Bohr Atomic Model: Principles and Quantum Mechanics

The Bohr Atomic Model (1913)

The Bohr model was developed based on studies of atomic emission spectra and quantum theory.

Key Foundations

• Emissions of Atoms: The light emitted by an element is called its spectrum, and each element possesses a unique one.
• Quantum Theory: In a chemical reaction, one cannot refer to a quantity of material less than one atom. Furthermore, there is a minimum amount of energy that can be emitted, known as a photon or quantum.

Bohr's Four Basic Tenets

• Electrons in atoms are located in specific orbits or energy levels around the nucleus.
• Electrons in orbits closer to the nucleus possess lower energy than those in more remote orbits.
• Any electron in an atom can have only certain allowed energy values, which determine its orbit.
• Electrons can move from one orbit to another by gaining or losing energy. To transition, an electron must gain or lose an exact amount of energy, known as a quantum of energy.

The Hydrogen Atom Model

Bohr used the hydrogen atom to develop his model, describing it as having one proton in the nucleus with one electron revolving around it. In this model, electrons occupy the lowest possible energy orbit closest to the nucleus. Each energy level is identified by the principal quantum number n, which takes integer values from 1 to 7.

Quantum Numbers

Bohr postulated that electrons spin at high speeds around the nucleus in circular orbitals, which determine different energy levels (n = 1 to 7).

• Principal Quantum Number (n): Indicates the energy level of the electron and determines the size or volume of the orbital.
• Azimuthal or Secondary Quantum Number (l): Defines the sub-level and the geometric shape of the orbital. Its value depends on n (ranging from 0 to n-1). For example, if n=1, l=0 (1s orbital); if n=2, l=1 (2s, 2p orbitals).
• Magnetic Quantum Number (m): Indicates the orientation of the orbital in space. Its values depend on l (ranging from -l to +l).
• Spin Quantum Number (ms): Represents the direction of electron rotation, with values of +1/2 (clockwise) or -1/2 (counter-clockwise).

Other Atomic Models

Thomson Atomic Model

This model proposed that the atom is composed of negative electrons embedded in a positive charge, similar to raisins in a pudding. It was initially thought that electrons were distributed evenly within a cloud of positive charge.

Rutherford Model

The significance of this model lies in proposing the existence of a dense atomic nucleus containing 99.9% of the atom's mass and a concentrated positive charge, with electrons revolving around it. Estimates of the core showed that the atom is mostly empty space.