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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