Transcript Chemical Changes PP

Chemical Changes
Matter and Chemical Reactions
• What is a chemical change?
– New substance formed; different properties from the
reactants
• How do we know a chemical reaction has taken
place?
– (1). Formation of a gas
– (2). Permanent color change
– (3). Temperature change
• Exothermic and endothermic
– (4). Precipitant formed
Matter and Chemical Reactions
• How do elements form compounds?
– Atoms of the reactant(s) are rearranged and bond in
different combinations
– A + B ----------- C + D
– Phase Changes and Chemical changes Animation
Electron Shells
Note that elements within the same group have the
same electron-dot structure.
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Pearson Addison Wesley
Electron Shells
• Valence electrons: Electrons in the outermost shell of
an atom. These are the ones that can participate in
chemical bonding.
• Electron-dot structure: A notation showing the valence
electrons surrounding the atomic symbol.
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Education, Inc., publishing as
Pearson Addison Wesley
Chemical Bonds
• (1). Ionic
– Metals transfer electrons to nonmetals
– Positive ions (cations) and Negative ions (anions) are
formed
• (2). Covalent
– Sharing of electrons between nonmetal atoms
– Equal sharing: nonpolar compound
– Unequal sharing: polar compound
• (3). Metallic
– Sea of electrons float between metal atoms
Ionic Bonding
• Characteristics
– (1). Electrostatic Attraction between positive and
negative ions
– (2). Formation of Crystal Structures
– (3). Strong Bonds – High Melting Points
– (4). Electrolytes
– Animation
Polar Bonds and Polar Molecules
• Electronegativity: The ability of a bonded atom
to pull on shared electrons. Greater
electronegativity means greater “pulling
power.”
High
Low
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Covalent Bonding
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Characteristics
(1). Sharing of electrons between atoms
(2). Electronegativity is similar
(3). Molecules formed
(4). Bonds not as strong as covalent
(5). May be equal or unequal sharing between
electrons
• (6). Gases form covalent bonds
The Covalent Bond
• The type of electrical attraction in which atoms
are held together by their mutual attraction for
shared electrons.
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The Covalent Bond
• The type of electrical attraction in which atoms
are held together by their mutual attraction for
shared electrons.
• There are two electrons within a single covalent
bond.
• The covalent bond is represented using a
straight line.
F
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F
F—F
Polar Bonds and Polar Molecules
• Electrons within a covalent bond are shared
evenly when the two atoms are the same.
• They may be shared unevenly, however, when
the bonded atoms are different.
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Education, Inc., publishing as
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Polar Bonds and Polar Molecules
• But if polar bonds within a molecule are facing
in equal and opposite directions…
…then the polarity may cancel itself out.
Copyright © 2007 Pearson
Education, Inc., publishing as
Pearson Addison Wesley
Polar Bonds and Polar Molecules
• But if polar bonds within a molecule are facing
in equal and opposite directions…
…then the polarity may cancel itself out.
…or not!
Copyright © 2007 Pearson
Education, Inc., publishing as
Pearson Addison Wesley
More Chemical Reactions
• Animation – Chemistry Comes Alive
• Dust Explosion Animation
Metallic Bonding
• Characteristics
• Electrons of metal atoms flow freely between
other metal atoms
• The more mobile the electrons, the better the
electrical conductor
• Higher the vibration of electrons more metal is
shiny.
• Malleable characteristic due to electrons not
being fixed, but freely flowing
• Alloy: White gold (molten gold and palladium are
blended)
Metals vs Nonmetals
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High luster, shiny
Malleable
Ductile
High Melting Points
Good Conductors
Poor Insulators
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Low luster and dull
Nonmalleable
Not Ductile
Low Melting Points
Poor Conductors
Good Insulators
Periodic and Group Trends
Atomic Size/Radius
• Half the distance between the centers of two
atoms of that element that are touching each
other.
: Decreases left to right
– Why?
: increases as you go down
– Why?
Ionization Energy
• Energy required to completely remove an
electron from a gaseous atom or ion.
: Increases from left to right
– Right side: increasing nuclear charge holds atoms
tighter
group
: Decreases as you go down the
– Atomic radius increases; electrons father away from
the nucleus