Binary Ionic Compounds & The Stock System

Ever wonder how chemists name all those chemical compounds? It's not as complicated as it seems! For binary ionic compounds (made of a metal cation and a nonmetal anion), the metal is named first, followed by the nonmetal with its ending changed to "-ide."

When transition metals are involved, we use the Stock System with Roman numerals to show the metal's charge. For example, FeCl₃ is named "iron(III) chloride" because iron has a +3 charge. Some transition metals can form multiple ions (like Cu⁺ and Cu²⁺), which is why specifying the charge is important.

To write formulas for ionic compounds, remember that the overall charge must be neutral. Use the "crisscross method" where the numerical value of each ion's charge becomes the subscript for the other ion. For example, aluminum oxide (Al³⁺ and O²⁻) becomes Al₂O₃.

Chemistry Tip: When you see a compound like KF or CaBr₂, the subscripts in the formula don't affect the name - they just tell you how many atoms are needed to balance the charges. Potassium fluoride is still potassium fluoride whether it's written as KF, K₂F₂, or K₃F₃!

Polyatomic ions are groups of atoms that act as a single unit with an overall charge. Most polyatomic ions are anions and often end in "-ate" or "-ite" (like nitrate NO₃⁻ or nitrite NO₂⁻). The most common polyatomic cation is ammonium (NH₄⁺).

Polyatomic Ions & Molecular Compounds

Transition metals form ions by losing electrons from their outer s-orbitals first. This is why they can form multiple different ion charges - they have unfilled inner d-electron shells that can also lose electrons.

Ternary ionic compounds contain three or more elements, usually with one type of cation and one type of anion, where at least one is a polyatomic ion. For example, NH₄Cl (ammonium chloride) or Fe(OH)₃ (iron(III) hydroxide). When writing formulas with polyatomic ions, use parentheses and a subscript to show how many of those ions are present.

For binary molecular compounds (made of two nonmetals), we use a prefix system to show how many atoms of each element are present. The first element doesn't use "mono-" but the second does (for one atom). For example, NO is nitrogen monoxide, and Cl₂O₇ is dichlorine heptoxide.

Remember this! Ionic compounds form between metals and nonmetals, while molecular compounds form between nonmetals only. This distinction helps you know which naming system to use.

Acids are molecular compounds that release H⁺ when dissolved in water. Binary acids $H + one other element$ are named "hydro-[element]ic acid" like hydrochloric acid (HCl). Oxoacids follow a pattern based on their related polyatomic ions: "-ate" becomes "-ic acid" and "-ite" becomes "-ous acid."

Bases produce hydroxide ions (OH⁻) in water and are named by writing the cation first, followed by "hydroxide." Common examples include calcium hydroxide Ca(OH)₂ and ammonium hydroxide NH₄OH.

Atomic Ions & Periodic Table Trends

Looking at the periodic table reveals patterns in how elements form ions. This visual guide shows the most common ion charges for elements across the table.

Group 1A (alkali metals) form +1 ions (like Na⁺), while Group 2A (alkaline earth metals) form +2 ions (like Ca²⁺). This makes sense because these elements want to lose their outer electrons to achieve a stable electron configuration.

On the right side of the table, Group 7A (halogens) typically form -1 ions (like Cl⁻), and Group 6A elements usually form -2 ions (like O²⁻). These elements gain electrons to fill their outer shells.

Transition metals in the middle of the table can form multiple ion charges. For instance, iron can form Fe²⁺ and Fe³⁺ ions. That's why we need the Stock System with Roman numerals to specify which ion we're talking about.

Quick Check: Notice how the charge of an ion often matches its group number or is calculated as $Group number - 8$ for nonmetals on the right side of the table. This pattern can help you predict ion charges!

Understanding these patterns makes it much easier to predict how elements will combine to form compounds, and how to name those compounds correctly.