Molecules and Bonding

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Molecules and Bonding
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Molecules, Compounds

• Compound a substance formed by the chemical
  combination of two or more different elements in
  definite proportions.
• Molecule
• when atoms share electrons
• Molecules make up the basic unit of a compound
• written as molecular formula showing the number
  of atoms of each element (H2O)

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Chemical Reactions

• Atoms react chemically to form a more stable
  arrangement of electrons
• Bonds hold together the atoms in molecules
• An atom with a full outer electron shell is
  stable and unlikely to form a bond with another
  atom
• The Octet rule states that biologically important
  elements interact to produce chemically stable
  arrangements of 8 electrons in the outermost
  occupied electron shell.
• Whether electrons are shared, donated or acquired
  determines the type of bond formed.

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Ions

• Ions are formed by ionization (gain or loss of
  electrons) and are electrically charged.
• An ion is
• an atom that gave up or gained an electron
• written with its chemical symbol and () or (-)
• positive ion cation
• negative ion anion

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Ionic Bonds

• Ionic bonds are formed when one or more electrons
  are transferred from one atom to another.
• Ionic bonds are only formed between metals and
  non-metals.

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The Ionic Bond in Sodium Chloride

• Sodium loses an electron to become Na (cation)
• Chlorine gains an electron to become Cl- (anion)
• Na and Cl- are attracted to each other to form
  the compound sodium chloride (NaCl) -- table salt
• Ionic compounds generally exist as solids
• An ionic compound that dissociates in water into
  and - ions is called an electrolyte
• the solution can conduct an electric current
• In the body, ionic bonds are found mainly in
  teeth and bones

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Covalent Bonds

• Atoms share electrons to form covalent bonds
• Electrons spend most of the time between the 2
  atomic nuclei
• single bond share 1pair
• double bond share 2 pair
• triple bond share 3 pair
• Polar covalent bonds share electrons unequally
  between the atoms involved

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Polar Covalent Bonds

• In a water molecule, oxygen attracts the hydrogen
  electrons more strongly
• Oxygen has greater electronegativity as indicated
  by the negative lower case Greek delta sign.

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van der Waals Forces

• When molecules are close together, a slight
  attraction can develop between oppositely charged
  regions of nearby molecules.
• These intermolecular forces of attraction are
  called van der Waals forces.
• Not as strong as ionic or covalent bonds, but can
  hold large molecules together.

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Hydrogen Bonds

• Polar covalent bonds between hydrogen and other
  atoms
• Useful in establishing links between molecules
• Large 3-D molecules areoften held together by a
  large number of hydrogen bonds.

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The States of Water
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The States of Water

• Water has three states.
• Below freezing water is a solid (ice or
  snowflakes).
• Between freezing and boiling water is a liquid.
• Above its boiling point water is a gas.
• Below, which is gas, liquid, or solid?

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The States of Water

• Most liquids contract (get smaller) when they get
  colder.
• Water is different. Water contracts until it
  reaches 4 C then it expands until it is solid.
• Solid water is less dense that liquid water
  because of this.
• If water worked like other liquids, then there
  would be no such thing as an ice berg, the ice in
  your soft drink would sink to the bottom of the
  glass, and ponds would freeze from the bottom up!

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Adhesion and Cohesion

• Water is attracted to other water.
• This is called cohesion.
• Water can also be attracted to other materials.
• This is called adhesion.

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Adhesion and Cohesion

• The oxygen end of water has a negative charge and
  the hydrogen end has a positive charge.
• This makes the water molecule a partially polar
  molecule
• The hydrogen atoms of one water molecule are
  attracted to the oxygen from other water
  molecules.
• This attractive force is what gives water its
  cohesive and adhesive properties.

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Surface Tension

• Surface tension is the name we give to the
  cohesion of water molecules at the surface of a
  body of water.
• Quick experiment place a drop of water onto a
  piece of wax paper. Look closely at the drop.
  What shape is it? Why do you think it is this
  shape?

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Surface Tension

• What is happening?
• Water is not attracted to wax paper (there is no
  adhesion between the drop and the wax paper).
  Each molecule in the water drop is attracted to
  the other water molecules in the drop.
• This causes the water to pull itself into a shape
  with the smallest amount of surface area, a bead
  (sphere).
• All the water molecules on the surface of the
  bead are 'holding' each other together or
  creating surface tension.

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Surface Tension

• Surface tension allows water striders to 'skate'
  across the top of a pond.
• You can experiment with surface tension.
• Quick experiment Try floating a paperclip on the
  top if a beaker of water.

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Surface Tension

• Why could you float the paperclip on the surface
  of the water?
• A paper clip is heavier than water, but because
  of the surface tension the water is able to hold
  up the metal.

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Capillary Action

• Surface tension is related to the cohesive
  properties of water.
• Capillary action however, is related to the
  adhesive properties of water.

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Capillary Action

• The water 'climbs' up the straw.
• What is happening is that the water molecules are
  attracted to the straw molecules.
• When one water molecule moves closer to the straw
  molecules the other water molecules (which are
  cohesively attracted to that water molecule) also
  move up into the straw.

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Capillary Action

• Capillary action is limited by gravity and the
  size of the straw.
• The thinner the straw or tube the higher up
  capillary action will pull the water.

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Capillary Action

• Critical thinking???????????
• What type of organism uses capillary action to
  pull water into their system?
• Plants take advantage of capillary action to pull
  water from the into themselves.
• From the roots water is drawn through the plant
  by another force, transpiration.

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Solutions and Suspensions

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