Atomic Structure Principles and Periodic Property Trends

Fundamental Concepts of Atomic Structure

• Atomic Orbital: The region of space around the nucleus in which there is a high probability of finding an electron with a particular energy.
• Effective Nuclear Charge ($Z_{eff}$): The net positive charge experienced by an electron in a multi-electron atom. It is the charge the core would have if the atom consisted only of the core and the electron considered.
• Screening (Shielding): The repulsion between electrons, which reduces the nuclear attraction experienced by outer electrons and affects their energy state.

Rules Governing Electronic Configuration

The fundamental electronic configuration is determined by three key principles:

1. Aufbau Principle (Rule of Construction): Electrons fill available atomic orbitals in order of increasing energy.
2. Pauli Exclusion Principle: No two electrons in an atom can have the same set of four quantum numbers.
3. Hund's Rule of Maximum Multiplicity: When electrons occupy degenerate orbitals (orbitals of the same energy), they will occupy different orbitals with parallel spins (unpaired electrons) as much as possible.

Magnetic Properties of Atoms

• Paramagnetism: Substances that are weakly attracted by an external magnetic field (due to the presence of unpaired electrons).
• Diamagnetism: Substances that are weakly repelled by an external magnetic field (all electrons are paired).

The Periodic Law and Chemical Periodicity

The Periodic Law

When elements are arranged in order of increasing atomic number, there is a periodic repetition of their physical and chemical properties.

Key Periodic Properties and Trends

Atomic Radius

Half the distance between the nuclei of two identical atoms bonded together.

• Period Trend: Decreases across a period (left to right).
• Group Trend: Increases down a group (top to bottom).
• Conclusion: Largest radii are found toward the bottom-left of the periodic table.

Ionic Radius

• Cations are smaller than their parent atoms.
• Anions are larger than their parent atoms.

Ionization Energy (IE)

The minimum energy required to remove one electron from a neutral atom (X) in its gaseous ground state, forming a monopositive ion ($X^+$).

• Period Trend: Increases across a period (left to right).
• Group Trend: Decreases down a group (top to bottom).
• Conclusion: Highest IE is found toward the upper-right of the periodic table.

Electron Affinity (EA)

The energy change that occurs when an electron is added to a neutral atom (X) in its gaseous ground state, forming a mononegative ion ($X^-$).

• Trend: Generally increases (becomes more negative/favorable) toward the upper-right of the periodic table.

Electronegativity (EN)

The relative tendency of an atom to attract electrons toward itself when chemically bonded to another atom.

• Period Trend: Increases across a period (left to right).
• Group Trend: Decreases down a group (top to bottom).
• Conclusion: Highest EN is found toward the upper-right of the periodic table.