Lecture 1

• What defines an element in chemistry?


  It is a substance that cannot be chemically interconverted or broken down into simpler substances.
• How is each element distinguished?

  By its atomic number, which is the number of protons in its nucleus.
• What is an atom?


  The smallest particle of a chemical element that can exist.
• What is a compound?


  A substance composed of atoms of more than one element.
• How are molecules formed?
• What is valency?


  The combining power of an element, especially as measured by the number of hydrogen atoms it can displace or combine with.
• Why is valency important?


  It determines how many bonds an atom can form with other atoms.
• How do you find the valency of an atom?


  By determining the number of electrons it needs to gain or lose to obtain a full outer shell.
• If an atom has four electrons in its outer shell, what is its valency?


  It has a positive valency and wants to give away electrons.
• What happens when an atom has five, six, or seven electrons in its outer shell?


  The valency is determined by subtracting the number of electrons from eight, indicating it wants to gain electrons.
• What is the valence shell?


  The outermost shell of electrons responsible for the formation of bonds.
• How do atoms join together?


  Atoms join to obtain a full complement of outer shell electrons.
• What is an example of an ionic bond?


  NaCl (sodium chloride)
• How are ionic bonds formed?


  By the transfer of electrons from one atom to another.
• What is a covalent bond?


  A bond formed between two atoms involving the sharing of electrons.
• What is an example of a covalent bond?


  H2O (water)
• What is a lone pair of electrons?


  A pair of electrons that are not involved in the formation of bonds.
• How do lone pairs influence molecular shape?

  Lone pairs affect the arrangement of bonding pairs of electrons, altering the shape of the molecule.
• What is a double bond?


  A bond formed when two pairs of electrons are shared between two atoms.
• What is the structure of benzene?


  A ring structure with alternating single and double bonds.
• What are delocalised electrons?


  Electrons that are free to move anywhere within a structure, such as in benzene.
• What is a dative (coordinate) bond?


  A type of covalent bond where one atom contributes both of the electrons.
• What is the significance of valency in covalent bonding?


  It indicates how many covalent bonds an element can form.
• What factors determine the shape of molecules?

  • Number of bonding pairs of electrons
  • Number of lone pairs of electrons
  • Valence electron pairs move as far apart as possible
• What is the molecular shape of methane (CH4)?


  Tetrahedral
• What is the molecular shape of ammonia (NH3)?


  Pyramidal
• What is the molecular shape of water (H2O)?


  Bent
• How does the molecular shape of carbon dioxide (CO2) differ from that of water (H2O)?


  CO2 is linear while H2O is bent.
• How do lone pairs affect the shape of a molecule?

  Lone pairs push bonding pairs closer together, altering the molecular geometry.
• What is the overall charge of an ionic compound?


  Zero charge overall.
• Why are ionic bonds strong in solids but weak in polar solvents?


  Because the electrostatic interactions are disrupted in polar solvents.
• What is the valency of hydrogen?


  1
• What is the valency of carbon?


  4
• What is the valency of oxygen?

  2
• What are the types of bonds discussed in the lecture?

  • Ionic bonds: Transfer of electrons
  • Covalent bonds: Sharing of electrons
  • Dative (coordinate) bonds: One atom contributes both electrons
• What are the differences between ionic and covalent bonds?

  • Ionic bonds involve transfer of electrons; covalent bonds involve sharing.
  • Ionic bonds form between metals and non-metals; covalent bonds form between non-metals.
  • Ionic compounds have high melting points; covalent compounds have lower melting points.
• What types of bonds involve the sharing of 1, 2, or 3 pairs of electrons?


  Single, double, and triple bonds
• What do aromatic compounds contain?


  Delocalized electrons
• What is a dative covalent bond?


  A bond formed when one atom contributes both bonding electrons
• What must be considered when determining the shape of molecules?

  The number of bonding pairs and lone pairs of electrons