Aufbau Principle:

• Electrons fill orbitals in order of increasing energy.
• Follows the sequence: ( 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, … ).

Pauli Exclusion Principle

• No two electrons in an atom can have the same set of four quantum numbers (n, l, m_l, m_s).

Hund’s Rule:

• Electrons occupy orbitals singly before pairing up, and all singly occupied orbitals have the same spin.

Examples

• Carbon (C): ( 1s² 2s² 2p² )
• Neon (Ne): ( 1s² 2s² 2p⁶ )
• Chlorine (Cl): ( 1s² 2s² 2p⁶ 3s² 3p⁵ )

Periodic Properties of Elements:

Atomic Radius

Electron Affinity

• Definition: Energy change when an electron is added to a neutral atom to form a negative ion.

Electronegativity

• Definition: Measure of the ability of an atom to attract electrons in a chemical bond

Metallic Character

• Property of elements associated with the ability to lose electrons and form positive ions (cations)

Chemical Reactivity

• Metals: Tend to lose electrons and form positive ions (cations).
• Non-Metals: Tend to gain electrons and form negative ions (anions)
• Electronic configuration defines how electrons are distributed in atoms based on quantum mechanical principles (Aufbau, Pauli exclusion, Hund’s rules).
• Periodic properties describe trends across the periodic table related to atomic size (radius), tendency to gain or lose electrons (ionization energy, electron affinity), ability to attract electrons (electronegativity), and behavior in chemical reactions (metallic character, reactivity).