Allotropes, Binary Covalent Compounds, and Ionic Compound Nomenclature

Allotropes

• Definition: Allotropes are different molecular forms of a single element.

• Examples:

  • Oxygen Allotropes:

    • Molecular oxygen ($O_2$)

    • Ozone ($O_3$)

  • Carbon Allotropes:

    • Diamond (known for being hard and colorless)

    • Graphite (known for being soft and gray)

    • Nanotubes

    • Fullerenes

Naming Binary Covalent Compounds

Purpose of Prefixes

• The same two elements can often bond together in different ratios (e.g., carbon monoxide, carbon dioxide).

• Covalent compound names use prefixes to specify the number of atoms of each type present in the compound.

Prefixes Used in Naming Binary Covalent Compounds

• $1$: mon(o)-

• $2$: di-

• $3$: tri-

• $4$: tetr(a)-

• $5$: pent(a)-

• $6$: hex(a)-

• $7$: hept(a)-

• $8$: oct(a)-

• $9$: non(a)-

• $10$: dec(a)-

• Note: The last 'o' or 'a' of the prefix is usually dropped when the element name that follows begins with 'o'.

Steps to Name a Binary Covalent Compound

1. First Element: Name the first element using its full element name. Add a prefix if its subscript is greater than $1$ (e.g., "dinitrogen" for $N_2$). The prefix "mono-" is generally omitted for the first element.

2. Second Element: Use a prefix to indicate the number of atoms of the second element. Modify the element name to end with "-ide" (e.g., "dioxide" for two oxygen atoms).

Binary Compounds of Hydrogen

• These compounds are named using different systems compared to standard binary covalent compounds.

• Many binary compounds of hydrogen are acids and are named according to the specific system for naming acids.

Ions

• Definition: Ions are atoms or groups of atoms that have gained or lost electrons.

• Characteristic: As a result of gaining or losing electrons, ions possess a net negative or positive charge.

• Monatomic Ions: Single atoms that gain or lose electrons form monatomic ions (e.g., $Cl^{-}$ or $Na^{+}$).

Binary Ionic Compounds

• Formation: These compounds form when metal atoms combine with nonmetal atoms.

• Mechanism: Metal atoms transfer electrons to nonmetal atoms, resulting in the formation of ions that are attracted to each other.

• Ion Variability: Atoms of some elements form only one type of ion (e.g., Group $1$ metals always form $+1$ ions), while others can form more than one type of ion (e.g., iron can form $Fe^{2+}$ and $Fe^{3+}$).

Chemical Formulas for Binary Ionic Compounds

• Electrical Neutrality: All compounds are electrically neutral overall. This means the total positive charge from the cations must balance the total negative charge from the anions.

• Example: When aluminum ($Al$) and chlorine ($Cl$) form a compound:

  • Aluminum typically forms an $Al^{3+}$ ion.

  • Chlorine typically forms a $Cl^{-}$ ion.

  • To balance the charges, three $Cl^{-}$ ions must be present for each $Al^{3+}$ ion, resulting in the formula $AlCl_3$.

Using Formulas and Charges to Determine the Charge of an Ion

• It is possible to determine the charge of an unknown ion if the formula of the compound and the charge of the other ion are known.

• Example: Iron and chlorine can form $FeCl<em>2$ and $FeCl</em>3$.

  • Known: The chloride ion ($Cl^{-}$) has a charge of $-1$.

  • Known: The total charge of the compound is $0$.

  • In $FeCl_2$: Let $x$ be the charge of iron. $x + 2(-1) = 0 \implies x - 2 = 0 \implies x = +2$. So, iron is $Fe^{2+}$.

  • In $FeCl_3$: Let $y$ be the charge of iron. $y + 3(-1) = 0 \implies y - 3 = 0 \implies y = +3$. So, iron is $Fe^{3+}$.

Polyatomic Ions

• Definition: Polyatomic ions are groups of two or more bonded atoms that, as a unit, have lost or gained electrons, consequently possessing an overall net charge.

Writing Formulas of Ionic Compounds with Polyatomic Ions

• Balancing Charges: Use the least common multiple of the charges of the cation and the polyatomic anion to determine the subscripts needed to balance the positive and negative charges.

• Parentheses Rule: When a subscript is needed for a polyatomic ion that is greater than $1$, the polyatomic ion must be enclosed in parentheses, and the subscript is placed outside of the parentheses.

  • Example: For calcium ($Ca^{2+}$) and phosphate ($PO<em>4^{3-}$) ions, the least common multiple of $2$ and $3$ is $6$. So, $3$ $Ca^{2+}$ ions and $2$ $PO</em>4^{3-}$ ions are needed, forming $Ca<em>3(PO</em>4)_2$. The phosphate ion is enclosed in parentheses.

Naming Ionic Compounds

• The names of ionic compounds consist simply of the name of the cation followed by the name of the anion present in the compound.

• Variations in Naming: There are three distinct types of cations (constant charge, variable charge monatomic metals, polyatomic) and two distinct types of anions (monatomic, polyatomic), each with specific naming conventions.

Naming Polyatomic Cations

• The only common polyatomic cation is the ammonium ion, $NH_4^{+}$. It is simply named "ammonium."

Naming Monatomic Metal Cations with Constant Charge

• Elements Covered: These are typically metal ions from Groups $1$ and $2$ (alkali metals and alkaline earth metals) on the periodic table, plus a few other specific elements like aluminum ($Al^{3+}$), zinc ($Zn^{2+}$), silver ($Ag^{+}$), and cadmium ($Cd^{2+}$).

• Naming Rule: Use the full name of the element followed by the word "ion" (e.g., $Na^{+}$ is sodium ion, $Ca^{2+}$ is calcium ion).

  • Visual Aid: A diagram shows elements like Li, Be, Na, Mg, K, Ca, Rb, Sr, Cs, Ba, Al, Zn, Ag, Cd typically forming constant charge cations.

Naming Monatomic Metal Cations with Variable Charge

• Elements Covered: These are typically metal ions from Groups $3-16$ (primarily transition metals and some post-transition metals).

• Naming Rule: Use the name of the element followed by a Roman numeral in parentheses to indicate the specific charge of the ion (e.g., $Fe^{2+}$ is iron(II) ion, $Fe^{3+}$ is iron(III) ion).

  • Visual Aid: A diagram shows elements like Cr, Mn, Fe, Co, Ni, Cu, Sn, Pb, Au, Hg typically forming variable charge cations.

Naming Monatomic Anions

• Charge Characteristic: All monatomic anions have a constant charge, which is predictable based on their group number.

• Elements Covered: They form from nonmetal elements, typically from Groups $5-7$ (e.g., $N^{3-}$, $O^{2-}$, $Cl^{-}$).

• Naming Rule: Name the monatomic anion by taking the root of the element name, changing the ending to "-ide," and adding the word "ion" (e.g., $O^{2-}$ is oxide ion, $Cl^{-}$ is chloride ion).

  • Examples: Fluoride ($F^{-}$), Chloride ($Cl^{-}$), Bromide ($Br^{-}$), Iodide ($I^{-}$), Oxide ($O^{2-}$), Sulfide ($S^{2-}$), Nitride ($N^{3-}$), Phosphide ($P^{3-}$).

Naming Polyatomic Anions

• "-ide" Ending Exceptions: Only two common polyatomic ions use the "-ide" ending: hydroxide ($OH^{-}$) and cyanide ($CN^{-}$).

• Oxyanions: These are polyatomic anions that contain one nonmetal atom bonded to a variable number of oxygen atoms.

  • Charge Consistency: All oxyanions within a specific series (e.g., chlorine oxyanions) will have the same overall charge.

  • Naming Conventions within a Series: The names within a series differ by their endings ("-ate" or "-ite") and sometimes by prefixes ("per-" or "hypo-"). These indicate the relative number of oxygen atoms.

    • "per-" + root + "-ate": most oxygen atoms (e.g., perchlorate $ClO_4^{-}$)

    • root + "-ate": next highest number of oxygen atoms (e.g., chlorate $ClO_3^{-}$)

    • root + "-ite": fewer oxygen atoms than "-ate" (e.g., chlorite $ClO_2^{-}$)

    • "hypo-" + root + "-ite": fewest oxygen atoms (e.g., hypochlorite $ClO^{-}$)

How to Name Ionic Compounds (Summary and Examples)

• Formula Implication: The chemical formula of an ionic compound inherently indicates the lowest whole-number ratio of cations to anions required for electrical neutrality.

• Naming Simplicity: Because the ratio is implied by the charges, there is no need to include ratio information (like prefixes) in the name of an ionic compound.

• Overall Rule: Simply state the name of the cation followed by the name of the anion.

Example Flowchart Breakdown

• Example 1: $FeSO_4$ (Iron(II) Sulfate)

  • Cation: $Fe^{2+}$. This is a monatomic metal cation with a variable charge (iron can be $+2$ or $+3$). Its name is iron(II).

  • Anion: $SO_4^{2-}$. This is a polyatomic anion (sulfate). Its name is sulfate.

  • Compound Name: iron(II) sulfate.

• Example 2: $CaCl_2$ (Calcium Chloride)

  • Cation: $Ca^{2+}$. This is a monatomic metal cation with a constant charge (calcium is always $+2$). Its name is calcium.

  • Anion: $Cl^{-}$. This is a monatomic anion. Its root is chlor, and it ends in -ide. Its name is chloride.

  • Compound Name: calcium chloride.