Chemical Formulas and Compounds

Significance of a Chemical Formula

  • A chemical formula indicates the relative number of atoms of each kind in a chemical compound.
  • For a molecular compound, the chemical formula reveals the number of atoms of each element contained in a single molecule of the compound.
    • Example: octane — C<em>8H</em>18C<em>8H</em>{18}
      • The subscript after the C indicates that there are 8 carbon atoms in the molecule.
      • The subscript after the H indicates that there are 18 hydrogen atoms in the molecule.
  • The chemical formula for an ionic compound represents one formula unit—the simplest ratio of the compound’s positive ions (cations) and its negative ions (anions).
    • Example: aluminum sulfate — Al<em>2(SO</em>4)3Al<em>2(SO</em>4)_3
      • Parentheses surround the polyatomic ion to identify it as a unit.
      • The subscript 3 refers to the unit.
      • Note also that there is no subscript for sulfur: when there is no subscript next to an atom, the subscript is understood to be 1.

Monatomic Ions

  • Many main-group elements can lose or gain electrons to form ions.
  • Ions formed form a single atom are known as monatomic ions.
  • Diatomic: Two atoms.
  • Polyatomic: More than 2 atoms.

Naming Monatomic Ions

  • Monatomic cations are identified simply by the element’s name.
    • Examples:
      • K+K^+ is called the potassium cation
      • Mg2+Mg^{2+} is called the magnesium cation
  • For monatomic anions, the ending of the element’s name is dropped, and the ending -ide is added to the root name.
    • Examples:
      • F–F^– is called the fluoride anion
      • N3–N^{3–} is called the nitride anion

Common Monatomic Ions

  • Main-group elements
    • 1+:
      • lithium Li+Li^+
      • sodium Na+Na^+
      • potassium K+K^+
      • rubidium Rb+Rb^+
      • cesium Cs+Cs^+
    • 2+:
      • beryllium Be2+Be^{2+}
      • magnesium Mg2+Mg^{2+}
      • calcium Ca2+Ca^{2+}
      • strontium Sr2+Sr^{2+}
      • barium Ba2+Ba^{2+}
    • 3+:
      • aluminum Al3+Al^{3+}
    • 1-:
      • fluoride F−F^-
      • chloride Cl−Cl^-
      • bromide Br−Br^-
      • iodide I−I^-
    • 2-:
      • oxide O2−O^{2-}
      • sulfide S2−S^{2-}
    • 3-:
      • nitride N3−N^{3-}
      • phosphide P3−P^{3-}
  • Continue:
    • 1+:
      • copper(I) Cu+Cu^+
      • silver Ag+Ag^+
    • 2+:
      • vanadium(II) V2+V^{2+}
      • chromium(II) Cr2+Cr^{2+}
      • manganese(II) Mn2+Mn^{2+}
      • iron(II) Fe2+Fe^{2+}
      • cobalt(II) Co2+Co^{2+}
      • nickel(II) Ni2+Ni^{2+}
      • copper(II) Cu2+Cu^{2+}
      • zinc Zn2+Zn^{2+}
      • cadmium Cd2+Cd^{2+}
      • tin(II) Sn2+Sn^{2+}
      • mercury(II) Hg2+Hg^{2+}
      • lead(II) Pb2+Pb^{2+}
    • 3+:
      • vanadium(III) V3+V^{3+}
      • chromium(III) Cr3+Cr^{3+}
      • iron(III) Fe3+Fe^{3+}
      • cobalt(III) Co3+Co^{3+}
    • 4+:
      • vanadium(IV) V4+V^{4+}
      • tin(IV) Sn4+Sn^{4+}
      • lead(IV) Pb4+Pb^{4+}

Binary Ionic Compounds

  • Compounds composed of two elements are known as binary compounds.
  • In a binary ionic compound, the total numbers of positive charges and negative charges must be equal.
  • The formula for a binary ionic compound can be written given the identities of the compound’s ions.
    • Example: magnesium bromide
      • Ions combined: Mg2+Mg^{2+}, Br–Br^–, Br–Br^–
      • Chemical formula: MgBr2MgBr_2
  • A general rule to use when determining the formula for a binary ionic compound is “crossing over” to balance charges between ions.

Naming Binary Ionic Compounds

  • Sample Problem A
    • Write the formulas for the binary ionic compounds formed between the following elements:
      • zinc and iodine
      • zinc and sulfur
  • Solution
    • Write the symbols for the ions side by side. Write the cation first.
      • a. Zn2+Zn^{2+} I−I^−
      • b. Zn2+Zn^{2+} S2−S^{2−}
    • Cross over the charges to give subscripts.
    • Check the subscripts and divide them by their largest common factor to give the smallest possible whole-number ratio of ions.
      • a. The subscripts give equal total charges of 1 × 2+ = 2+ and 2 × 1− = 2−. The largest common factor of the subscripts is 1. The smallest possible whole-number ratio of ions in the compound is 1:2. The formula is ZnI2ZnI_2.
      • b. The subscripts give equal total charges of 2 × 2+ = 4+ and 2 × 2− = 4−. The largest common factor of the subscripts is 2. The smallest whole-number ratio of ions in the compound is 1:1. The formula is ZnS.
  • The Stock System of Nomenclature
    • Some elements such as iron, form two or more cations with different charges.
    • To distinguish the ions formed by such elements, scientists use the Stock system of nomenclature.
    • The system uses a Roman numeral to indicate an ion’s charge.
      • Examples:
        • Fe2+Fe^{2+} iron(II)
        • Fe3+Fe^{3+} iron(III)
    • Sample Problem B
      • Write the formula and give the name for the compound formed by the ions Cr3+Cr^{3+} and F–F^–.
    • Solution
      • Write the symbols for the ions side by side. Write the cation first.
        • Cr3+Cr^{3+} F−F^−
      • Cross over the charges to give subscripts.

Polyatomic Ions

  • Common Polyatomic Ions
    • Name, Formula, Charge
      • ammonium, NH4+NH_4^+, 1+
      • hydroxide, OH−OH^-, 1-
      • nitrate, NO3−NO_3^-, 1-
      • carbonate, CO32−CO_3^{2-}, 2-
      • sulfate, SO42−SO_4^{2-}, 2-
      • phosphate, PO43−PO_4^{3-}, 3-
  • Compounds Containing Polyatomic Ions
    • Sample Problem C
      • Write the formula for Nickel (III) sulfate.

Naming Binary Molecular Compounds

  • The old system of naming molecular compounds is based on the use of prefixes.
    • Examples:
      • CCl4CCl_4 — carbon tetrachloride (tetra- = 4)
      • COCO — carbon monoxide (mon- = 1)
      • CO2CO_2 — carbon dioxide (di- = 2)
  • Prefixes for Naming Covalent Compounds
    • Number of Atoms, Prefix, Example, Name
      • 1, mono-, CO, carbon monoxide
      • 2, di-, SiO2SiO_2, silicon dioxide
      • 3, tri-, SO3SO_3, sulfur trioxide
      • 4, tetra-, SCl4SCl_4, sulfur tetrachloride
      • 5, penta-, SbCl5SbCl_5, antimony pentachloride
      • 6, hexa-, CeB6CeB_6, cerium hexaboride
      • 7, hepta-, IF7IF_7, iodine heptafluoride
      • 8, octa-, NP<em>3O</em>8NP<em>3O</em>8, trineptunium octoxide
      • 9, nona-, I<em>4O</em>9I<em>4O</em>9, tetraiodine nonoxide
      • 10, deca-, S<em>2F</em>10S<em>2F</em>{10}, disulfur decafluoride
  • Sample Problem D
    • a. Give the name for As<em>2O</em>5As<em>2O</em>5.
    • b. Write the formula for oxygen difluoride.
  • Solution
    • a. A molecule of the compound contains two arsenic atoms, so the first word in the name is diarsenic. The five oxygen atoms are indicated by adding the prefix pent- to the word oxide. The complete name is diarsenic pentoxide.
    • b. Oxygen is first in the name because it is less electronegative than fluorine. Because there is no prefix, there must be only one oxygen atom. The prefix di- in difluoride shows that there are two fluorine atoms in the molecule. The formula is OF2OF_2.

Covalent-Network Compounds

  • Some covalent compounds do not consist of individual molecules.
  • Instead, each atom is joined to all its neighbors in a covalently bonded, three-dimensional network.
  • Subscripts in a formula for covalent-network compound indicate smallest whole-number ratios of the atoms in the compound.
    • Examples:
      • SiCSiC, silicon carbide
      • SiO2SiO_2, silicon dioxide
      • Si<em>3N</em>4Si<em>3N</em>4, trisilicon tetranitride.