The Periodic Table and Electronic Configurations

Overview of the Periodic Table

The periodic table is a systematic arrangement of elements based on their atomic number, electron configurations, and recurring chemical properties. It is divided into blocks (s, p, d, f) corresponding to the subshells being filled by electrons.

• Main-group elements are found in the s and p blocks.

• Transition elements occupy the d block.

• Lanthanides and actinides are in the f block.

Rules for Assigning Electrons to Orbitals

Principles Governing Electron Configuration

Electron configuration describes the distribution of electrons in atomic orbitals. Three fundamental rules guide this process:

• Aufbau Principle: Electrons fill orbitals in order of increasing energy, starting from the lowest energy level.

• Pauli Exclusion Principle: No two electrons in an atom can have the same set of four quantum numbers. Each orbital can hold a maximum of two electrons with opposite spins.

• Hund’s Rule: Electrons occupy degenerate (equal energy) orbitals singly before pairing, maximizing the number of unpaired electrons with parallel spins.

Order of Filling Electronic Subshells

The order in which subshells are filled is determined by their energy levels. The filling sequence can be visualized using the diagonal rule.

• Filling order: 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p → 7s → 5f → 6d → 7p

• Maximum electrons per subshell: s (2), p (6), d (10), f (14)

Electronic Configurations: Notation and Diagrams

spdf Notation and Electron-in-Box Diagrams

Electronic configurations can be represented in different ways:

• spdf notation: Shows the number of electrons in each subshell, e.g., 1s22s22p2.

• Electron-in-box diagrams: Visualize electron spins and orbital occupancy.

For example, the electron configuration of carbon (Z=6) is:

• spdf notation: 1s22s22p2

• Electron-in-box diagram:

Electrons in singly occupied orbitals of the same subshell have parallel spins, which is the lowest energy (ground-state) configuration.

Expanded Electron Configurations

Expanded notation specifies the distribution of electrons among individual orbitals, e.g., 1s22s22px12py1.

Electron Configurations for Elements

Examples for elements N, O, F, Ne (Z = 7–10):

• N: [He]2s22p3

• O: [He]2s22p4

• F: [He]2s22p5

• Ne: [He]2s22p6

For transition elements (Z = 21–30), electrons fill the 3d orbitals. Exceptions include Cr ([Ar]3d54s1) and Cu ([Ar]3d104s1).

Periodic Trends

Introduction to Periodic Trends

Periodic trends are patterns observed in the properties of elements across periods and groups in the periodic table. These include:

• Electronegativity

• Ionisation energy

• Electron affinity

• Atomic radius

• Ionic radius

• Melting point

• Metallic character

Ionisation Energy

Ionisation energy (I.E.) is the energy required to remove an electron from a gaseous atom. Successive ionisation energies increase as electrons are removed, especially after removing all valence electrons.

• Sharp rises in I.E. occur when removing electrons from a new shell closer to the nucleus.

• Example: Sodium shows sharp increases after the first and tenth electrons are removed.

General equation:

Electron Affinity

Electron affinity (E.A.) is the energy change when an electron is added to a gaseous atom. A more negative E.A. indicates a greater tendency to gain electrons.

• First E.A. is usually negative (energy released).

• Second E.A. is positive due to repulsion (energy required).

Example equations:

E.A. increases across a period and decreases down a group. Smaller atoms to the right of the periodic table have more negative E.A.

Atomic Radius

The atomic radius is a measure of the size of an atom. It can be defined as:

• Metallic radius: Half the distance between nuclei of adjacent atoms in a metallic lattice.

• Covalent radius: Half the distance between nuclei of identical atoms bonded together.

Ionic Radius

Ionic radius refers to the size of an ion (cation or anion). It is related to the distance between neighboring cations and anions in a crystal lattice.

• Cations are smaller than their parent atoms due to loss of electrons.

• Anions are larger than their parent atoms due to gain of electrons.

Example:

• Mg: 1s22s22p63s2 → Mg2+: 1s22s22p6

• F: 1s22s22p5 → F-: 1s22s22p6

Summary Table: Periodic Trends

The following table summarizes the main periodic trends:

Practice Problems

Electronic Configurations

• Write the electronic configurations for P, Fe, Ag, Ho, Os.

• Represent configurations in spdf notation and electron-in-box diagrams.

Periodic Trends

• Arrange As, Sn, Br, Sr in order of increasing first ionisation energy.

• Arrange K+, Cl-, S2-, Ca2+ in order of increasing ionic size.

Additional info: These notes expand on brief points from the original materials, providing definitions, examples, and visual aids for clarity and completeness.