Electron Configurations of Ions

Introduction to Electron Configurations

Electron configurations describe the arrangement of electrons in an atom or ion. Understanding these configurations is essential for predicting chemical behavior, bonding, and periodic trends. When atoms form ions, they gain or lose electrons to achieve more stable electron arrangements, often resembling those of noble gases.

Noble-Gas Electron Configurations

Stability of Noble Gases

• Noble gases have completely filled s and p sublevels in their outermost energy level.

• This configuration is particularly stable, making noble gases largely unreactive.

• Other elements tend to gain or lose electrons to achieve a similar configuration, known as the noble-gas configuration.

Example: Neon (Ne) has the electron configuration , a full second shell.

Abbreviated Electron Configurations

Using Noble-Gas Notation

• Electron configurations can be abbreviated by using the symbol of the nearest noble gas in brackets to represent core electrons.

• This notation simplifies the representation of electron configurations, especially for larger atoms and ions.

Example: Sodium (Na):

Alkali-Metal Cations

Alkali metals (Group 1) lose one electron to form cations with a noble-gas configuration.

• Li+: or

• Na+: or

• K+: or

• Rb+:

• Cs+:

Alkaline-Earth-Metal Cations

Alkaline earth metals (Group 2) lose two electrons to form cations with a noble-gas configuration.

• Be2+: or

• Mg2+: or

• Ca2+:

• Sr2+:

• Ba2+:

Predicting Charges of Main-Group Metals

• Metals in Groups 1 and 2 form cations with charges equal to their group number (1+ for Group 1, 2+ for Group 2).

• Aluminum (Al), in Group 13, typically forms a 3+ ion by losing three electrons: .

Example: Aluminum: after losing three electrons.

Transition Metal Cations

• Transition metals can form cations with various charges.

• Electrons are lost first from the s orbital of the highest principal quantum number, then from the d orbitals.

Example: Iron (Fe): ; : ; :

Unpaired Electrons in Ions

• The number of unpaired electrons in an ion can be determined from its electron configuration.

• Example: Rhodium(III) ion, : ; in a low-spin complex, all electrons may be paired, but in a high-spin complex, there may be unpaired electrons.

Application: The number of unpaired electrons affects magnetic properties (paramagnetism vs. diamagnetism).

Periodic Trends: Atomic and Ionic Radii, Ionization Energy

Atomic Radius

• The atomic radius is the distance from the nucleus to the outermost electron shell.

• Atomic radius increases down a group (more electron shells) and decreases across a period (increased nuclear charge pulls electrons closer).

Example: Li (167 pm) < Na (190 pm) < K (243 pm) < Rb (265 pm) < Cs (298 pm)

Ionic Radius

• Cations are smaller than their parent atoms due to loss of electrons and reduced electron-electron repulsion.

• Anions are larger than their parent atoms due to increased electron-electron repulsion.

• For isoelectronic species (same number of electrons), the more positive the charge, the smaller the ion.

Example Table: Sizes of Atoms and Ions (pm)

Ionization Energy

• Ionization energy is the energy required to remove an electron from a gaseous atom or ion.

• Ionization energy increases across a period (left to right) and decreases down a group.

• Elements with low ionization energies (e.g., alkali metals) lose electrons easily to form cations.

Example: Na has a lower ionization energy than Mg or Ne.

Summary Table: Periodic Trends

Practice and Application

• Predict the charge of main-group elements based on their group number.

• Write abbreviated electron configurations for ions using noble-gas notation.

• Compare atomic and ionic radii for isoelectronic species.

• Determine the number of unpaired electrons in a given ion.

Example: Which is larger: Na+ or F-? Both are isoelectronic, but F- (more negative charge) is larger.

Additional info: The number of unpaired electrons in transition metal ions depends on the specific electron configuration and whether the ion is in a high-spin or low-spin state (relevant in coordination chemistry).