Fundamental Principles of Atomic Structure and Quantum Rules

Quantum Mechanical Rules for Electron Configuration

The Aufbau Principle

Electrons will fill the lowest available energy level first.

The Pauli Exclusion Principle

Each orbital can hold a maximum of two electrons, and they must have opposite spins.

Hund’s Rule (Rule of Maximum Multiplicity)

Orbitals of equal energy (degenerate orbitals) will fill with one electron at a time before pairing up. Single electrons in these orbitals should have the same spin.

Foundational Discoveries in Atomic Theory

Rutherford's Gold Foil Experiment

This experiment demonstrated that atoms possess a positively charged nucleus orbited by electrons. Very few positively charged alpha particles were greatly deflected back from the metal foil, indicating a small, dense, positive core.

Rutherford's Key Discoveries:

• The atom has a positively charged core, called the nucleus.
• Electrons surround the nucleus.
• The rest of the atom is mostly empty space.

J.J. Thomson's Cathode Ray Experiment

Thomson discovered electrons and used the cathode ray tube to show that these particles carry negative charges.

J.J. Thomson's Key Discoveries:

• The atom consists of charged particles.
• Negatively charged particles are called electrons.
• Positively charged particles are called protons.

Experiment details: Electricity is passed through a gas tube, and the resulting gas beam can be bent with a magnet.

Dalton: The Father of Atomic Theory

Dalton proposed that all matter was composed of atoms, which he considered indivisible and indestructible building blocks. He theorized that while all atoms of an element were identical, different elements had atoms of differing size and mass.

Dalton's Principles and Modern Status

• All matter is composed of indestructible atoms: False (Disproved by nuclear chemistry).
• Atoms of the same element are identical: False (Disproved by the discovery of isotopes).
• Atoms of different elements have different physical and chemical features: True.
• Atoms of different elements combine in simple, whole-number ratios to form chemical compounds: True.
• Atoms cannot be created or destroyed in an ordinary chemical reaction: True (Law of Conservation of Mass).

Bohr's Model of the Atom

Bohr discovered that electrons are only found at specific, fixed energy levels (orbits), and each level contains a certain amount of energy.

• Lower energy levels are closer to the nucleus.
• Higher energy levels are farther away from the nucleus.
• Concepts introduced: Ground State and Excited State.

Note: Bohr's model failed to accurately describe the electron configuration for every element besides Hydrogen.

Schrödinger and Atomic Orbitals

Schrödinger developed the quantum mechanical model, discovering the five types of atomic orbitals: s, p, d, f, and g. These orbitals are often described as electron clouds, representing regions of probability where electrons can be found (often depicted with complex shapes).

James Chadwick's Discovery

Chadwick discovered the neutron, a neutrally charged subatomic particle located in the nucleus.

Key Definitions in Electron Energy

Defining Ground State

The ground state is the condition where all electrons in an atom are in their lowest possible energy levels.

Defining Excited State

The excited state occurs when electrons absorb energy and jump to a higher, unstable energy level.