Notes on Molecular vs Ionic Compounds and Binary Ionic Compound Naming
Molecular vs Ionic Substances
Nonmetals tend to form molecules, not crystals. Molecular substances form molecules with specific formulas (e.g., ext{H}_2 ext{O}).
Example: water has two hydrogens and one oxygen: ext{H}_2 ext{O}. These are not ions; they are atoms bound together by covalent bonds.
Hydrogen can behave as a metal or a nonmetal depending on context; some periodic tables place it near the boundary, which can be confusing.
In molecular substances you usually have discrete molecules rather than a continuous ionic lattice. You can crystallize molecular substances, but that does not make them ionic crystals.
Concept of molecules is reinforced by Lewis dot structures (mentioned as a teachable tool you may or may not know yet). The idea is to show that these are physical molecules, not extended crystals.
A well-known molecular example is hydrogen peroxide: ext{H}2 ext{O}2.
Uses: disinfectant, hair bleaching, and can be used to make explosives.
Note: It is not HO in a simple unit; it has a fixed molecular ratio.
In molecular compounds you cannot simply reduce the subscripts like you might in an ionic context. For H$2$O$2$, the ratio 2:2 is fixed in the molecular formula (the speaker notes you cannot reduce the coefficients here, which contrasts with how you might simplify some ionic formulas). In contrast, ionic compounds form a crystal lattice with a formula unit rather than discrete molecules.
The course will return to molecular chemistry later, with more naming and concepts.
When you’re learning, it helps to recall that you may have seen Lewis dot structures before, which can make understanding these ideas easier.
Section 2.7: Naming and Formulas (overview)
Naming can be difficult because you must first identify what kind of compound you’re dealing with; each type has its own naming rules.
The first unit discussed is binary ionic compounds.
Binary means there are two kinds of ions involved: a cation (positive) and an anion (negative).
The key requirement: the total positive charge must balance the total negative charge.
Example given: aluminum in the +3 oxidation state and chlorine in the -1 oxidation state.
To balance +3 with -1, you need three chloride ions: Al$^{3+}$ with 3 Cl$^-$.
In the solid (ionic lattice), the ions are held together in a crystal; when dissolved, you think of separate ions (Al$^{3+}$ and Cl$^-$) in solution.
Naming binary ionic compounds (cation first, anion second):
The cation keeps its name.
The anion takes the -ide ending, e.g., chloride from chloride ion.
Example: aluminum chloride is written as ext{AlCl}_3 (Aluminum is the cation, chloride is the anion).
In contrast to the ionic lattice, you would not write the formula with the ions separated when describing the solid; the written formula unit reflects the ratio in the crystal.
Some notes on terminology and exceptions:
If the anion is a simple monatomic ion, its name ends with -ide (e.g., chloride, oxide, sulfide).
There are exceptions to the naming pattern; cyanide (CN$^-$) is given as a specific case where the -ide suffix is part of a named polyatomic anion (CN$^-$), which can be seen as an inconsistency in the naming system.
The term “oxo ions” refers to polyatomic ions containing oxygen and is used here to note that many such ions have names that differ from the simple -ide pattern and use -ite/-ate endings rather than -ide for the ion name.
A quick, practical method to get the formula for a binary ionic compound is to criss-cross the charges of the ions:
For example, aluminum(III) ion (Al$^{3+}$) and sulfide (S$^{2-}$) → criss-cross the charges to obtain the formula: ext{Al}2 ext{S}3.
Rationale: 2 × (+3) = +6 and 3 × (−2) = −6, so the total charge is zero; the solid formula unit is Al$2$S$3$.
Another explicit example discussed: for Al$^{3+}$ and Cl$^-$, the criss-cross gives AlCl$_3$ (Al:1, Cl:3), which balances to zero charge in the formula unit.
In practice, when writing formulas for binary ionic compounds, you balance the charges to zero in the formula unit, and you name the compound by the cation name followed by the -ide anion name (for monatomic anions).
Occasionally, instructors might mention zinc and silver as examples where the ionic charges can be less obvious in some contexts, which is why some courses emphasize formal charges or common oxidation states on the periodic table during problem solving.
Quick reference: common rules and examples
Binary ionic compounds:
Components: a cation (positive) and an anion (negative).
Charges must balance to zero in the formula unit.
Name: + .
Example: ext{AlCl}_3 for aluminum chloride; cation Al$^{3+}$, anion Cl$^-$ (three of them).
Criss-cross rule (to determine empirical formula): use the absolute values of the ion charges as subscripts and then simplify if possible (if a common factor exists in the subscripts, reduce to smallest whole-number ratio).
Exceptions and notes:
Not all anions are monatomic; many are polyatomic (oxo ions) and use other naming conventions (-ite/-ate endings).
Cyanide (CN$^-$) is an exception to the simple -ide pattern in naming; it is a polyatomic anion with its own established name.
Example problems mentioned:
Aluminum chloride: ext{AlCl}_3 (Al$^{3+}$ with Cl$^-$, 3 Cl$^-$ balance the +3).
Aluminum sulfide: ext{Al}2 ext{S}3 (Al$^{3+}$ and S$^{2-}$ balance via criss-cross to 2 Al and 3 S).
The criss-cross reasoning: 2 × (+3) and 3 × (−2) balance to zero, giving Al$2$S$3$.
Connections and takeaways
Molecular vs ionic bonding represents fundamentally different types of chemical bonding: covalent bonds within molecules vs ionic bonds in a lattice.
Understanding the correct naming convention requires identifying whether the compound is ionic or molecular, then applying the appropriate rules.
The criss-cross method is a practical tool for predicting empirical formulas for binary ionic compounds, but remember the resulting subscripts reflect the simplest whole-number ratio that balances charges in the formula unit.
The -ide suffix indicates a simple, monatomic anion in many binary ionic compounds, but real chemistry includes polyatomic ions with special names, which you should memorize as needed.
The content ties back to foundational ideas about charge balance, ionic lattices, molecular structures, and how chemical formulas encode composition and structure.