Study Notes on Atomic Structure and Orbitals

Structure of the Atom

Presented by Dr. Windsor

Bohr Model

• Model Description:
  • According to Bohr, electrons “orbit” the nucleus in well-defined “allowed” energy levels.
• Suitability:
  • The concept of orbits is adequate for describing single electron atoms and ions such as:
  • Hydrogen (H)
  • Helium ion (He+)
  • Lithium ion (Li2+)

Orbitals

• Characteristics:
  • Orbitals possess characteristic energy levels and distinct shapes.
• Energy Variation:
  • The energy of the orbital increases with the size of the orbital.
• Periodic Table Relation:
  • The size of the orbital is related to the period of the periodic table.

Size of Orbitals

• Energy Levels (n):
  • n=1: Core electrons (closest to nucleus)
  • n=2: Example -
  • Helium (He)
  • Electron Configuration: 1s¹
  • n=3: Example -
  • Sodium (Na) situated in the Neon (Ne) core, Electron Configuration: 3s¹
  • n=4: Core Ne (Ar)
  • n=5: Core Kr
  • n=6: Core Xe
  • n=7: Core Rn

Summary of Elements and Their Valence Electron Configurations

• Hydrogen (H): 1s¹
• Lithium (Li): [He] 2s¹
• Sodium (Na): [Ne] 3s¹
• Magnesium (Mg): 3s²
• Aluminum (Al): 3s² 3p¹
• Each of these elements belongs to specific groups in the periodic table based on their outer shell configurations.

Orbitals Shapes

• Characteristic Shapes of Orbitals:
  • The shape of orbitals is denoted by letters:
  • s, p, d, f
  • Each shape corresponds with distinct blocks in the periodic table.

s Shape Orbitals

• 1s Orbital:
  • Spherical shape
• s Block Elements:
  • Elements have their valence electrons in the s orbitals, includes:
  • All alkali metals and alkaline earth metals (Li, Na, K, Be, Mg, Ca, etc.)

p Shape Orbitals

• p Block Elements:
  • Involves p orbitals, includes:
  • Elements such as C, N, O, F, Ne, Al, Si, etc.

d Shape Orbitals

• d Block Elements:
  • Includes transition metals, which have d orbitals in their configurations.
  • Examples include:
  • Iron (Fe), Nickel (Ni), Copper (Cu), etc.

f Shape Orbitals

• f Block Elements:
  • Includes lanthanides and actinides, characterized by the filling of f orbitals in their electron configurations.
  • Examples include:
  • Uranium (U), Plutonium (Pu), etc.

Principles Governing Electron Configuration

Pauli Exclusion Principle

• Definition:
  • States that no two electrons in an atom can have the same set of four quantum numbers.
• Implication:
  • An orbital can hold a maximum of two electrons, and they must have opposite spins.

Aufbau Principle

• Definition:
  • Describes the order in which orbitals are filled with electrons: lower energy levels are filled before higher energy levels.
• Energy Order of Orbitals:
  • A qualitative energy level diagram displays the general trend of energy levels and allows prediction of electron configurations without precise energy values.

Hund's Rule

• Definition:
  • For orbitals of the same energy (degenerate), the lowest energy configuration is achieved when the number of electrons having the same spin is maximized.
• Example:
  • The filling of 2p orbitals in nitrogen (N) and oxygen (O) illustrates the application of Hund's rule.

Anomalous Electron Configurations

• Explanation:
  • Occur due to the close energy levels of 3d and 4s orbitals.
  • Elements such as chromium and copper exhibit these configurations:
  • Chromium: 3d⁵ 4s¹
  • Copper: 3d¹⁰ 4s¹