L18. Writing Chemical Formulas

Chemical Names and Formulas

R-report

L18. Writing Chemical Formulas

How do we write the correct recipe of atoms and charges so a compound is neutral and shows the actual particles present?

What a chemical formula shows

A chemical formula is a compact recipe that tells which elements are present and how many atoms of each appear in the smallest stable unit of a substance. You already learned how to name ionic and molecular compounds; now we focus on writing the formula itself. A formula uses element symbols (like Na, O, or S), subscripts to show atom counts (the 2 in H2), parentheses for repeated groups (as in Ca3(PO4)2), and — for charged ions — a plus or minus sign with a number to show charge (Na+ or SO4 2−). Read a formula as a particle-level description: molecules for covalent substances, and simplest whole-number ratios for ionic substances.

• Element symbol: one- or two-letter code (C, Cl) Subscript: small number after a symbol = count of that atom Parentheses: repeat a group; multiply subscripts inside Charge: shown on isolated ions (not on neutral formulas)

Writing formulas for ionic compounds (balance the charge)

Ionic formulas show the simplest whole-number ratio of positive and negative ions that gives a neutral compound. You do not list charges in the final neutral formula — charges guide how many of each ion combine. Always check that total positive charge + total negative charge = 0 (we'll show charge units in the calculation). Use parentheses around polyatomic ions when more than one is needed, and reduce subscripts to the simplest ratio.

• Step 1: write the ion symbols with their charges (e.g., Al3+, SO4 2−). Step 2: criss-cross charges or use least common multiple to get counts (Al3+ needs 2 to balance 3− charges of 3 SO4 2−? instead use LCM). Step 3: put counts as subscripts; use parentheses if the anion is polyatomic (Al2(SO4)3). Step 4: check charges with simple arithmetic and show charge units (e.g., 2 × (+3) = +6 charge units).

Writing molecular formulas for covalent compounds

For covalent (molecular) compounds, the formula lists the element symbols with subscripts that show the number of each atom in one molecule. There is no overall ionic charge to balance; instead, use the molecular information you have (names with prefixes, experimental composition, or structural clues) to put the correct subscripts. For example, from the name 'dinitrogen pentoxide' (you have seen naming rules) write N2O5: two N atoms, five O atoms. If given an empirical formula (the simplest ratio), you may need experimental data to find the actual molecular formula — that step appears later in the course.

Everyday application: reading formulas on household salts

Look at labels on common crystalline salts and decode the formula to see the particles. Epsom salt is magnesium sulfate, MgSO4: Mg2+ balances SO4 2− in a 1:1 ratio so formula is MgSO4. Table salt is NaCl: Na+ and Cl− combine 1:1 so formula is NaCl. Knowing the formula tells you exactly how many atoms of each element are in one formula unit, and you can use that to calculate relative masses later (we will show moles and mass in future lessons). When writing formulas from ions, always show simple whole-number subscripts and check that atoms and total charge balance.

Putting it together

Chemical formulas are a particle-level shorthand: symbols, subscripts, and parentheses record exactly how many atoms or ions are combined. For ionic compounds we use the charges of the ions as a counting tool to reach overall neutrality; show this by simple arithmetic with charge units and by counting atoms to confirm the formula unit.

This lesson prepares you for drawing Lewis models and predicting structure: once you can write correct formulas (like Al2(SO4)3 or N2O5), you will place the right numbers of atoms and electrons in later lessons to show bonds and geometry.