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Ionic and Covalent Compounds: Periodic Trends, Lewis Symbols, and Nomenclature

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Trends in Periodic Properties

Group Numbers and Valence Electrons

The periodic table is organized into groups (vertical columns) and periods (horizontal rows). The group number indicates the number of valence electrons for main group elements (Groups 1A–8A).

  • Valence electrons are the electrons in the outermost energy level of an atom and are responsible for chemical bonding.

  • Elements in the same group have similar chemical properties due to the same number of valence electrons.

  • Example: Group 1A elements (e.g., Na, K) have 1 valence electron; Group 2A elements (e.g., Mg, Ca) have 2 valence electrons.

Lewis Symbols

Lewis symbols are a simple way to represent the valence electrons of an atom using dots around the element symbol.

  • Each dot represents one valence electron.

  • Lewis symbols help visualize how atoms bond in molecules and compounds.

  • Example: Mg is shown as Mg: with two dots for its two valence electrons.

Element

Lewis Symbol

Na

Na·

Mg

Mg··

Al

Al···

Cl

Cl·······

Metallic Character

Metallic character refers to how easily an element loses electrons to form positive ions (cations).

  • Metallic character increases down a group and decreases across a period (left to right).

  • Metals are found on the left and center of the periodic table; nonmetals are on the right.

  • Example: Sodium (Na) is more metallic than magnesium (Mg).

Ions: Transfer of Electrons

Formation of Ions

Atoms gain or lose electrons to achieve a stable electron configuration, often resembling the nearest noble gas.

  • Cations are positive ions formed when atoms lose electrons (usually metals).

  • Anions are negative ions formed when atoms gain electrons (usually nonmetals).

Ionic and Covalent Bonds

  • Ionic bonds occur between metals and nonmetals, involving the transfer of electrons from metal to nonmetal.

  • Covalent bonds occur between nonmetals, involving the sharing of electrons.

  • Example: NaCl is an ionic compound; H2O is a covalent compound.

Positive Ions: Loss of Electrons

  • Metals lose electrons to form cations with a positive charge equal to the number of electrons lost.

  • Example: Na → Na+ + e-

  • Group 1A metals form 1+ ions, Group 2A metals form 2+ ions.

Negative Ions: Gain of Electrons

  • Nonmetals gain electrons to form anions with a negative charge equal to the number of electrons gained.

  • Example: Cl + e- → Cl-

  • Group 7A elements form 1- ions, Group 6A elements form 2- ions.

Ionic Charges and Group Numbers

Group

Ion Charge

1A

1+

2A

2+

3A

3+

5A

3-

6A

2-

7A

1-

Ionic Compounds

Properties of Ionic Compounds

  • Composed of positive and negative ions held together by electrostatic forces (ionic bonds).

  • Form crystalline solids with high melting points.

  • Conduct electricity when dissolved in water (as ions are free to move).

Chemical Formulas of Ionic Compounds

  • The total positive charge must balance the total negative charge.

  • Subscripts indicate the ratio of ions in the compound.

  • Example: In NaCl, one Na+ balances one Cl-.

Writing Ionic Formulas from Ionic Charges

  • Balance the charges so the total positive and negative charges are equal.

  • Example: For Mg2+ and Cl-, the formula is MgCl2.

Naming and Writing Ionic Formulas

Naming Ionic Compounds

  • Name the cation (metal) first, then the anion (nonmetal) with an -ide ending.

  • Example: NaCl is sodium chloride.

  • For metals with variable charges (transition metals), indicate the charge with Roman numerals (e.g., FeCl2 is iron(II) chloride).

Formula

Name

NaCl

Sodium chloride

CaO

Calcium oxide

FeCl3

Iron(III) chloride

Metals with Variable Charge

  • Some metals (mainly transition metals) can form more than one type of positive ion.

  • The charge is specified in the compound name using Roman numerals.

  • Example: Cu+ is copper(I), Cu2+ is copper(II).

Polyatomic Ions

Definition and Examples

  • Polyatomic ions are ions composed of two or more atoms covalently bonded, carrying a net charge.

  • Common examples: NO3- (nitrate), SO42- (sulfate), NH4+ (ammonium).

Ion

Name

Formula

Nitrate

NO3-

Sulfate

SO42-

Ammonium

NH4+

Writing Formulas Containing Polyatomic Ions

  • Balance the total positive and negative charges.

  • If more than one polyatomic ion is needed, use parentheses and a subscript.

  • Example: Calcium nitrate is Ca(NO3)2.

Naming Compounds with Polyatomic Ions

  • Name the cation first, then the polyatomic ion.

  • Example: Na2SO4 is sodium sulfate.

Summary Flow Chart: Naming Ionic Compounds

  • Identify if the compound contains a polyatomic ion.

  • Determine if the metal has a variable charge.

  • Name the cation and anion appropriately, using Roman numerals if necessary.

Practice and Application

  • Practice writing formulas and names for ionic compounds, including those with polyatomic ions and metals with variable charges.

  • Check that the total charges balance in each formula.

Additional info: These notes cover material relevant to Ch. 4 (Atoms), Ch. 6 (Ionic and Molecular Compounds), and Ch. 7 (Chemical Reactions and Quantities) of a GOB Chemistry course, including periodic trends, Lewis structures, ion formation, and nomenclature.

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