Week 3: Ionic and Covalent Bonding, Periodic Trends, and Polyatomic Ions

Trends in Periodic Properties

Group Numbers: Valence Electrons

Understanding the arrangement of electrons in atoms is essential for predicting chemical behavior. The valence electrons are the electrons in the outermost energy level of an atom and are primarily responsible for chemical properties.

• Valence electrons: Electrons in the highest occupied energy level of an atom.

• Group number: For representative elements (Groups 1A–8A), the group number equals the number of valence electrons.

• Example: Magnesium (Mg) is in Group 2A and has 2 valence electrons.

Lewis Symbols

Lewis symbols visually represent valence electrons as dots placed around the symbol of an element. They are useful for predicting bonding behavior.

• Each dot represents one valence electron.

• Lewis symbols help in understanding how atoms bond to achieve stable electron configurations.

• Example: Lewis symbol for magnesium: Mg with two dots.

Metallic Character

Metallic character refers to how easily an atom loses valence electrons. It is a key periodic trend.

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

• Metals are found on the left side of the periodic table and tend to lose electrons easily.

• Nonmetals are found on the right side and do not lose electrons easily.

Ions: Transfer of Electrons

Formation of Ions

Atoms gain or lose electrons to achieve a stable octet, forming ions. This process is fundamental to ionic bonding.

• Cation: A positively charged ion formed by loss of electrons (usually metals).

• Anion: A negatively charged ion formed by gain of electrons (usually nonmetals).

• Example: Sodium (Na) loses one electron to form Na+; chlorine (Cl) gains one electron to form Cl-.

Positive Ions: Loss of Electrons

Metals lose electrons to form cations. The charge is equal to the number of electrons lost.

• Group 1A metals lose 1 electron: $\text{Na} \rightarrow \text{Na}^+ + e^-$

• Group 2A metals lose 2 electrons: $\text{Mg} \rightarrow \text{Mg}^{2+} + 2e^-$

Negative Ions: Gain of Electrons

Nonmetals gain electrons to form anions. The charge is equal to the number of electrons gained.

• Group 7A nonmetals gain 1 electron: $\text{Cl} + e^- \rightarrow \text{Cl}^-$

• Group 6A nonmetals gain 2 electrons: $\text{O} + 2e^- \rightarrow \text{O}^{2-}$

Ionic Charges and Group Numbers

The charge of ions formed by representative elements can be predicted from their group number.

Group	Valence Electrons	Ion Formed
1A	1	+1
2A	2	+2
6A	6	-2
7A	7	-1

Ionic Compounds

Formation and Properties

Ionic compounds are formed from the electrostatic attraction between positive and negative ions. They have distinct physical properties.

• Consist of a lattice of positive and negative ions.

• High melting points and are solid at room temperature.

• Conduct electricity when dissolved in water.

Chemical Formulas of Ionic Compounds

The chemical formula of an ionic compound reflects the ratio of ions needed to balance charges.

• Sum of positive and negative charges must be zero.

• Subscripts indicate the number of each ion in the formula.

• Example: $\text{Na}^+ + \text{Cl}^- \rightarrow \text{NaCl}$

Writing Ionic Formulas from Ionic Charges

To write the formula for an ionic compound, balance the total positive and negative charges.

• Example: $\text{Mg}^{2+} + 2\text{Cl}^- \rightarrow \text{MgCl}_2$

Naming and Writing Ionic Formulas

Naming ionic compounds follows specific rules to ensure clarity.

• Name the cation (metal) first, then the anion (nonmetal).

• The anion name ends in "-ide".

• Example: NaCl is named sodium chloride.

Compound	Cation Name	Anion Name	Compound Name
NaCl	Sodium	Chloride	Sodium chloride
KBr	Potassium	Bromide	Potassium bromide
MgO	Magnesium	Oxide	Magnesium oxide

Metals with Variable Charge

Some transition metals can form more than one type of cation. Their charge is indicated by a Roman numeral in parentheses.

• Example: Fe2+ is iron(II); Fe3+ is iron(III).

• Cu+ is copper(I); Cu2+ is copper(II).

Metal	Possible Charges	Name
Fe	2+, 3+	Iron(II), Iron(III)
Cu	1+, 2+	Copper(I), Copper(II)
Cr	2+, 3+	Chromium(II), Chromium(III)

Writing Formulas for Ionic Compounds with Variable Charge

Balance the charges to determine the correct formula.

• Example: Iron(III) chloride: $\text{Fe}^{3+} + 3\text{Cl}^- \rightarrow \text{FeCl}_3$

Polyatomic Ions

Definition and Examples

Polyatomic ions are groups of covalently bonded atoms that carry an overall charge. They are common in many ionic compounds.

• Examples include sulfate ($\text{SO}_4^{2-}$), nitrate ($\text{NO}_3^-$), phosphate ($\text{PO}_4^{3-}$), and ammonium ($\text{NH}_4^+$).

• Most polyatomic ions are negatively charged, except ammonium.

Ion	Formula	Charge
Sulfate	SO4	2-
Nitrate	NO3	1-
Phosphate	PO4	3-
Ammonium	NH4	1+

Naming Polyatomic Ions

Polyatomic ions have specific naming conventions.

• Most common polyatomic ions end in "-ate" (e.g., sulfate, nitrate).

• When a related ion has one less oxygen, its name ends in "-ite" (e.g., nitrite, sulfite).

Writing Formulas Containing Polyatomic Ions

When writing formulas for compounds containing polyatomic ions, balance the charges as for simple ionic compounds.

• Use parentheses around the polyatomic ion if more than one is needed.

• Example: Magnesium nitrate: $\text{Mg}^{2+} + 2\text{NO}_3^- \rightarrow \text{Mg(NO}_3)_2$

Properties and Applications of Ionic Compounds

Ionic compounds are widely used in daily life and industry.

• Examples: Table salt (NaCl), potassium iodide (KI), sodium chloride (NaCl) in medical treatments.

• Polyatomic ions are found in fertilizers, cleaning agents, and biological systems.

Additional info: Some context and examples have been expanded for clarity and completeness.