small molecules

• What is a covalent bond?

  A covalent bond is formed when two atoms share a pair of electrons.
• In which types of elements does covalent bonding typically occur?

  Covalent bonding occurs in most non-metal elements and in compounds formed between non-metals.
• Where are the shared electrons found in a covalent bond?

  The shared electrons are found in the outer shells of the atoms.
• How many electrons does each atom usually contribute to a shared pair in a covalent bond?

  Usually, each atom contributes one electron to the shared pair of electrons.
• What is the process of forming hydrogen chloride through covalent bonding?

  • A hydrogen atom shares its one electron with a chlorine atom.
  • This results in a shared pair of electrons forming a covalent bond.
  • The compound formed is hydrogen chloride (HCl).
• What is a molecule?

  A molecule is a group of two or more atoms joined together by covalent bonds.
• Do substances made up of ions form molecules?

  No, substances that are made up of ions do not form molecules.
• What is the typical size range of small molecules?

  Small molecules are typically around 0.1 nm or $1 \times 10^{-10} \text{ m}$ across.
• Why are individual atoms and molecules too small to see with a light microscope?

  Because they are smaller than the resolution limit of the most powerful light microscope.
• What is the size of a water molecule?

  A water molecule (H2O) is about 0.3 nm or $3 \times 10^{-10} \text{ m}$ across.
• What do chemical formulas of substances with small molecules indicate?

  • The number of atoms of each element in one molecule of the substance.
  • Example formulas:
  • Hydrogen: H2
  • Oxygen: O2
  • Nitrogen: N2
  • Hydrogen chloride: HCl
  • Water: H2O
  • Ammonia: NH3
• What does a dot and cross diagram represent?

  A dot and cross diagram shows the bonding in a small molecule, with electrons from one atom as dots and from another as crosses.
• How is a hydrogen molecule (H2) formed according to dot and cross diagrams?

  A hydrogen molecule forms when two hydrogen atoms each share their outer electron.
• How does an ammonia molecule (NH3) form according to dot and cross diagrams?

  • One nitrogen atom shares its outer electrons with three hydrogen atoms.
  • There are three covalent bonds formed in the ammonia molecule.
• What does a structural formula show?

  A structural formula shows the bonds between the atoms in a molecule.
• How are covalent bonds represented in structural formulae?

  Each covalent bond is shown as a straight line in structural formulae.
• What are three-dimensional ball and stick models used for?

  • To show how the atoms and bonds are arranged in space.
  • They provide a visual representation of molecular structure.
• How do atoms form covalent bonds to achieve a full outer shell?

  Atoms form covalent bonds by sharing electrons to get a full outer shell.
• How many covalent bonds can an atom form based on its electrons?

  • The number of covalent bonds an atom can form is the same as the number of electrons needed to get a full outer shell.
  • For most elements, a full outer shell is eight electrons.
• What is the number of covalent bonds formed by elements in groups 4 to 7?

  • Group 4 (Carbon, C): 4 bonds
  • Group 5 (Nitrogen, N): 3 bonds
  • Group 6 (Oxygen, O): 2 bonds
  • Group 7 (Chlorine, Cl): 1 bond
  • Hydrogen: 1 bond
• How do you determine how many circles to draw for a simple molecular substance like ammonia (NH3)?

  Draw four circles: one labelled N and three labelled H, overlapping the N circle.
• What do dot and cross diagrams show about electrons in covalent bonds?

  Dot and cross diagrams show how electrons are shared in covalent bonds.
• What is a double bond in molecules?

  A double bond consists of two shared pairs of electrons.
• How do oxygen molecules form a double bond?

  Oxygen molecules consist of two oxygen atoms joined together by a double bond.
• What is a triple bond in nitrogen molecules?

  A triple bond consists of three shared pairs of electrons between two nitrogen atoms.
• What are the properties of substances with small molecules?

  • Strong covalent bonds hold the atoms together in molecules.
  • Weak forces exist between molecules and their neighbors.
  • These properties explain their low melting and boiling points.
• Why do most substances with simple molecules have low melting and boiling points?

  Because weak intermolecular forces are overcome, not the strong covalent bonds.
• What happens to a substance when it melts or boils?

  Energy must be transferred to a substance to make it melt or boil.
• What is the role of energy in melting and boiling substances?

  The energy overcomes the attractive forces between the particles in the substance.
• What are intermolecular forces?

  Intermolecular forces are weak forces between small molecules and their neighbors.
• What is the state of oxygen (O2) at 20°C?

  Oxygen (O2) is a gas at 20°C.
• What is the state of water (H2O) at 20°C?

  Water (H2O) is a liquid at 20°C.
• What is required for a substance to conduct electricity?

  A substance must contain charged particles that are free to move from place to place.
• Why can't small molecules conduct electricity?

  Small molecules have no overall electric charge, so they cannot conduct electricity.
• What happens when simple molecular substances melt or boil?

  • The weak intermolecular forces are overcome.
  • The strong covalent bonds remain intact.