Comprehensive Guide to Chemical Nomenclature: Molecular Compounds, Ionic Compounds, and Acids

Chemical compounds fall into three distinct classifications based on composition, charge behavior, and nomenclature rules: molecular compounds, ionic compounds, and acids.
Molecular compounds are composed entirely of nonmetal elements and do not involve ionic charges.
Ionic compounds are composed of metals paired with nonmetals or polyatomic ions, and their nomenclature and formula construction depend strictly on balancing positive and negative ionic charges.
Acids represent a distinct class of hydrogen-containing compounds that balance a positively charged hydrogen cation with a negatively charged species.

Molecular compounds consist of combinations of two nonmetals.
On the periodic table, nonmetals are categorized in the yellow section located on the right-hand side.
Molecular compounds do not possess ionic charges; therefore, ionic charges are not utilized when constructing their chemical formulas or writing their names.
The sequence in which elements are written in molecular formulas is determined by their periodic position:
- The nonmetal element situated furthest to the left on the periodic table is written first.
- The element furthest to the left is considered the most metallic of the nonmetals.
- When comparing elements in similar regions, the element positioned furthest toward the bottom of the periodic table is also prioritized.
Prefixes indicating numerical quantities from $1$ through $10$ are prefixed to the element names during nomenclature:
- The numerical prefix for $1$ is $\text{mono-}$ .
- Intermediate prefixes include $\text{di-}$ for $2$ and $\text{tri-}$ for $3$ .
- The numerical prefix for $10$ is $\text{deca-}$ .
Prefixes explicitly dictate the number of atoms present in the molecular compound, replacing the charge-balancing rules used in ionic compounds.

Ionic compounds are structured from positively charged and negatively charged species, divided into three specific categories:
- Simple binary ionic compounds: Composed of a basic metal cation paired with a nonmetal anion.
- Transition metal ionic compounds: Composed of a transition metal paired with a nonmetal. Transition metals are characterized by their ability to form ions with more than one possible charge, which is why they are called transition metals.
- Polyatomic ion ionic compounds: Composed of a metal cation combined with a polyatomic ion. A polyatomic ion is a distinct group of bonded atoms carrying an overall negative charge.
Determining the formulas and names of transition metal and polyatomic compounds requires referencing standardized charge and ion tables.

Acids are uniquely identified by their initial chemical symbol: they always start with hydrogen ( $H$ ).
The hydrogen component in an acid carries a $+1$ charge, functioning as the hydrogen cation ( $H^+$ ).
Because the hydrogen species carries a positive charge ( $H^+$ ), it must always combine with a species that carries a negative charge.
Acid categories include:
- Oxyacids: Acids that explicitly contain oxygen within their chemical formula.
- Binary acids: Acids where hydrogen combines with a negative nonmetal anion, predominantly the halogens in Group 7A.
Group 7A nonmetals carry an ionic charge of $-1$ when forming simple binary acids:
- Hydrochloric acid ( $HCl$ ) is formed by the combination of $H^+$ and $Cl^-$ .
Halogens and nonmetals involved in acid formation often correlate with the "seven-shaped" cluster on the periodic table:
- Element numbers $7$ , $8$ , and $9$ .
- Downward along Group 7A encompassing elements $9$ , $17$ , $35$ , and $53$ .
Deviations and exceptions occur where the nonmetal anion carries a charge other than $-1$ , requiring multiple hydrogen ions for balance:
- An example exception is $H_2S$ , which combines two $H^+$ ions with a sulfur species.
The mnemonic phrase utilized for identifying relevant elemental groups is: have no fear, bicycle beer.

The examination provides dedicated reference charts printed on the back of the test paper:
- A transition metals chart containing the various possible charges for each transition metal.
- A polyatomic ions chart identifying the formulas, names, and negative charges for groups of atoms.
- A list of numerical prefixes from $1$ to $10$ (including $\text{mono-}$ and $\text{deca-}$ ).
Students are not required to commit the full tables of transition metal charges, polyatomic ions, or numerical prefixes to rote memory.
Students are expected to understand the application and interpretation of these reference charts to successfully determine formulas and names.

Instructional review is structured across three core educational resources:
- Resource 1: Instructional video on writing chemical formulas for ionic compounds and molecular compounds (reviewed directly in class).
- Resource 2: Instructional video detailing formula writing for ionic compounds containing polyatomic ions (assigned for review outside of class).
- Resource 3: Instructional video demonstrating formula writing and systematic naming for acids.
Supplementary lecture slides containing video mixes should be referenced alongside the main slides.

Question: What is not needed when writing the formula and name of a molecular compound?
- Response: Charges are not needed. Molecular compounds do not possess charges; instead, prefixes from $1$ through $10$ (such as $\text{mono-}$ , $\text{di-}$ , and $\text{tri-}$ ) are written out in the name.
Question: What is unique about an acid, and how can an acid be identified immediately?
- Response: Acids always start with hydrogen ( $H$ ). When oxygen is present in the compound, it is categorized as an oxyacid.
Question: What is the charge on the hydrogen atom in an acid?
- Response: The charge is $1$ , represented as the cation $H^+$ .
Question: When writing a molecular compound, which nonmetal element is written first?
- Response: The nonmetal element situated furthest to the left on the periodic table is written first because it is the most metallic. If necessary, the element furthest toward the bottom is also prioritized.