The Chemical Context of Life

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Chapter 2

• The Chemical Context of Life

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Why are we studying chemistry?

• Chemistry is the foundation of Biology

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Chemical Context of Life

• Matter (space mass)
• Element compound
• 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)

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• Everything is made of matter
• Matter is made of atoms

Hydrogen 1 proton 1 electron
Oxygen 8 protons 8 neutrons 8 electrons
Proton
Neutron
Electron

0

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The World of Elements
H
C
O
N
P
S
Na
Mg
K
Ca
Different kinds of atoms elements
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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 numberof electrons in atoms
  outermost shell
• valence shell

How does this atom behave?
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Whats themagic number?
Bonding properties

• Effect of electrons
• chemical behavior of an atom depends on number of
  electrons in its valence shell

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
Remember some food chainsare 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
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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)

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Covalent Bonding

• Sharing pair of valence electrons
• Number of electrons required to complete an
  atoms valence shell determines how many bonds
  will form
• Ex Hydrogen oxygen bonding in water methane

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Nonpolar covalent bond

• Pair of electrons shared equally by 2 atoms
• example hydrocarbons CxHx
• methane (CH4 )

balanced, stable,good building block
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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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Hydrogen bonds

• Hydrogen atom covalently bonded to one
  electronegative atom is also attracted to another
  electronegative atom (oxygen or nitrogen)

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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
H
O
Lets go to the videotape!
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van der Waals interactions

• Weak interactions between molecules or parts of
  molecules that are brought about by localized
  change fluctuations
• Due to the fact that electrons are constantly in
  motion and at any given instant, ever-changing
  hot spots of negative or positive charge may
  develop