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?

  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 do most covalently bonded substances consist of?

  Most covalently bonded substances consist of small molecules.
• What is a molecule?

  A molecule is a group of two or more atoms joined together by covalent bonds.
• Do molecules of the same element or compound always contain the same number of atoms of each element?

  Yes, molecules of the same element or compound always contain the same number of atoms of each element.
• What type of substances do not form molecules?

  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?

  They are too small to see even with the most powerful light microscope.
• How large is a water molecule, H2O?

  A water molecule is about $0.3 \text{ nm} (3 \times 10^{-10} \text{ m})$ across.
• What does the chemical formula of a substance with small molecules show?

  The chemical formula shows the number of atoms of each element in one molecule of the substance.
• What are the chemical formulas and number of atoms for common substances with small molecules?

  • Hydrogen: H2 (Two hydrogen atoms)
  • Oxygen: O2 (Two oxygen atoms)
  • Nitrogen: N2 (Two nitrogen atoms)
  • Hydrogen chloride: HCl (One hydrogen atom and one chlorine atom)
  • Water: H2O (Two hydrogen atoms and one oxygen atom)
  • Ammonia: NH3 (One nitrogen atom and three hydrogen atoms)
• What is a dot and cross diagram used for?

  A dot and cross diagram shows the bonding in a small molecule.
• 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 is a hydrogen molecule, H2, formed?

  A hydrogen molecule 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 are covalent bonds represented in a structural formula?

  Each covalent bond is shown as a straight line.
• What do three-dimensional ball and stick models represent?

  They show how the atoms and bonds are arranged in space.
• How do atoms form covalent bonds?

  Atoms form covalent bonds by sharing electrons to get a full outer shell.
• What is the maximum number of electrons needed for a full outer shell for most elements?

  A full outer shell is usually eight electrons.
• How many covalent bonds can elements in groups 4 to 7 form?

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

  Look at the formula to determine how many circles to draw and label them accordingly.
• What is the significance of non-bonding outer electrons in a dot and cross diagram?

  Non-bonding outer electrons are those that do not take part in bonding.
• What do dot and cross diagrams show about covalent bonds?

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

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

  Oxygen atoms can form two covalent bonds, so a double bond forms between them.
• What is a triple bond in a molecule?

  A triple bond consists of three shared pairs of electrons.
• How do nitrogen molecules form a triple bond?

  Nitrogen atoms can form three covalent bonds, so a triple bond forms between them.
• What are the properties of substances with small molecules?

  • Strong covalent bonds within molecules
  • Weak forces between molecules
  • Low melting points and boiling points
  • Cannot conduct electricity
• Why do most substances with simple molecules have low melting and boiling points?

  They have weak intermolecular forces that require less energy to overcome.
• What happens during melting and boiling of small molecular substances?

  Energy must be transferred to overcome the attractive forces between the particles.
• What are intermolecular forces?

  Intermolecular forces are the weak forces between small molecules and their neighbors.
• What is the melting point of oxygen (O2)?

  • 218°C
• What is the boiling point of water (H2O)?

  100°C
• Why can small molecules not conduct electricity?

  Small molecules have no overall electric charge and their particles cannot move freely.
• What happens when simple molecular substances melt or boil?

  When simple molecular substances melt or boil, their weak intermolecular forces are overcome, not their strong covalent bonds.