Atomic Structure, Models, and Electron Configuration Rules

Fundamental Concepts of Atomic Structure

The Bohr Atomic Model and Energy Levels

The Bohr atom only allows electrons to move in certain, quantized orbits around the nucleus. The lowest energy level corresponds to the orbit closest to the nucleus. As the orbit moves away from the core, the energy levels increase.

Hydrogen Spectra Explanation

The interaction of electrons with energy explains the hydrogen spectra:

• The electron absorbs energy when moving from a lower energy level to a higher energy level.
• When passing from a higher level to a lower level, the electron emits energy in the form of radiation (a photon).
• The energy absorbed or emitted by the photon equals the energy difference between the levels.
• The frequency of the emitted or absorbed radiation is given by the equation: (Equation implied)

Historical Atomic Models

Thomson Model (Plum Pudding Model)

A sphere with the mass and positive charge distributed uniformly, with electrons inserted throughout.

Rutherford Nuclear Model

This model assumes the existence of a core 10⁴ times smaller than the total size of the atom. This core concentrates the entire mass and positive charge of the atom, surrounded by electrons (which have a negative charge and negligible mass).

To explain why electrons do not fall upon and collapse the core, the atom proposed a dynamic model in which electrons move, describing orbits around the nucleus. The electrostatic attraction provides the centripetal force needed to maintain these circular orbits.

Defining Atomic Components

• Atomic Number (Z): The number of protons that any atom of that element possesses.
• Number of Neutrons (N): The count of neutrons in the nucleus.
• Mass Number (A): The total number of protons plus neutrons (A = Z + N).
• Isotopes: Atoms with the same number of protons (same Z) but different numbers of neutrons (different N).

Atomic Orbitals

An orbital is a region of space where there is a high probability of finding an electron.

• The form (shape) of an orbital depends on the sublevel to which it belongs:
  • s orbitals: Spherical shape.
  • p orbitals: Dumbbell shape (differing only in their orientation in space).
• The size of the orbital depends on the principal quantum number (n); the larger the value of n, the larger the orbital.

Electron Configuration Principles

The electron configuration of an atom is written by indicating the sublevels that contain electrons, using a superscript to denote the number of electrons hosted in each sublevel:

Sublevel Energy Order (Mnemonic Diagram)

1s
2s 2p
3s 3p 3d
4s 4p 4d 4f
5s 5p 5d 5f
6s 6p 6d 6f

Rules Governing Electron Placement

• Aufbau Principle: In a ground-state atom, electrons occupy the lowest energy orbitals available first.
• Pauli Exclusion Principle: Each orbital can hold only 2 electrons, and they must have different spins.
• Hund's Rule: When electrons occupy several orbitals of the same energy (the same sublevel), they are arranged so that there is the maximum number of unpaired electrons (all having the same spin initially).