Electron Configurations of Ions

Formation of Cations and Anions

Atoms can gain or lose electrons to form ions, which are charged particles. The loss of electrons results in cations (positively charged ions), while the gain of electrons results in anions (negatively charged ions). The electron configuration of the resulting ion is closely related to the stability and chemical properties of the element.

• Cations: Formed by loss of electrons, typically from the highest energy (valence) orbitals.

• Anions: Formed by gain of electrons, filling the valence orbitals to capacity.

• Example: Sodium (Na) loses one electron to become Na+, achieving a noble gas configuration. Chlorine (Cl) gains one electron to become Cl-, also achieving a noble gas configuration.

Equation Examples:

Reoccurring Themes for Representative Elements

Noble Gas Configurations and Ion Formation

The noble gases are chemically stable due to their filled valence s and p orbitals. Many representative elements (Groups 1A, 2A, 3A, 6A, 7A) form ions that achieve a noble gas-like electron configuration, which is a driving force for their chemical behavior.

• Metals: Tend to lose electrons to form cations with noble gas configurations.

• Nonmetals: Tend to gain electrons to form anions with noble gas configurations.

• Example: Group 1A (Na) loses one electron to become Na+ ([Ne]), Group 2A (Mg) loses two electrons to become Mg2+ ([Ne]), Group 6A (S) gains two electrons to become S2- ([Ne]), Group 7A (Cl) gains one electron to become Cl- ([Ne]).

Common Main-Group Ions Summary

This table summarizes the main-group ions and their corresponding noble-gas electron configurations.

Additional info: Some ions (marked with *) do not have a true noble-gas configuration due to an additional filled d subshell.

Multivalent Elements and Transition Metals

Formation of Transition Metal Ions

Transition metals and some heavier metals can form ions with multiple possible charges. They lose their valence-shell s electrons first, followed by d electrons. For example, iron can lose two 4s electrons to form Fe2+ or lose two 4s and one 3d electron to form Fe3+.

• Fe:

• Fe:

• Note: s electrons are removed before d electrons when forming ions.

Ionic Radii

Cations

When a cation forms, the removal of valence electrons reduces the ionic radius compared to the neutral atom. The trend down a group remains: ionic radius increases as you move down the group.

Anions

When an anion forms, the addition of valence electrons increases the ionic radius compared to the neutral atom. The trend down the group is the same: ionic radius increases as you move down the group.

Across a period: Ionic radius contracts for cations, then expands for anions.

Ionization Energy

Definition and Trends

Ionization energy (Ei) is the energy required to remove the highest-energy electron from a neutral atom in the gaseous state. Metallic elements generally have lower ionization energies, while nonmetals have higher ionization energies. Across a period, ionization energy increases.

Irregularities in Ionization Energy Trends

Some elements show irregularities in ionization energy trends due to their electron configurations. For example, Be has a higher Ei than B, and N has a higher Ei than O.

Additional info: These irregularities are explained by the stability of filled and half-filled subshells.

Higher Ionization Energies

Atoms can have multiple ionization energies, corresponding to the removal of successive electrons. The energy required increases with each electron removed.

Electron Affinity

Definition and Trends

Electron affinity (Eea) is the energy change when an electron is added to a neutral atom in the gaseous state. Most electron affinities are negative, indicating energy is released when an atom gains an electron. The more negative the value, the greater the tendency to accept an electron.

Additional info: Electron affinity values are influenced by nuclear attraction and electron-electron repulsion.

The Octet Rule

Valence Electrons and Covalent Bond Formation

The octet rule states that main group elements tend to gain, lose, or share electrons to fill their valence s and p orbitals, achieving eight electrons (an octet). This rule predicts the number of covalent bonds formed by elements, especially Period 2 nonmetals (C, N, O, F).

• Carbon: 1s22s22p2 (4 valence electrons) forms 4 covalent bonds to complete its octet.

• Hydrogen: Exception; forms only one bond to fill its n=1 shell (2 electrons).

• Transition metals, lanthanides, actinides: Often do not follow the octet rule.

Ionic Bonds and Formation of Ionic Solids

Mechanism of Ionic Bonding

Ionic bonding occurs when one atom transfers electrons to another, resulting in electrostatic attraction between cations and anions. This process forms ionic solids.

Formation of NaCl

The formation of NaCl involves both ionization energy (removal of electron from Na) and electron affinity (addition of electron to Cl). The overall stability of the ionic solid is due to the electrostatic attraction between Na+ and Cl- ions.

Born-Haber Cycle for Ionic Solids

Energy Changes in Ionic Solid Formation

The Born-Haber cycle is used to analyze the energy changes involved in forming an ionic solid from its elements. It includes steps such as sublimation, ionization, dissociation, electron affinity, and lattice energy.

Lattice Energy

Definition and Factors Affecting Lattice Energy

Lattice energy (ΔUlattice) is the energy released when ions form a crystalline solid. It is given by:

• C: Constant depending on crystal structure

• Z+, Z-: Charges on the ions

• Ro: Interionic distance

Lattice energy increases with higher ion charges and smaller ion sizes.

Lattice Energy Reference Table

This table shows lattice energies for various ionic solids.

Comparing Ionic Solids

Visual representations of ionic solids help distinguish between compounds like NaCl and MgO, which differ in lattice energy due to ion charges and sizes.

Additional info: MgO has a larger lattice energy than NaCl due to higher ion charges.