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Use the Periodic Table

Writing chemical formulas begins with a solid grasp of the periodic table. The table lists every known element, pairing its full name with a concise symbol—H for hydrogen, Cl for chlorine, and so forth. Some symbols follow the element’s English name (e.g., O for oxygen), while others come from Latin or Greek roots (Na for sodium, derived from *natrium*). When a symbol isn’t immediately obvious, the periodic table is your go‑to reference.

Identifying Chemical Symbols

Before drafting a formula, identify every atom in the compound or molecule. For instance, the common table salt is sodium chloride; write Na for sodium and Cl for chloride. The resulting formula is NaCl. For covalent compounds—those formed between two nonmetals—the names often translate directly into symbols. Prefixes indicate multiple atoms: carbon dioxide becomes CO₂ because the “di‑” prefix signals two oxygen atoms.

Determining the Valence

Ionic compounds involve charged atoms. Many periodic tables list typical valences. Group 1 metals are +1 cations, group 2 are +2, and transition metals (groups 3–12) can exhibit a range of charges. Elements in groups 13–18 may have variable charges, while groups 15–17 typically form anions with negative charges. Knowing these charges is essential for balancing the overall formula.

Balancing the Charges

Balance the total charge to ensure the compound is neutral. For example, magnesium (Mg) carries a +2 charge, and the oxide ion (O²⁻) carries –2. Combining one Mg²⁺ with one O²⁻ yields MgO, a neutral formula. Similarly, balancing magnesium chloride requires two chloride ions (Cl⁻) for every magnesium ion, producing MgCl₂.

Writing the Chemical Formula

Subscripts denote the number of atoms of each element. A single atom is implied, so no subscript “1” is written. In MgO, the absence of a subscript indicates one magnesium and one oxygen atom. When multiple atoms of the same element are present, the subscript follows the element’s symbol—MgCl₂ shows two chlorine atoms per magnesium.

Additional Tips

• Compounds ending in “‑ide” usually belong to groups 15–17.
• Roman numerals in parentheses, such as Fe(II), specify the ion’s charge (here +2).
• Polyatomic ions (e.g., hydroxide OH⁻) are grouped in parentheses when they appear multiple times: Al(OH)₃ is aluminum hydroxide.

With regular practice, these guidelines will become second nature, enabling you to write accurate chemical formulas confidently.