The Atom

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The Atom
electron
proton
neutron
Hydrogen 1p, 1e_
Helium 2p, 2e_
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Sub Atomic Particles
Name Mass Charge Location
Proton 1 AMU 1 Nucleus
Neutron 1 AMU N/A Nucleus
Electron N/A -1 Electron Cloud
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Carbon 6p, 6e_
Oxygen 8p, 8e_
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Molecules

• Chemical bonds hold together two or more atoms of
  the same or different elements
• Ex Nitrogen gas N2
• Chemical bond a union between the electron
  clouds of two atoms
• Molecules of compounds-elements in proportions
  never vary.

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1 mm
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• Molecules can be represented in many ways

Table 2.8
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Covalent bonding
Click to view animation.
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Hydrogen bond
Water molecule
Ammonia molecule
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Slight negative charge at this end
O
H
H
Slight positive charge at this end
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Multiple H bonds holding two molecules together
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Water is a polar molecule

• Atoms in a covalently bonded molecule may share
  electrons equally, creating a nonpolar molecule
• If electrons are shared unequally, a polar
  molecule is created

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• In a water molecule, oxygen exerts a stronger
  pull on the shared electrons than hydrogen

• This makes the oxygen end of the molecule
  slightly negatively charged
• The hydrogen end of the molecule is slightly
  positively charged
• Water is therefore a polar molecule

()
()
O
H
H
()
()
Figure 2.9
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Hydrogen Bonding

• The charged regions on water molecules are
  attracted to the oppositely charged regions on
  nearby molecules
• This attraction forms weak bonds called hydrogen
  bonds

Hydrogen bond
Figure 2.10A
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• Like no other common substance, water exists in
  nature in all three physical states

• as a solid
• as a liquid
• as a gas

Figure 2.10B
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Liquid Water Cohesion

• Due to hydrogen bonding, water molecules can move
  from a plants roots to its leaves
• Insects can walk on water due to surface tension
  created by cohesive water molecules

Figure 2.11
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evaporation
cohesion
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Moderate Temperature

• It takes a lot of energy to disrupt hydrogen
  bonds
• Therefore water is able to absorb a great deal of
  heat energy without a large increase in
  temperature
• As water cools, a slight drop in temperature
  releases a large amount of heat

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• A water molecule takes a large amount of energy
  with it when it evaporates

• This leads to evaporative cooling

Figure 2.12
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Ice is less dense water

• Molecules in ice are farther apart than those in
  liquid water

Hydrogen bond
ICE Hydrogen bonds are stable
LIQUID WATER Hydrogen bonds constantly break and
re-form
Figure 2.13
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Ice
Liquid water
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In-text FigurePage 20
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2.14 Water is a versatile solvent

• Solutes whose charges or polarity allow them to
  stick to water molecules dissolve in water
• They form aqueous solutions

Na


Na


Cl
Cl





Ions in solution
Salt crystal
Figure 2.14
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Na







Cl











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Acids and Bases

• A compound that releases H ions in solution is
  an acid, and one that accepts H ions in solution
  is a base
• Acidity is measured on the pH scale
• 0-7 is acidic
• 8-14 is basic
• Pure water and solutions that are neither basic
  nor acidic are neutral, with a pH of 7

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pH scale

• The pH scale

H
OH
Lemon juice gastric juice
Increasingly ACIDIC (Higher concentration of H)
Grapefruit juice
Acidic solution
Tomato juice
Urine
NEUTRAL H OH
PURE WATER
Human blood
Seawater
Neutral solution
Increasingly BASIC (Lower concentration of H)
Milk of magnesia
Household ammonia
Household bleach
Oven cleaner
Basic solution
Figure 2.15
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Buffers and Salts

• Cells are kept close to pH 7 by buffers

• Buffers are substances that resist pH change
• They accept H ions when they are in excess and
  donate H ions when they are depleted
• Buffers are not foolproof
• Salts Compounds that release ions other than H
  or OH-
• NaCl, KCl