IB MYP 4-5 Chemistry -Writing and naming chemical formulas- Study Notes - New Syllabus

Writing and Naming Chemical Formulas

A chemical formula shows the types and numbers of atoms present in a compound. It represents the simplest ratio in which elements combine chemically to form a substance.

Writing and naming chemical formulas correctly is essential for understanding reactions, predicting products, and communicating chemical information accurately.

Types of Compounds

• Ionic Compounds: Formed by the transfer of electrons between metals and non-metals. (e.g., \( \mathrm{NaCl,\ MgO} \))
• Covalent Compounds: Formed by sharing of electrons between non-metals. (e.g., \( \mathrm{CO_2,\ H_2O} \))

Rules for Writing Formulas of Ionic Compounds

Step 1: Write the symbol of the metal (cation) first, then the non-metal (anion).

Step 2: Write the valency (charge) of each ion above its symbol.

Step 3: Balance the total positive and negative charges so that the compound is neutral overall.

Step 4: Write the formula by combining the symbols and using subscripts to show the ratio of ions.

Example:

• \( \mathrm{Na^+} \) and \( \mathrm{Cl^-} \) → one of each → \( \mathrm{NaCl} \)
• \( \mathrm{Mg^{2+}} \) and \( \mathrm{O^{2-}} \) → charges balance → \( \mathrm{MgO} \)
• \( \mathrm{Ca^{2+}} \) and \( \mathrm{Cl^-} \) → two Cl⁻ needed → \( \mathrm{CaCl_2} \)

Polyatomic ions: Some ions consist of more than one atom (act as a single charged unit). Examples include:

When combining polyatomic ions: If more than one polyatomic ion is required, use brackets around it. Example: \( \mathrm{Ca^{2+}} \) and \( \mathrm{NO_3^-} \) → \( \mathrm{Ca(NO_3)_2} \)

Naming Ionic Compounds

• Name the metal first, followed by the non-metal.
• Change the ending of the non-metal to “–ide.” e.g., \( \mathrm{NaCl} \): sodium chloride
• For compounds with polyatomic ions, use their full names: e.g., \( \mathrm{Na_2SO_4} \): sodium sulfate
• For transition metals, indicate the oxidation state (charge) with Roman numerals: e.g., \( \mathrm{FeCl_2} \): iron(II) chloride, \( \mathrm{FeCl_3} \): iron(III) chloride

Rules for Writing Formulas of Covalent Compounds

Step 1: Write symbols of the elements involved.

Step 2: Use prefixes to show how many atoms of each element are present.

Prefixes:

Examples:

• \( \mathrm{CO} \): carbon monoxide
• \( \mathrm{CO_2} \): carbon dioxide
• \( \mathrm{N_2O_3} \): dinitrogen trioxide
• \( \mathrm{PCl_5} \): phosphorus pentachloride

Common Mistakes to Avoid

• Do not write charges in the final formula (e.g., \( \mathrm{NaCl} \), not \( \mathrm{Na^+Cl^-} \)).
• For covalent compounds, use prefixes only — not charges.
• Always ensure ionic compounds are neutral overall.