U3 Ionic Compounds

Ionic Compounds

• Definition and Characteristics

  • Ionic compounds are formed from metals and non-metals or polyatomic ions.

  • A metal loses electrons and becomes a positive ion, while a non-metal or polyatomic ion gains electrons and becomes a negative ion.

  • The only positive polyatomic ion that is not a metal is the ammonium ion.

  • Ionic compounds consist of metals located to the left of the imaginary staircase on the periodic table.

• Structure of Ionic Compounds

  • Typically involve a metal cation and a non-metal anion or a polyatomic ion.

  • Example of a metal: Lithium (Li), Sodium (Na), Copper (Cu), Iron (Fe)

  • Example of a non-metal or polyatomic ion: Bromine (Br), Sulfate (SO₄²⁻), Nitrate (NO₃⁻)

• Identification of Ionic Compounds

  • To determine if a compound is ionic, check for presence of a metal or the ammonium ion.

  • Example question: Which compound is ionic? Select the one with a metal ion.

• Formation of Ionic Compounds

  • Ionic compounds are formed through the transfer of electrons.

  • The metal -- desiring to lose electrons -- transfers electrons to the non-metal. This creates positive ions (cations) and negative ions (anions).

  • Because opposite charges attract, these ions bond together.

• Formula Representation

  • The formula of an ionic compound indicates the types of ions present and their relative ratios.

  • The overall compound is electrically neutral. Sum of positive and negative charges = 0.

  • Example: +2 for one cation requires two -1 anions to balance: formula = A₂B.

Example: Strontium and Bromine Ionic Compound

• Procedure to Write Formula

  1. Look-up the periodic table for strontium (Sr: +2 charge) and bromine (Br: -1 charge).

  2. Strontium needs to transfer 2 electrons to form 2 Bromide ions (Br⁻).

  3. Resulting formula: SrBr₂ (implied 1:2 ratio).

• Crisscross Method for Charging

  • Identify charges: Sr +2, Br -1.

  • Crisscross the charges into subscripts: Sr becomes 1, Br becomes 2 (implied, not written).

  • Final chemical representation: SrBr₂.

Second Example: Aluminum and Sulfur Ionic Compound

• Aluminum (Al: +3 charge) and Sulfur (S: -2 charge).

• Use crisscross method:

  • Resulting formula: Al₂S₃ from +3 and -2 charges after simplification.

Naming Ionic Compounds

• Follow rules based on ions composition:

  • Name metal as is, for anions, change name's ending to -ide if they are single-element ions.

  • If transition metals are involved, indicate their charge in Roman numerals (e.g., Iron(II)).

• Example Names:

  • Strontium Bromide from Sr +2 and Br⁻ → strontium bromide.

  • Aluminum Sulfide from Al +3 and S -2 → aluminum sulfide.

  • If the compound includes polyatomic ions, use their standard names, e.g., sulfate, nitrate.

Polyatomic Ions

• Can be involved in forming ionic compounds alongside metals.

• Always enclose polyatomic ion in parentheses when multiple are present:

  • Example: Aluminum Sulfate from Al +3 and sulfate (SO₄²⁻) results in Al₂(SO₄)₃ which signifies two aluminum to three sulfates.

Simplifying Ratios

• Ratios should be presented in simplest form; e.g., if both subscripts are equal, reduce them to the lowest whole numbers.

  • 2:2 simplifies to 1:1 (do not write the '1').

Conclusively: Mastering Ionic Compounds

• It is essential for exams to practice how to identify, write, and name ionic compounds fostered by understanding composition of ions.

• Utilize periodic tables for charge assignments and refer to mnemonics for popular polyatomic ions.

• The knowledge of charges guides the accurate formulation of compound names and structures.

Understanding Covalent Compounds

• Covers compounds formed from two or more non-metals.

• In contrast to ionic compounds, covalent compounds involve sharing electrons instead of transferring, producing neutral molecules rather than charged ions.

• Will explore naming conventions and specific attributes in upcoming sessions.

• Comparison of ionic (charged ions) to covalent (neutral molecules) will be detailed further.