Chemical naming and formula writing connect the name of a substance to the atoms and ions it contains. Students need this cheat sheet because many chemistry errors come from mixing up charges, prefixes, Roman numerals, and acid rules. A clear reference helps students quickly decide whether a compound is ionic, covalent, acidic, or contains a polyatomic ion.
The most important idea is that ionic compounds must have a total charge of . Covalent compounds use prefixes to show the number of atoms, while ionic compounds usually do not use prefixes. Acids have special naming patterns based on whether the anion contains oxygen.
Correct formulas require careful use of subscripts, parentheses, and charges.
Key Facts
- Ionic compounds are electrically neutral, so the total positive charge plus the total negative charge must equal .
- Write the cation first and the anion second in an ionic formula, such as .
- Use subscripts to balance ion charges, so .
- Use parentheses when more than one polyatomic ion is needed, as in .
- A Roman numeral in a metal name gives the metal ion charge, so iron(III) chloride contains and has the formula .
- Binary covalent compounds use prefixes such as mono-, di-, tri-, tetra-, and penta-, so is carbon dioxide.
- For binary acids, use hydro- plus the nonmetal root plus -ic acid, so is hydrochloric acid.
- For oxyacids, anions ending in -ate form -ic acids and anions ending in -ite form -ous acids, so is nitric acid and is nitrous acid.
Vocabulary
- Cation
- A cation is a positively charged ion that has lost one or more electrons.
- Anion
- An anion is a negatively charged ion that has gained one or more electrons.
- Ionic compound
- An ionic compound is a neutral compound made from cations and anions held together by electrostatic attraction.
- Covalent compound
- A covalent compound is a compound made of nonmetals that share electrons in chemical bonds.
- Polyatomic ion
- A polyatomic ion is a charged group of covalently bonded atoms that acts as one ion.
- Oxidation number
- An oxidation number is the charge an atom has or appears to have in a compound or ion.
Common Mistakes to Avoid
- Writing subscripts without balancing charge is wrong because ionic formulas must have a total charge of . For example, is incorrect and should be .
- Changing the subscripts inside a polyatomic ion is wrong because the ion must stay together as a unit. Calcium sulfate is , not .
- Using covalent prefixes for ionic compounds is wrong because ionic names are based on ion charges, not atom counts. is calcium chloride, not calcium dichloride.
- Forgetting Roman numerals for transition metals is wrong because many transition metals can have more than one charge. is iron(II) chloride, while is iron(III) chloride.
- Naming acids as ordinary ionic compounds is wrong because aqueous acids follow special acid naming rules. is hydrochloric acid, not hydrogen chloride.
Practice Questions
- 1 What subscript is needed on to balance in aluminum chloride?
- 2 Write the formulas for aluminum oxide and calcium nitrate.
- 3 Name the compounds and .
- 4 Explain how you can tell whether a compound name should use covalent prefixes, ionic charges, or acid naming rules.
Understanding Chemical Naming & Formulas
A useful first step is to identify what kind of particles are present. Metals tend to lose electrons and form positive ions. Nonmetals tend to gain electrons and form negative ions.
This makes a metal paired with a nonmetal a strong clue for an ionic compound. A compound made only from nonmetals is usually covalent. In covalent substances, atoms share electrons rather than becoming separate ions.
The name gives a direct count of atoms in each molecule. This difference explains why the same naming rule cannot be used for every compound. Sodium chloride is a repeating crystal of ions, while carbon dioxide is made of separate molecules.
Formula writing is really a counting problem based on charge. The small charge written on an ion tells how many units of positive or negative charge it has. A formula must contain enough ions for these amounts to cancel.
For calcium chloride, one calcium ion has a two positive charge. Each chloride ion has a one negative charge. Two chloride ions are needed for one calcium ion.
Students often use a shortcut called crisscrossing charges. It can work, but it is safer to check the result afterward. Charges are not copied into the final formula.
The subscripts must be reduced to the smallest whole number ratio. For example, a two to two balance becomes one to one, not two to two. This matters because formulas show the simplest ratio in an ionic compound.
Polyatomic ions need special care because they stay together as a group during formula writing. Nitrate, hydroxide, carbonate, sulfate, and ammonium are common examples. Treat each group like one charged unit first.
Then decide how many groups are required. Parentheses show that a subscript applies to the entire group, not only to its last element. Without parentheses, a formula can describe a different substance.
Learning common polyatomic ions saves time in later topics such as reactions, solubility, and acid base chemistry. Their charges should be memorized as part of the ion name, since the charge is what controls the formula.
Names can be checked by reading them in both directions. Start with a formula and see whether every part of the name is represented. Then start with the name and rebuild the formula from the clues it gives.
A Roman numeral is especially important because some metals form more than one ion. Iron can form a two positive ion or a three positive ion, so the numeral prevents confusion. In covalent names, prefixes matter because they describe the actual number of atoms in a molecule.
The ending of the second element changes to ide. Acid names require attention to the ending of the negative ion and to whether the substance is dissolved in water.
The aqueous label is important because hydrogen chloride gas and hydrochloric acid are named differently. Careful reading is more reliable than guessing from a familiar-looking formula.