Chemical Compounds, Formulas, and Nomenclature

Ions: The Building Blocks of Ionic Compounds

Not all chemical species are neutral. Many atoms and groups of atoms carry an electrical charge, known as an ion.

Cations and Anions

An ion is formed when an atom gains or loses electrons.

• Cations are positively charged ions ($M^+$) that have lost electrons, such as the sodium ion ($\text{Na}^+$) or the magnesium ion ($\text{Mg}^{2+}$).
• Anions are negatively charged ions ($X^-$) that have gained electrons, such as the chloride ion ($\text{Cl}^-$) or the oxide ion ($\text{O}^{2-}$).

Simple and Polyatomic Ions

Ions can be single atoms, known as simple ions (e.g., $\text{Li}^+$, $\text{F}^-$, $\text{S}^{2-}$), or they can be groups of atoms that act as a single unit, known as polyatomic ions. Common polyatomic ions include the ammonium ion ($\text{NH}_4^+$), the nitrate ion ($\text{NO}_3^-$), and the sulfate ion ($\text{SO}_4^{2-}$).

Forming Ionic Compounds

Ionic compounds are formed through the transfer of electrons from one species to another, resulting in an electrostatic attraction between cations and anions. To write a neutral formula unit for an ionic compound, the total positive charge must balance the total negative charge.

Exam Tip: When determining the formula for an ionic compound, always ensure the sum of the charges equals zero. For example, if you have $\text{Mg}^{2+}$ and $\text{Cl}^-$, you need two chloride ions to balance one magnesium ion, resulting in $\text{MgCl}_2$.

Chemical Nomenclature

Nomenclature is the systematic method of naming chemical substances. The rules change depending on the type of bond present in the compound.

Ionic Compounds

For simple ionic compounds composed of a metal and a non-metal, the name of the cation is written first, followed by the name of the anion with its ending changed (e.g., $\text{NaCl}$ is sodium chloride). If a metal can form multiple different ions (like transition metals), a Roman numeral is used to specify the charge, such as $\text{Fe}^{2+}$ being iron(II).

Binary Molecular Compounds

Molecular compounds consist of non-metals held together by covalent bonds. These are named using Greek prefixes to indicate the number of atoms of each element present. For example, $\text{N}_2\text{O}_3$ is named dinitrogen trioxide.

Acids

Acids are named based on their composition:

1. Binary Acids: These consist of hydrogen and one other element (e.g., $\text{HBr}$ is hydrobromic acid).
2. Oxyacids and their Salts: These contain hydrogen, oxygen, and another element. The name is derived from the polyatomic ion (oxyanion) they contain. For instance, $\text{HClO}_4$ (perchlorate) becomes perchloric acid.

By mastering these formulas and naming conventions, you gain the ability to translate between the visual structures of molecules and the mathematical descriptions used in chemical equations.