Ionic and Molecular (Covalent) Compounds: Ions, Electron Transfer, and Periodic Trends

Ch 6: Ionic and Molecular (Covalent) Compounds

Overview

This chapter introduces the fundamental concepts of ionic and molecular (covalent) compounds, focusing on the formation of ions, electron transfer, and the relationship between electron configuration and the periodic table. The following notes summarize key definitions, trends, and examples relevant to General Chemistry.

Ions: Transfer of Electrons

Definition of Ions

• Ion: An atom or group of atoms that has lost or gained one or more electrons, resulting in a net electrical charge.

• Cation: A positively charged ion formed when an atom loses electrons. Metals tend to form cations. Examples: Na+, K+, Mg2+, Ca2+, Al3+

• Anion: A negatively charged ion formed when an atom gains electrons. Nonmetals tend to form anions. Examples: F-, Cl-, O2-, S2-, N3-, P3-

• Noble gases: Elements with full valence shells; they are stable and non-reactive.

Electron Shell Arrangements of Anions

Nonmetals gain electrons to fill their valence shell and achieve stability similar to noble gases.

Element	Ne	O	O2-	F
# Protons	10	8	8	9
# Electrons	10	8	10	9
Valence Electrons	8	6	8	7
Charge	0	0	-2	0

Electron Shell Arrangements of Cations

Metals lose electrons to achieve a full valence shell, becoming stable like noble gases.

Element	Ne	Na	Na+	Mg
# Protons	10	11	11	12
# Electrons	10	11	10	12
Valence Electrons	8	1	8	2
Charge	0	0	+1	0

Charge on an Ion

• The charge of an ion is equal to the difference between the number of protons (positive) and electrons (negative).

• For main group elements, ions in the same group have the same charge.

	Na	Na+	O	O2-
# Protons	11	11	8	8
# Electrons	11	10	8	10
Charge	0	+1	0	-2

Periodic Table and Charges on Ions

The periodic table can be used to predict the charges of ions formed by main group elements.

Group #	# Valence e-	To be like Noble Gas	Ionic charge	Example
1A	1	Lose 1 e-	1+	Na+
2A	2	Lose 2 e-	2+	Mg2+
3A	3	Lose 3 e-	3+	Al3+
5A	5	Gain 3 e-	3-	N3-
6A	6	Gain 2 e-	2-	O2-
7A	7	Gain 1 e-	1-	F-

Note: Elements in Group 4A typically do not form ions.

Periodic Table Reference

The periodic table is a key tool for determining the properties and charges of elements and their ions. Main group elements (s- and p-block) have predictable charges, while transition metals (d-block) often have variable charges.

Practice: Determining Valence Electrons and Noble Gas Configuration

• To determine the number of valence electrons, locate the element's group number (for main group elements).

• Identify the nearest noble gas to predict the stable electron configuration the ion will achieve.

• Calculate the charge needed for the element to achieve the noble gas configuration.

Key Equations

• Ion charge: $\text{Ion charge} = \text{Number of protons} - \text{Number of electrons}$

Summary Table: Main Group Ion Charges

Group	Typical Ion Charge	Example
1A	+1	Na+
2A	+2	Mg2+
3A	+3	Al3+
5A	-3	N3-
6A	-2	O2-
7A	-1	F-

Example: Sodium and Oxygen

• Sodium (Na): Group 1A, loses 1 electron to form Na+ (like Ne).

• Oxygen (O): Group 6A, gains 2 electrons to form O2- (like Ne).

Additional info: Transition metals (d-block) can form multiple ions with different charges, which are not as easily predicted from the periodic table position.