2.3 : Chemical Formula

Chemical Formula

A chemical formula is a representation of the types and numbers of atoms in a chemical compound using symbols and numbers. Each element is denoted by its chemical symbol (one or two letters), and subscripts indicate the number of atoms of each element present in a compound molecule.

• A subscript (the small number after a chemical symbol) shows how many atoms of that element are in a compound molecule.
  • Example: CO₂ represents one carbon atom and two oxygen atoms.
• A coefficient (a number before the chemical formula) shows how many molecules of a compound are present.
  • Example: 2H₂O means two water molecules containing four hydrogen and two oxygen atoms.

Science and Engineering Practices (SEP): Analyzing and Interpreting Data

Scientists use chemical formulas to create molecular models and visualize bonding patterns and molecular structures. By analyzing formulas, they can predict how elements will react and combine. Understanding these formulas allows chemists to design chemical reactions and create new compounds with desired properties, making chemical formulas a universal language for sharing information about compounds and reactions across scientific fields.

Activity Ideas:

• Build a Formula Puzzle – Use flashcards with chemical symbols and numbers to build chemical formulas for common compounds (e.g., H₂O, CO₂, NaCl). Identify how many atoms of each element are present in the compounds.
• Create Your Compound – Combine element cards (showing atomic numbers and symbols) to form new chemical formulas. Write down the name of the compound (if known) and explain its potential properties based on the elements involved.

Crosscutting Concepts (CCC): Patterns

Patterns help scientists understand and predict how elements combine to form compounds in chemistry. Chemical formulas follow specific rules, showing the ratio of atoms in a molecule. By recognizing these patterns, scientists can determine molecular structures, predict chemical reactions, and classify compounds with similar properties.

• Pattern: Elements in the same group of the periodic table form compounds with similar formulas.
  Example: Group 1 metals (e.g., sodium and potassium) form compounds like NaCl and KCl, which bond with chlorine in a 1:1 ratio.
• Pattern: Many nonmetals form diatomic molecules, meaning they naturally exist as pairs of atoms.
  Example: O₂, N₂, and H₂ are all diatomic molecules because these elements bond with themselves to become more stable.
• Pattern: Compounds containing polyatomic ions follow predictable formulas based on charge balance.
  Example: Sulfates (SO₄²⁻) combine with calcium (Ca²⁺) to form CaSO₄, maintaining a neutral charge.