Ionic and Molecular Compounds: Formation, Electron Transfer, and Ion Nomenclature

Ionic and Molecular Compounds

Introduction to Compounds

Chemical compounds are formed when elements combine through the transfer or sharing of electrons. The two main types of compounds are ionic compounds and molecular (covalent) compounds. Understanding how these compounds form is fundamental in GOB Chemistry.

• Ionic compounds are formed by the transfer of electrons from one atom (usually a metal) to another (usually a nonmetal).

• Molecular compounds are formed by the sharing of electrons between nonmetal atoms.

Ions and Electron Transfer

Formation of Ions

An ion is an atom or molecule with a net electric charge due to the loss or gain of one or more electrons.

• Cations: Positively charged ions formed when atoms lose electrons (typically metals).

• Anions: Negatively charged ions formed when atoms gain electrons (typically nonmetals).

Atoms tend to lose or gain electrons to achieve a stable electron configuration, often that of the nearest noble gas (the octet rule).

Electron Transfer and Ionic Bonding

Ionic bonds form due to the electrostatic attraction between oppositely charged ions. This usually occurs between metals (which lose electrons) and nonmetals (which gain electrons).

• Metals in Groups 1A, 2A, and 3A lose 1, 2, or 3 electrons, respectively, to form cations.

• Nonmetals in Groups 5A, 6A, and 7A gain 3, 2, or 1 electrons, respectively, to form anions.

Example: Sodium (Na) loses one electron to form Na+, and chlorine (Cl) gains one electron to form Cl-. These ions combine to form NaCl (table salt).

Valence Electrons and the Octet Rule

Valence Electrons

Valence electrons are the outermost electrons of an atom and are involved in chemical bonding. The group number on the periodic table often indicates the number of valence electrons for main group elements.

• Group 1A: 1 valence electron

• Group 2A: 2 valence electrons

• Group 7A: 7 valence electrons

The Octet Rule

Atoms tend to gain, lose, or share electrons to achieve a full set of eight valence electrons, similar to the electron configuration of noble gases.

• Metals lose electrons to achieve the octet in the previous energy level.

• Nonmetals gain electrons to complete their octet.

Example: Sodium (Na) has 11 electrons (1 valence electron). By losing one electron, it achieves the electron configuration of neon (10 electrons).

Formation of Cations and Anions

Cations (Positive Ions)

• Formed when an atom loses one or more electrons.

• The number of protons remains unchanged; only electrons are lost.

• The resulting ion has a smaller radius than the neutral atom due to increased attraction between the remaining electrons and the nucleus.

Example: Na → Na+ + e-

Anions (Negative Ions)

• Formed when an atom gains one or more electrons.

• The number of protons remains unchanged; only electrons are gained.

• The resulting ion has a larger radius than the neutral atom due to increased electron-electron repulsion.

Example: Cl + e- → Cl-

Trends in Ion Formation

Group Trends for Metals and Nonmetals

• Group 1A metals lose 1 electron to form +1 cations.

• Group 2A metals lose 2 electrons to form +2 cations.

• Group 7A nonmetals gain 1 electron to form -1 anions.

• Group 6A nonmetals gain 2 electrons to form -2 anions.

These trends are predictable based on the periodic table and help in determining the formulas of ionic compounds.

Atomic Radius Changes Upon Ion Formation

• Cations have smaller atomic radii than their parent atoms due to the loss of electrons and increased effective nuclear charge.

• Anions have larger atomic radii than their parent atoms due to the gain of electrons and increased electron-electron repulsion.

Ionization Energy and Electron Affinity

Ionization Energy

Ionization energy is the energy required to remove an electron from a neutral atom in the gas phase. Metals have low ionization energies, making it easier for them to lose electrons and form cations.

Electron Affinity

Electron affinity is the energy change that occurs when an atom gains an electron. Nonmetals have high electron affinities, making it easier for them to gain electrons and form anions.

Examples of Ion Formation

• Sodium (Na): Neutral atom has 11 protons and 11 electrons. Loses 1 electron to form Na+ (11 protons, 10 electrons).

• Magnesium (Mg): Neutral atom has 12 protons and 12 electrons. Loses 2 electrons to form Mg2+ (12 protons, 10 electrons).

• Chlorine (Cl): Neutral atom has 17 protons and 17 electrons. Gains 1 electron to form Cl- (17 protons, 18 electrons).

Common Ions and Their Names

Cations

Element	Symbol	Name
Sodium	Na+	Sodium ion
Magnesium	Mg2+	Magnesium ion
Calcium	Ca2+	Calcium ion
Potassium	K+	Potassium ion
Lithium	Li+	Lithium ion

Anions

Element	Symbol	Name
Chlorine	Cl-	Chloride ion
Fluorine	F-	Fluoride ion
Oxygen	O2-	Oxide ion
Sulfur	S2-	Sulfide ion
Nitrogen	N3-	Nitride ion

Naming Ions

• Cations: The name is the same as the element (e.g., sodium ion, calcium ion).

• Anions: The ending of the element name is changed to -ide (e.g., chloride, oxide, nitride).

Example: Oxygen becomes oxide, nitrogen becomes nitride, fluorine becomes fluoride.

Summary Table: Ion Formation and Noble Gas Configuration

Element	Electrons Lost/Gained	Ion Formed	Noble Gas Configuration Achieved
Na	Lose 1	Na+	Neon (Ne)
Mg	Lose 2	Mg2+	Neon (Ne)
Cl	Gain 1	Cl-	Argon (Ar)
O	Gain 2	O2-	Neon (Ne)

Key Equations

• Formation of a cation: $\text{Na} \rightarrow \text{Na}^+ + e^-$

• Formation of an anion: $\text{Cl} + e^- \rightarrow \text{Cl}^-$

Additional info:

• Ionization energy and electron affinity are important periodic trends that help explain why metals lose electrons and nonmetals gain electrons.

• When naming simple anions, the suffix -ide is always used.

• Proton count never changes during ion formation; only electrons are gained or lost.