Ions: Monatomic Anions and Cations

• Atoms can gain or lose electrons to form ions
• Ions are atoms or groups of atoms with an electrical charge.
• If an atom gains electrons, it becomes a negatively charged ion
• If an atom loses electrons, it becomes a positively charged ion.
• Monatomic ions are formed from a single atom.
• Polyatomic ions are formed from multiple atoms
• Naming and determining the formulas of monatomic and polyatomic ions are crucial for correctly naming and writing formulas for ionic compounds

Monatomic Anions

• Monatomic anions are negatively charged ions formed from a single atom.
• Anions are named by taking the root of the element name and adding the suffix "-ide."
• A chlorine atom gaining an electron becomes a chloride ion (Cl⁻).
• Fluorine (F) becomes fluoride (F⁻).
• Bromine (Br) becomes bromide (Br⁻).
• Iodine (I) becomes iodide (I⁻).
• Oxygen (O) becomes oxide (O²⁻).
• Sulfur (S) becomes sulfide (S²⁻).
• Nitrogen (N) becomes nitride (N³⁻).
• Phosphorus (P) becomes phosphide (P³⁻).
• Anions are negatively charged ions formed when an atom gains one or more electrons

Monatomic Cations

• Monatomic cations are positively charged ions formed from a single atom.
• Naming monatomic cations is generally straightforward.
• For elements that typically form only one type of cation, the ion is named after the element, followed by the word "ion."
• Na⁺ is named the sodium ion.
• K⁺ is the potassium ion.
• Mg²⁺ is the magnesium ion.
• Ca²⁺ is the calcium ion.
• Al³⁺ is the aluminum ion.
• Some metals, particularly transition metals, can form cations with different charges and Roman numerals indicate the charge.
• Iron can form two common cations: Fe²⁺ and Fe³⁺
• Fe²⁺ is named iron(II) ion, and Fe³⁺ is named iron(III) ion.
• Copper(I) ion (Cu⁺)
• Copper(II) ion (Cu²⁺)
• Lead(II) ion (Pb²⁺)
• Lead(IV) ion (Pb⁴⁺)
• Tin(II) ion (Sn²⁺)
• Tin(IV) ion (Sn⁴⁺)
• Chromium(II) ion (Cr²⁺)
• Chromium(III) ion (Cr³⁺)
• Cations are positively charged ions formed when an atom loses one or more electrons
• Roman numeral notation for charges is known as the Stock system
• An older naming system uses the suffixes -ous and -ic to denote lower and higher charges
• Fe2+ is also known as ferrous ion, and Fe3+ is known as ferric ion
• The Stock system is preferred because it is less ambiguous
• Group 1A elements (alkali metals) form +1 ions (e.g., Li+, Na+, K+)
• Group 2A elements (alkaline earth metals) form +2 ions (e.g., Mg2+, Ca2+, Ba2+)
• Group 7A elements (halogens) form -1 ions (e.g., F-, Cl-, Br-, I-)
• Oxygen typically forms a -2 ion (O2-)
• Aluminum forms a +3 ion (Al3+)
• Zinc typically forms a +2 ion (Zn2+)
• Silver typically forms a +1 ion (Ag+)

Polyatomic Ions

• Polyatomic ions consist of two or more atoms covalently bonded with an overall charge and act as a single unit.
• Naming them can be more complex.
• Some common polyatomic ions have specific names that need to be memorized.
• Ammonium ion (NH₄⁺)
• Hydroxide ion (OH⁻)
• Nitrate ion (NO₃⁻)
• Nitrite ion (NO₂⁻)
• Sulfate ion (SO₄²⁻)
• Sulfite ion (SO₃²⁻)
• Phosphate ion (PO₄³⁻)
• Carbonate ion (CO₃²⁻)
• Cyanide ion (CN⁻)
• Acetate ion (CH₃COO⁻ or C₂H₃O₂⁻)
• Permanganate ion (MnO₄⁻)
• Dichromate ion (Cr₂O₇²⁻)
• Chromate ion (CrO₄²⁻)
• Hypochlorite ion (ClO⁻)
• Chlorite ion (ClO₂⁻)
• Chlorate ion (ClO₃⁻)
• Perchlorate ion (ClO₄⁻)
• Some polyatomic ions are related, differing by the presence or absence of hydrogen ions (H⁺).
• Sulfate (SO₄²⁻) and hydrogen sulfate/bisulfate (HSO₄⁻) are related.
• Phosphate (PO₄³⁻), hydrogen phosphate (HPO₄²⁻), and dihydrogen phosphate (H₂PO₄⁻) form a series of related polyatomic ions.
• The prefix "hydrogen" or "dihydrogen" indicates one or two hydrogen ions, respectively.
• The prefix "bi-" is also used sometimes instead of "hydrogen," especially for older names like bicarbonate (HCO₃⁻).
• Polyatomic ions are ions composed of two or more atoms covalently bonded together that have an overall charge
• Polyatomic ions act as a single unit in forming ionic compounds
• Some common polyatomic cations include ammonium (NH4+) and hydronium (H3O+)

Oxyanions

• Oxyanions are polyatomic ions containing oxygen and another element.
• For a series of oxyanions with the same central element but different numbers of oxygen atoms, the one with more oxygen atoms takes the suffix "-ate," while the one with fewer takes "-ite."
• Nitrate (NO₃⁻) has one more oxygen atom than nitrite (NO₂⁻).
• If there are more than two oxyanions in the series, prefixes "per-" and "hypo-" are used.
• "Per-" indicates one more oxygen atom than the "-ate" ion, and "hypo-" indicates one fewer oxygen atom than the "-ite" ion.
• The chlorine oxyanions include:
  • Hypochlorite (ClO⁻): one oxygen atom
  • Chlorite (ClO₂⁻): two oxygen atoms
  • Chlorate (ClO₃⁻): three oxygen atoms
  • Perchlorate (ClO₄⁻): four oxygen atoms
• For oxyanions of the same element, the one with more oxygen atoms is named with the suffix "-ate"
• The one with fewer oxygen atoms is named with the suffix "-ite"
• When there are more than two oxyanions in a series:
• The prefix "per-" is used for the ion with the most oxygen atoms (e.g., perchlorate, ClO4-)
• The prefix "hypo-" is used for the ion with the fewest oxygen atoms (e.g., hypochlorite, ClO-)

Hydrated Ions

• Some ionic compounds incorporate water molecules into their crystal structure, called hydrates.
• The number of water molecules is indicated by a prefix followed by "hydrate."
  • Mono (1)
  • Di (2)
  • Tri (3)
  • Tetra (4)
  • Penta (5)
  • Hexa (6)
  • Hepta (7)
  • Octa (8)
  • Nona (9)
  • Deca (10)
• CuSO₄ · 5H₂O is named copper(II) sulfate pentahydrate, with five water molecules per formula unit.

Guidelines for Naming Ionic Compounds

• Identify the cation and anion.
• Name the cation first, followed by the anion.
• For monatomic cations with a fixed charge (Group 1A, 2A, and aluminum), simply use the element name.
• For monatomic cations with variable charges (transition metals), indicate the charge with Roman numerals in parentheses.
• For monatomic anions, use the root of the element name with the suffix "-ide."
• For polyatomic ions, use the memorized name of the ion.
• If the compound is a hydrate, indicate the number of water molecules using the appropriate prefix followed by "hydrate."

Common Mistakes and Confusions

• Confusing monatomic and polyatomic ions
• Ensure recognition of whether an ion is composed of a single atom or multiple atoms bonded together
• Using the wrong Roman numeral for transition metal cations
• Determine the charge correctly based on the other ions in the compound
• Forgetting the charge on common polyatomic ions
• Memorizing the charges is essential for writing correct formulas
• Confusing "-ate" and "-ite" endings for oxyanions
• "-ate" indicates more oxygen atoms, while "-ite" indicates fewer oxygen atoms
• Incorrectly applying prefixes like "per-" and "hypo-"
• Understand when to use these prefixes based on the number of oxygen atoms in the series of oxyanions

Summary of Key Rules

• Monatomic cations are named after the element, with Roman numerals for multiple charges
• Monatomic anions are named with the element root and the "-ide" suffix
• Polyatomic ions have specific names that must be memorized
• "-ate" and "-ite" suffixes indicate the relative number of oxygen atoms in oxyanions
• Prefixes like "bi-" or "hydrogen" can be added to indicate the presence of an acidic hydrogen ion (hydrogen carbonate (bicarbonate) is HCO3-)

Ions are atoms that gain or lose electrons, resulting in a charge. Anions are negatively charged ions formed when atoms gain electrons. Cations are positively charged ions formed when atoms lose electrons. Monatomic ions consist of a single atom.