

so

da

(8.4

)

anta

cid

(10

)

was

hin

g s

od

a (1

2)

ove

n c

lean

er (

13.0

)

1 m

ola

r so

diu

mh

ydro

xid

e (N

aOH

)

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

pH value

H concentration in moles/liter

100 10–1 10–2 10–3 10–4 10–5 10–6 10–7 10–8 10–9 10–10 10–11 10–12 10–13 10–14

neutral(H OH)

(H OH) (H < OH)

vin

eg

ar,

cola

(3.

0)

uri

ne

(5.7

)

blo

od

, s

wea

t (7

.4)

chlo

rin

e b

leac

h (

12.6

)

dra

in c

lea

ner

(14

.0)

ora

ng

e (3

.5)

increasingly acidic increasingly basic

ho

use

ho

ld a

mm

on

ia (

11.9

)



A buffer helps maintain a relatively constant pH in a solution– A buffer is a compound that accepts or releases H+ in

response to a pH change– If the solution becomes too acidic, a buffer accepts (and

absorbs) H+ which creates an acidic molecule– If the solution becomes too basic, an acidic molecule

liberates hydrogen ions to combine with OH– to form water

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