Bohr’s Atomic Model: Postulates and Applications

Posted on Feb 14, 2025 in Chemistry

Bohr’s Theory

Bohr’s Atomic Model

What did Bohr Do?

In 1913, Bohr applied quantum theory to Rutherford’s atom and proposed a model that explains the hydrogen atom’s emission and absorption spectrum. He produced a revolutionary theory.

What are the Postulates of Bohr’s Theory?

  • The electron moves around the nucleus in specific circular orbits called energy levels or principal quantum levels.
  • While the electron moves in the same energy level, it does not absorb or emit energy. The electron is in a steady state.
  • The energy of the electron in each principal energy level is the sum of its kinetic energy and its electric potential energy.
  • The total energy of the electron cannot have any value, but only certain allowed values. That is to say that the energy is quantized.
  • Each principal energy level is associated with a natural number n = 1, 2, 3, 4…, called the principal quantum number. The level closest to the nucleus is n = 1, which corresponds to the lowest energy.
  • When the electrons are located in the lowest possible energy state, it is said to be in the ground state. In this state, the atom is stable. The other states are called excited states, which occur when an atom ionizes.
  • When an electron moves from a higher energy quantum level to a lower one, it emits energy, not gradually, but in a single step, discontinuously and quantized, in the form of a photon. Its value is equal to the difference in energy between the two quantum levels.
  • The passage of an electron from one level to another is called an electronic transition. The energy emitted or absorbed in the electronic transition is equal to a quantum or photon.

Interpretation of the Hydrogen Emission Spectrum

Bohr provided a reasoned explanation for the discontinuity of the hydrogen atom’s emission spectra through the transition of electrons between quantum levels using various series.

  • The electrons of atoms have different energy levels.
  • The transition between two energy levels occurs by absorption or emission of a photon.

Extension of Bohr’s Theory

Bohr’s theory not only explains the spectrum of the hydrogen atom but can also be extended to any element with a single electron, such as hydrogenoid ions (He+, Li2+, or Be3+). However, this is not the case with the spectra of polyelectronic atoms.

Sommerfeld extended Bohr’s work on the hydrogen atom and proposed that, in addition to circular orbits, there could also be elliptical orbits, which are also quantized. Each was assigned a quantum number. This number is called the secondary or azimuthal quantum number and is represented by the letter l.

Zeeman, analyzing light from a hydrogen lamp placed inside an intense magnetic field, discovered that certain spectral lines split into a few. This phenomenon is called the Zeeman effect. This effect allowed us to establish that electron orbits have different orientations in space. The quantum number that determines the orientation of orbits in space is called the magnetic quantum number and is represented by m.

In summary, experimental facts require us to modify the Bohr model to show that the electron’s energy state is not only determined by the principal quantum number. To clarify the electron’s energy state, the hydrogen atom requires four parameters: four quantum numbers.