Modelling molecules

• What is a covalent bond?

  A covalent bond is a shared pair of electrons.
• What do covalent bonds form?

  Covalent bonds form molecules.
• What are the properties of substances with small molecules?

  They have low melting and boiling points and do not conduct electricity.
• What is a small molecule?

  • A collection of two or more atoms held together by chemical bonds.
• How is an atom represented in a dot and cross diagram?

  An atom is drawn as a circle.
• What does a covalent bond represent in a dot and cross diagram?

  A covalent bond is represented where circles overlap.
• How are electrons represented in a dot and cross diagram?

  Electrons from one atom are drawn as dots, and electrons from another atom as crosses.
• How does a hydrogen molecule, H₂, form?

  It forms when two hydrogen atoms each share their outer electron.
• What does a structural formula show?

  A structural formula shows the bonds between the atoms in a molecule.
• How is each covalent bond represented in a structural formula?

  Each covalent bond is shown as a straight line.
• How many covalent bonds does an ammonia molecule have?

  An ammonia molecule has three covalent bonds.
• What are the different ways to represent the structure of small molecules?

  • Dot and cross diagrams
  • Structural formulae
  • Three-dimensional ball and stick models
• What is an advantage of using a dot and cross diagram?

  It shows the pairs of outer electrons and which atoms they have come from.
• What is a limitation of using a dot and cross diagram?

  It does not show how the atoms are arranged in space.
• What is an advantage of using a structural formula?

  It shows the bonds between the atoms.
• What is a limitation of using a structural formula?

  It does not show which atoms the electrons in the bonds have come from.