Chemical Ions, Compounds, Formulas, and Nomenclature

Posted on Apr 6, 2025 in Chemistry

Ions, Compounds, and Chemical Formulas

Understanding Ions

An ion is a charged particle formed when a neutral atom or group of atoms gains or loses electrons.

Cations: Positively Charged Ions

A cation is a positively charged ion formed when a neutral atom loses electrons (e.g., Na⁺).

Anions: Negatively Charged Ions

An anion is a negatively charged ion formed when a neutral atom gains electrons (e.g., Cl⁻).

Chemical Compound Basics

Ionic vs. Molecular Compounds

An ionic compound is formed by cations and anions. Ionic compounds consist of electrically charged particles, whereas molecular compounds are composed of neutral particles (molecules).

Monatomic, Diatomic, Polyatomic

Monatomic refers to one atom, diatomic refers to two atoms, and polyatomic refers to two or more atoms.

Representing Compounds: Formulas

Chemical formulas express the composition of molecules and ionic compounds using chemical symbols.

Molecular and Empirical Formulas

A molecular formula indicates the exact number of atoms of each element present in the smallest unit (molecule) of a substance.

An empirical formula indicates the elements present and the simplest whole-number ratio of their atoms. Example: The empirical formula for N₂H₄ is NH₂.

Allotropes: Different Elemental Forms

An allotrope is one of two or more different forms of the same element in the same physical state (e.g., O₂ and O₃ are allotropes of oxygen).

Classifying Compounds

Organic vs. Inorganic Compounds

Organic compounds contain carbon, typically combined with elements like hydrogen, oxygen, nitrogen, and sulfur. However, some carbon-containing compounds are considered inorganic, such as carbon monoxide (CO), carbon dioxide (CO₂), carbon disulfide (CS₂), compounds containing the cyanide group (CN⁻), carbonates (CO₃²⁻), and bicarbonates (HCO₃⁻).

Inorganic compounds encompass all other compounds.

Binary and Tertiary Compounds

Binary compounds consist of only two elements. Tertiary compounds are formed from three elements.

Transition Metals & Oxidation Numbers

Transition metals are elements in groups IB and IIIB through VIIIB of the periodic table. Oxidation numbers represent the apparent charge an atom would have if electrons were completely transferred.

Chemical Nomenclature Rules

Naming Ionic Compounds

Ionic compounds are formed from cations and anions. With the exception of the ammonium ion (NH₄⁺), most common cations are derived from metal atoms.

Basic Binary Ionic Naming

For binary ionic compounds like NaCl, the anion is named by taking the root of the element name (Chlorine → Chlor-) and adding the suffix ‘-ide’ (Chloride). The cation name (Sodium) comes first, resulting in Sodium Chloride.

Metal cations are typically named after the element. Example:

Stock System for Metals

The Stock System is used for metals that can form more than one type of cation. The name consists of the metal name, followed by its oxidation state (charge) in Roman numerals in parentheses, and then the anion name (e.g., Iron(II) oxide, Iron(III) oxide).

Naming Molecular Compounds

Molecular compounds are composed of discrete molecular units. To name binary molecular compounds:

1. The second element in the formula is named first, using the root of its name plus the suffix ‘-ide’.
2. The first element in the formula is named second, using its full element name.
3. Greek prefixes are used to indicate the number of atoms of each element. The prefix ‘mono-‘ is usually omitted for the first element.

Examples: HCl – Hydrogen chloride, SiC – Silicon carbide.

Greek Prefixes in Naming

When a pair of elements forms multiple compounds, prefixes are essential to avoid confusion: CO – Carbon monoxide, CO₂ – Carbon dioxide, SO₃ – Sulfur trioxide.

Common Names for Hydrogen Compounds

An exception to using Greek prefixes involves many molecular compounds containing hydrogen. These are often called by their common or non-systematic names: B₂H₆ – Diborane, CH₄ – Methane, SiH₄ – Silane, NH₃ – Ammonia, PH₃ – Phosphine, H₂O – Water, H₂S – Hydrogen sulfide.

Naming Specific Compound Classes

Hydrides

A hydride is a binary compound containing hydrogen, typically with hydrogen as the anion (H⁻) or in a covalent bond (e.g., NaH – Sodium hydride).

Oxides

An oxide is a binary compound of an element with oxygen (e.g., CaO – Calcium oxide).

Hydroxides / Bases

Hydroxides (Bases) are substances that typically release hydroxide ions (OH⁻) in water or accept hydrogen ions (H⁺). They are often named with the word hydroxide following the name of the metal cation (e.g., NaOH – Sodium hydroxide).

Acids

An acid can be described as a substance releasing hydrogen ions (H⁺) when dissolved in water. Acids typically have a sour taste, change the color of plant dyes (indicators), and react with certain metals (like zinc, magnesium, iron) to produce hydrogen gas. Anions ending in ‘-ide’ form acids with the prefix ‘hydro-‘ and the suffix ‘-ic acid’. Example:

Salts

A salt is an ionic compound formed from the reaction between an acid and a base. The general reaction is: Base + Acid → Salt + Water (e.g., NaOH + HCl → NaCl + H₂O). In this reaction, the H⁺ from the acid combines with the OH⁻ from the base (or the base accepts the H⁺).