The Nature of Molecules

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The Nature of Molecules

• Chapter 2

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Outline

• Structure of Atoms
• Isotopes
• Radioactive Isotopes
• Electrons and Chemical Behavior
• Chemical Bonds
• Ionic versus Covalent
• Water Chemistry
• Hydrogen Bonding
• pH

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Atoms

• All matter is composed of atoms.
• Protons () and neutrons (neutral) are found on
  the atoms nucleus, while electrons (-) circle
  the nucleus.
• Atomic number - number of protons
• Atoms with the same atomic number belong to the
  same element, and thus have the same inherent
  properties.

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Atoms
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Atomic Weight

• Mass refers to the amount of a substance.
• Weight refers to the force exerted on a substance
  by gravity.
• Atomic mass of an atom refers to the sum of the
  masses of protons and neutrons.
• measured in Daltons

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Isotopes

• Isotopes - Atoms of an element that possess a
  different number of neutrons.
• Radioactive isotopes - Spontaneously decay into
  elements of lower atomic number.
• emit energy and/or subatomic particles
• Half-life refers to the amount of time necessary
  to decay half the atoms of a given sample.

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Electrons

• Atoms with the same number of protons as
  electrons are electrically neutral.
• Ions - Atoms in which the number of protons and
  electrons differ.
• Cation - Contains more protons than electrons,
  and carries a positive charge.
• Anion - Contains fewer protons than electrons,
  and carries a negative charge.

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Electrons and Atomic Behavior

• Orbital refers to the area around a nucleus where
  an electron is most likely found.
• Chemical behavior of an atom is determined by the
  number and arrangement of its orbitals.
• Electrons are attracted to the positively charged
  nucleus, thus it takes energy to hold electrons
  in place.
• potential energy of position

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Atomic Energy Levels
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Electrons and Atomic Behavior

• During some chemical reactions, electrons are
  transferred between atoms, while still retaining
  their energy of position.
• Oxidation - loss of an electron
• Reduction - gain of an electron

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Kinds of Atoms

• Ninety-two naturally occurring elements
• Periodic table arranged by grouping atoms based
  on valence electrons (electrons in the outer
  energy levels).
• Octet rule
• Inert atoms have outer level filled.
• Reactive atoms do not have outer level filled.

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Periodic Table of the Elements
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Chemical Bonds

• A molecule refers to a group of atoms held
  together by energy in a stable association.
• Compound is composed of two or more different
  types of atoms.
• Atoms in a molecule are joined by chemical bonds.

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Chemical Bonds

• Ionic bonds are formed because ions of opposite
  charge attract one another.
• table salt
• Covalent bonds are formed when two or more atoms
  share pairs of valence electrons.
• Strength depends on number of shared electrons.

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Chemical Reactions

• A chemical reaction occurs during the formation
  or breaking of chemical bonds.
• Reactants refer to original molecules.
• Products refer to resultant molecules.
• Chemical reactions can be influenced by
• temperature
• concentration of reactants and products
• catalysts

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Water Chemistry

• Water consists of an oxygen atom bound to two
  hydrogen atoms by two single covalent bonds.
• stable atom with no electrical charge
• most outstanding property is its ability to form
  weak chemical associations

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Hydrogen Bonding

• In a water molecule, both the oxygen and hydrogen
  atoms attract the shared electrons in the
  covalent bond (electronegativity).
• Oxygen atom is more electronegative than the
  hydrogen atoms.
• distinct ends with partial charge (polar)
• hydrogen bonds

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Water Chemistry

• Water clings to polar molecules.
• Cohesion refers to attraction to other water
  molecules.
• responsible for surface tension
• Adhesion refers to attraction to other
  substances.
• Water is adhesive to any substance with which it
  can form hydrogen bonds.

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Water Chemistry

• Water stores heat.
• High specific heat
• Amount of heat that must be absorbed or expended
  to change the temperature of 1g of a substance 1o
  C.
• High heat of vaporization
• Amount of energy required to change 1g of liquid
  water into a gas (586 calories).
• large number of hydrogen bonds

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Water as a Solvent

• Water is an effective solvent as it can form
  hydrogen bonds.
• Water clings to polar molecules causing them to
  be soluble in water.
• Hydrophilic - attracted to water
• Water tends to exclude nonpolar molecules.
• Hydrophobic - repelled by water

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pH

• pH scale expresses hydrogen ion (H)
  concentration in a solution.
• logarithmic scale ranging from 0-14
• neutral 7

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pH

• Acids dissociate in water to increase the
  concentration of H.
• pH values lower than 7
• Bases combine with H ions when dissolved in
  water, thus decreasing H concentration.
• pH values above 7
• Buffers act as a reservoir for hydrogen ions,
  donating or removing them from solution as
  necessary.

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pH
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Summary

• Structure of Atoms
• Isotopes
• Radioactive Isotopes
• Electrons and Chemical Behavior
• Chemical Bonds
• Ionic versus Covalent
• Water Chemistry
• Hydrogen Bonding
• pH

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