In chemistry, ions can be single atoms or clusters of atoms acting together—known as polyatomic ions. Each ion carries a distinct charge determined by the valence electrons of its constituent atoms. While numerous methods exist for calculating these charges, the most reliable approach is memorization, supported by systematic techniques and mnemonic aids.
Calculate the Charge Using Oxidation Numbers
- Assign oxidation numbers to every atom in the ion. For example, in hydroxide (OH⁻), oxygen is –2 and hydrogen is +1.
- Sum the oxidation numbers of all atoms. In OH⁻, –2 + (+1) = –1.
- Write the charge as a superscript to the right of the formula. A single unit is written as – or +, e.g., OH⁻.
Determine the Charge by Drawing a Lewis Structure
- Draw Lewis dots for each atom. For ammonium (NH₄⁺), nitrogen is represented with five dots and each hydrogen with one dot.
- Form covalent bonds between the atoms. The four hydrogens bond to nitrogen, using nitrogen’s valence electrons.
- Convert shared pairs to lines to denote covalent bonds.
- Count remaining electrons after achieving octet/duet rules. Excess electrons denote a negative charge; a deficit indicates a positive charge. Ammonium has one extra electron, giving it a +1 charge.
Mnemonic Devices for Quick Recall
- Use a memorable sentence that embeds the ion’s suffix. For instance, “Nick the Camel ate a Clam for Supper in Phoenix” helps recall the "‑ate" series.
- Identify the main atom from the sentence’s key word: “Nick” → Nitrogen for nitrate.
- Count the consonants in the word to determine the number of oxygen atoms. “Nick” has three consonants → 3 O atoms.
- Count the vowels to find the negative charge. “Nick” has one vowel → –1 charge.
Essential Tools
- Periodic table (digital or print)
- Pencil and paper for quick sketching
TL;DR
Oxidation numbers are reliable for simple ions like NH₄⁺, OH⁻, CN⁻, and CH₃COO⁻. For ions containing elements with variable oxidation states (e.g., S, N), use mnemonic methods or memorization to determine charges.
