Chapter 2 – Atoms, Molecules and Life 2.1 What Are Atoms? 2.2 How Do Atoms Interact to Form...

Chapter 2 – Atoms, Molecules and Life

• 2.1 What Are Atoms?

• 2.2 How Do Atoms Interact to Form Molecules?

• 2.3 Why Is Water So Important to Life?

2.1 What Are Atoms?

• Atoms are the fundamental structural units of matter

• Atoms are composed of a nucleus, protons, neutrons and electrons

• We use models to simplify talking about atoms

Helium (He)

nn

Hydrogen (H)

electronshell

nucleus

e-

p+ p+

p+

e-

e-

Radioactivity in Research

• Isotope is a form of an element where there is a different number of neutrons than protons

• Some isotopes are radioactive which means the nucleus is unstable and will break down releasing radiation

detector ring

PET = positron emission tomography

Atoms• Positive and negative attract

• Electrons are in held around the nucleus in “shells”

• The first “shell” holds two electrons

• The next “shells” hold eight electrons

Copyright © 2005 Pearson Prentice Hall, Inc.

Copyright © 2005 Pearson Prentice Hall, Inc.

Energy Capture and Release

• Life depends on electrons capturing and releasing energy– Electron shells correspond to energy levels– Energy exciting an atom causes an electron

jump from a lower- to higher-energy shell– Later, the electron falls back into its original

shell, releasing the energy

Copyright © 2005 Pearson Prentice Hall, Inc.

Copyright © 2005 Pearson Prentice Hall, Inc.

Copyright © 2005 Pearson Prentice Hall, Inc.

2.2 How Do Atoms Interact to Form Molecules?

• A molecule consists of two or more atoms of the same or different elements

• A compound means two different elements

• Inert vs. reactive

Helium (He)

nn

Hydrogen (H)

electronshell

nucleus

e-

p+ p+

p+

e-

e-

Inert (Nobel Gases)Reactive

2.2 How Do Atoms Interact to Form Molecules?

• Chemical bonds are atoms gaining stability by losing, gaining or sharing electrons

• Chemical bonds are attractive forces

• Chemical reactions are the making or breaking of chemical bonds

An ionic compound: NaCl

Na+

Cl–

11p+

11n17p+

18n

Sodium ion (+) Chloride ion (–)

Cl–Na+

Sodium atom (neutral)

Na

Chlorine atom (neutral)

Cl

11p+

11n17p+

18n

• Atoms that have lost or gained 1 or 2 electrons are charged and called ions

• Ions interact to form ionic bonds

2.2 How Do Atoms Interact to Form Molecules?

• Uncharged atoms can become stable by sharing electrons, forming covalent bonds

• Covalent bonds vary in strength but are always stronger than ionic bonds

• Nonpolar vs. polar covalent bonds

Nonpolar covalent bonding

Hydrogen (H–H or H2)

Oxygen (O=O or O2)

8p+8n

8p+8n

Polar covalent bonding

Water (H–O–H or H2O)

(slightly negative)

(slightly positive)

8p+8n

Free Radicals• A molecule with an unpaired electron

• Steals electrons from other molecules

• Free radicals contribute to cancer and Alzheimer’s disease

• Free radicals can be increased by exposure to the sun (radiation) and many chemicals

Antioxidants• React with free radicals to render them

harmless

• Vitamin C and Vitamin E

• Many can be found in a healthy diet

• Your risk of cancer can be lowered 50% simply by eating 5 fruits and veggies a day

O(–)

H(+)

hydrogenbonds

H(+)

H(+)

H(+)

O(–)

2.3 Why Is Water So Important to Life?

• Water interacts with many other molecules

• A solvent is capable of dissolving a wide range of substances

• Water is a polar solvent and can dissolve proteins, salts and sugars

Cl–

Cl–

Na+

Na+

Na+

H

H

H

H

O

O–

solution Water as a solvent

solutes

Polar vs. Nonpolar

• Ions and polar molecules are hydrophilic (Greek for “water-loving”)

• Uncharged and nonpolar molecules are hydrophobic (“water-fearing”)

• Think of oil and vinegar

Copyright © 2005 Pearson Prentice Hall, Inc.

hydrogen bond

hydroxylgroup

glucose

water

2.3 Why Is Water So Important to Life?

• Cohesion is the tendency of molecules to stick together

• Water’s high cohesion creates surface tension

• (Water also has a high specific heat)

O O

hydrogen ion(H+)

hydroxide ion(OH–)

water(H2O)

+(+)

(–)

H H H

H

2.3 Why Is Water So Important to Life?

• Water-Based Solutions Can Be Acidic, Basic, or Neutral

– If H+ exceeds OH-, the solution is acidic

– If OH- exceeds H+, the solution is basic

– If they are equal, the solution is neutral

1-molar hydrochloricacid (HCI)

stomach acidlime juice

lemon juice

"acid rain" (2.5–5.5)vinegar, cola, orange juice,tomatoes

beer

black coffee, tea

normal rain (5.6)urine (5.7)

pure water (7.0)salivablood, sweat (7.4)

seawater (7.8–8.3)

baking soda

phosphate detergentschlorine bleachmilk of magnesia

household ammoniasome detergents(without phosphates)

washing soda

oven cleaner

1-molar sodiumhydroxide (NaOH)

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

pHvalueH+ concentration

(moles/liter)100

10–1

10–2

10–3

10–4

10–5

10–6

10–7

–6

–7

–8

–9

–10

–11

–12

–13

–14

10–8

10–9

10–10

10–11

10–12

10–13

10–14

neutral

(H+ = OH–)

incr

easi

ngly

aci

dic

(H+ >

OH

– )in

cre

asi

ng

ly b

asi

c (

H+ <

OH

– )

2.3 Why Is Water So Important to Life?

• A buffer helps maintain a solution at a relatively constant pH (homeostatis)

• Water moderates the effects of temperature changes (due to high specific heat)

• 1 calorie of energy will rise the temperature of water 1 ºC (only 0.6 calories are needed for alcohol, 0.2 for salt, 0.02 for granite)

Water forms an unusual solid -> ICE