CHE-3: Chapter 6

Chemical Formula

• Represents elements or compounds in written form.
• Includes element symbols.
• A subscript indicates the number of atoms of an element in a formula unit.
• No number is shown if there is only one atom.
• Examples: H₂O (water), NaCl (sodium chloride).

Formulas of Elements

• Most elements exist as single-atom particles.
• The elemental symbol is the formula, reflecting that the basic unit is an uncombined atom, such as He (helium) and Li (lithium).
• Diatomic molecules are two-atom molecules: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂.

Binary Molecular Compounds

• Formed by two nonmetal elements or a metalloid-nonmetal pair.
• The name has two words.
• The first word is the element that appears first in the chemical formula.
• A prefix indicates the number of atoms of the element in the molecule.
• Also known as a Stoichiometric Coefficient Ex: 2 NH3 is equivalent to N2H6
• Includes a prefix to denote the number of atoms in the molecule.
• The second word is the element that appears second, changed to end in -ide.
• Two compounds with traditional names: H₂O is water and NH3 is ammonia.

Number Prefixes Used in Chemical Names

• mono- = 1
• di- = 2
• tri- = 3
• tetra- = 4
• penta- = 5
• hexa- = 6
• hepta- = 7
• octa- = 8
• nona- = 9
• deca- = 10

Monatomic Ion

• An ion formed from a single atom.
  • Ion: Atom or group of covalently bonded atoms that is electrically charged due to an excess or deficiency of electrons.
  • Cation: Positively charged, because cats are paws-itive.
  • Anion: Negatively charged.
• Rules for naming a monatomic ion depend on:
  • Location in the periodic table.
  • Whether it is a main group or a transition element.
• Writing the name and formula of a monatomic cation:
  • Name - Name of element followed by the word "ion."
  • Formula - Elemental symbol followed by electrical charge written in superscript (number first, + sign second).
• Writing the name and formula of a monatomic anion:
  • Name - Name of element changed to end in -ide followed by the word "ion."
  • Formula - Elemental symbol followed by electrical charge written in superscript (number first, - sign second).
• The periodic table can assist when determining the electrical charge.

Additional Metals

• Some elements form two or more different monatomic ions that have different charges, often transition metals.
  • To distinguish among the ions, the magnitude of the charge is included with its name, appearing in Roman numerals enclosed in parentheses.
  • Example: Iron (Fe). If a neutral atom loses two electrons, the ion has a 2+ charge (Fe²⁺), represented as iron(II) ion.
• If the atom loses three electrons, the ion has a 3+ charge (Fe³⁺), represented as iron(III) ion.
• Mercury forms a diatomic ion.
  • Hg₂²⁺ is called mercury(I) ion.
  • Named as if single atoms exist separately even though there are no individual Hg⁺ ions under common conditions.

Polyatomic Ions

• Ions that consist of more than one atom, like Ammonium ion, NH₄⁺.
  • Positively charged
  • A nitrogen atom is chemically bonded to four hydrogen atoms.
• Hydroxide ion, OH⁻.
  • Negatively charged.
  • An oxygen atom and hydrogen atom are chemically bonded to one another.
• Composed of two or more atoms and have an overall charge.

Ionic Compounds

• A compound in which ions are held by ionic bonds.
• Stability of the ionic compound depends on the interaction between positively and negatively charged ions.
• The formula of an ionic compound expresses the ratio of positive and negative ions.
  • Formula unit: Smallest electrically neutral collection of ions represented by a chemical formula.
• Writing the formula of an ionic compound:
  • Write the formula of the cation and the formula of the anion, balance the charges.
  • Decide what the fewest number of ions is that will make the compound electrically neutral.
  • Write the formula (cation first, anion second, with subscripts as needed for charge balance).
  • Omit the subscript if only one ion is needed. If a polyatomic ion is needed more than once, enclose the formula of the ion in parentheses and place the subscript after the closing parentheses.

Names of Ionic Compounds

• Names of ionic compounds do not include prefixes.
• Involves writing the name of the cation and the anion.
• A compound containing a metal that can have more than one ionic charge includes the charge of the metal in its name.
  • Example: FeCl₃ is iron(III) chloride.

Naming Compounds: Ionic vs. Molecular

• Naming compounds is a crucial skill in chemistry.
• First identify the compound type: ionic or molecular (covalent).

Identifying Compound Types

• Ionic compounds are composed of a metal and a nonmetal.
• Covalent compounds consist of two nonmetals.
• Use the periodic table to identify metals and nonmetals.

Naming Ionic Compounds: Without Transition Metals

• Structure: name of the metal + name of the nonmetal ending in "-ide".
• Balance charges of ions to form a neutral compound.
• An example is aluminum oxide (Al₂O₃).
• Aluminum has a 3+ charge, and oxygen has a 2- charge.
• Balance by using subscripts to equalize the total positive and negative charges.
• The "lasso" method involves swapping the numerical values of the charges to determine the subscripts.

Naming Ionic Compounds: With Transition Metals

• Structure: name of the transition metal + (Roman numeral indicating charge) + name of the nonmetal ending in "-ide".
• Roman numeral indicates the charge to specify which ion charge the transition metal has in the compound, as many can have multiple charges.
• Iron (III) oxide (Fe₂O₃) represents this type.
  • The Roman numeral (III) indicates that iron has a 3+ charge.
• Balancing charges involves finding the lowest common multiple for the charges.

Naming Ionic Compounds: With Polyatomic Ions

• Polyatomic ions consist of two or more elements.
• Common polyatomic ions should be memorized.
• Structure: name of the metal + name of the polyatomic ion.
• The nonmetal does not end in "ide".
• Calcium phosphate (Ca₃(PO₄)₂) exemplifies this.
  • Calcium has a 2+ charge, and phosphate (PO₄) has a 3- charge.
• Parentheses often enclose the polyatomic ion when applying a subscript to balance charges.
• To name: name of the metal + name of the polyatomic ion, all in lowercase letters.
• Always balance the charges for ionic compounds when finding the chemical formula.

Naming Ionic Compounds: Transition Metals with Polyatomic Ions

• Structure: name of the transition metal + (Roman numeral indicating charge) + name of the polyatomic ion.
• An example is copper (II) nitrate (Cu(NO₃)₂).
  • The (II) indicates that copper has a 2+ charge, and nitrate (NO₃) has a -1 charge.
• Parentheses are used around the nitrate ion, and a subscript of 2 balances the 2+ charge of copper.

Writing Names from Formulas: Ionic Compounds

• Identify the ions present and their charges.
• If there is a transition metal, determine its charge based on the charge of the other ion(s).
• Use the right nomenclature rules to write the name.
• FeBr₂ as iron (II) bromide exemplifies this; the 2+ charge of iron is deduced from the two -1 charged bromine ions.

Naming Covalent (Molecular) Compounds

• Naming molecular compounds involves using prefixes to indicate the number of atoms of each element.
• The prefixes for 1 to 10 should be memorized: mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca-.
• Structure: (prefix + name of 1st nonmetal) + (prefix + name of 2nd nonmetal ending in "-ide").
• Balancing charges is NOT required for covalent compounds.
• The prefixes directly indicate the number of atoms of each element in the compound.
• For example, diphosphorus pentoxide (P₂O₅) - "di-" means two phosphorus atoms, and "penta-" means five oxygen atoms.
• Another example, trinitrogen hexaoxide (N₃O₆) - "tri-" means three nitrogen atoms, and "hexa-" means six oxygen atoms.

Examples: Naming Ionic Compounds with Polyatomic Ions

• Magnesium Carbonate (MgCO₃): Magnesium (Mg²⁺) and Carbonate (CO₃²⁻) have the same charge, so no additional subscript is needed.
• Lithium Acetate (LiC₂H₃O₂): Li is Lithium, C₂H₃O₂ is Acetate.
• Aluminum Sulfate (Al₂(SO₄)₃): Aluminum (Al³⁺) and Sulfate (SO₄²⁻) require balancing charges, resulting in subscripts of 2 for Aluminum and 3 for Sulfate, with the Sulfate ion in parenthesis. Whenever a polyatomic ion requires a subscript, the entire polyatomic ion will be placed in parenthesis with the added subscript outside of the parenthesis.
• Calcium Hydroxide (Ca(OH)₂): Ca is Calcium, OH is Hydroxide.
• Ammonium Sulfate ((NH₄)₂SO₄): Ammonium (NH₄⁺) and Sulfate (SO₄²⁻) require balancing charges; Ammonium needs a subscript of 2 and will be in parenthesis.
• Ammonium Nitrate (NH₄NO₃): NH₄ is Ammonium, NO₃ is Nitrate.

Trick to Memorize Polyatomic Ions Faster

• Most polyatomic ions end in either -ite or -ate.
• The ones ending in -ate have one more oxygen than the ones ending in -ite.
• Focusing on remembering the -ates reduces memorization by half.