Molecular Compounds, Naming, and Formula Calculations

Molecular Compounds and Covalent Bonds

• Ionic Compounds: Formed by ionic bonds (metal + nonmetal) involving electron transfer.
• Molecular Compounds: Formed by covalent bonds (nonmetal + nonmetal) involving electron sharing.
• The primary goal is to learn how to name molecular compounds.

Types of Covalent Bonds

• Covalent bonds involve the sharing of electrons between atoms.
• Lewis dot structures or lines represent shared electrons.
• Bonding Pair: Two electrons shared between two atoms, forming a covalent bond (e.g., in water, hydrogen and oxygen share electrons).
• Lone Pair Electrons: Electrons on an atom that are not shared with other atoms (e.g., oxygen in water has two lone pairs).
• Each single line in a Lewis structure represents two shared electrons.

Single Covalent Bond

• Involves the sharing of $2$ electrons.
• Represented by a single line between two atoms.
• Exception: Hydrogen, a nonmetal, will only ever form a duet (have $2$ electrons).
• Most other atoms aim to achieve an octet ($8$ electrons) in their valence shell through sharing and lone pairs.

Double Covalent Bond

• Involves the sharing of $4$ electrons.
• Represented by two lines between two atoms.
• Elements capable of forming double bonds: Carbon ($C$), Nitrogen ($N$), Oxygen ($O$), Sulfur ($S$), and Phosphorus ($P$).

Triple Covalent Bond

• Involves the sharing of $6$ electrons.
• Represented by three lines between two atoms (e.g., in $N_{2}$ molecule).
• Elements capable of forming triple bonds: Carbon ($C$), Nitrogen ($N$), Oxygen ($O$), and Sulfur ($S$).

Naming Molecular Compounds

• Molecular compounds can form in various ratios (e.g., $NO$ and $NO_{2}$) due to their ability to form single, double, and triple bonds. This requires explicitly stating the number of each element.
• Prefixes are used to indicate the number of atoms of each element (similar to hydrate naming, but 'hemi' is not used for atoms).
  • $1$ - mono
  • $2$ - di
  • $3$ - tri
  • $4$ - tetra
  • $5$ - penta
  • $6$ - hexa
  • $7$ - hepta
  • $8$ - octa
  • $9$ - nona
  • $10$ - deca
• Rules for Naming:
  1. Use a prefix for both elements present.
  2. Exception: Do not use 'mono' if it is the first element and there is only one atom of it.
  3. Do use 'mono' if it is the second element and there is only one atom of it.
  4. The first element keeps its full elemental name.
  5. Add '-ide' to the root of the second element's name.
  6. Name the elements in the same order they appear in the chemical formula.
• Examples:
  • $NI_{3}$: Nitrogen triiodide (nitrogen is first, so no 'mono')
  • $PCl_{5}$: Phosphorus pentachloride (phosphorus is first, so no 'mono')
  • $N<em>{2}O</em>{5}$: Dinitrogen pentaoxide (two nitrogens, five oxygens)
• Writing Formulas from Names:
  • Tetraphosphorus decasulfide: $P<em>{4}S</em>{10}$
    • This is a molecular formula (exact number of atoms).
    • Its empirical formula (smallest whole-number ratio) would be $P<em>{2}S</em>{5}$.
  • Dihydrogen monoxide: $H_{2}O$ (also known as water)
  • Carbon dioxide: $CO_{2}$
• Important Note: This naming method is strictly for molecular compounds (nonmetals only); it does not apply to compounds involving metals.

Percent Composition

• Definition: The percentage by mass of each element in a compound.
• Methods to determine:
  1. Using the compound's formula (main focus in this lecture).
  2. Experimental mass analysis (can be done in a lab setting).
• Formula for Percent Mass of an Element:
  ext{Percent Mass} = rac{ ext{Molecular mass of the element (in the compound)}}{ ext{Mass of one mole of the compound}} imes 100
  (This is essentially