Chemistry
Atoms and Atomic Structure
Niels Bohr

# What is the Bohr model of the atom?

The Bohr model of the atom is based on four fundamental

postulates. Although he did not claim to explain or prove these

postulates, Bohr found that by applying them, he could predict the

properties of single-electron atoms and ions to an amazing degree

of accuracy. Using the Bohr model, it is possible to accurately

predict the energy levels of all one-electron atoms and ions, such

as H, He+, Li2+ Be3+, B4+, C5+, etc.

The four key postulates of Bohr's theory are:

1) Atoms have well-defined electron orbits.

3) Electrons travel in circular orbits with specific angular

momenta, and only certain values are possible (angular momentum is

quantized).

4) As electrons go from one orbit to the next, energy is either

absorbed or released by the atom.

The quantized angular momentum can be written as:

I = n * (h/2*pi)

where I is the angular momentum, n is any integer, h is Planck's

constant, and pi is the number pi, or 3.14159.

See the Related Questions and Web Links to the left for the answer.

### "Overview_of_the_Bohr_Model" id="Overview_of_the_Bohr_Model">Overview of the Bohr Model

Niels Bohr proposed the Bohr Model of the Atom in 1915. Because the

Bohr Model is a modification of the earlier Rutherford Model, some

people call Bohr's Model the Rutherford-Bohr Model. The modern

model of the atom is based on quantum mechanics. The Bohr Model

contains some errors, but it is important because it describes most

of the accepted features of atomic theory without all of the

high-level math of the modern version. Unlike earlier models, the

Bohr Model explains the Rydberg formula for the spectral emission

lines of atomic hydrogen.

The Bohr Model is a planetary model in which the

negatively-charged electrons orbit a small, positively-charged

nucleus similar to the planets orbiting the Sun (except that the

orbits are not planar). The gravitational force of the solar system

is mathematically akin to the Coulomb (electrical) force between

the positively-charged nucleus and the negatively-charged

electrons.

### "Main_Points_of_the_Bohr_Model" id="Main_Points_of_the_Bohr_Model">Main Points of the BohrModel

• Electrons orbit the nucleus in orbits that have a set size and

energy.

• The energy of the orbit is related to its size. The lowest

energy is found in the smallest orbit.

• Radiation is absorbed or emitted when an electron moves from

one orbit to another.

### "Bohr_Model_of_Hydrogen" id="Bohr_Model_of_Hydrogen">Bohr Model ofHydrogen

The simplest example of the Bohr Model is for the hydrogen atom (Z

= 1) or for a hydrogen-like ion (Z > 1), in which a

negatively-charged electron orbits a small positively-charged

nucleus. Electromagnetic energy will be absorbed or emitted if an

electron moves from one orbit to another. Only certain electron

orbits are permitted. The radius of the possible orbits increases

as n2, where n is the principal quantum number. The 3 → 2

transition produces the first line of the Balmer series. For

hydrogen (Z = 1) this produces a photon having wavelength 656 nm

(red light).

### "Problems_with_the_Bohr_Model" id="Problems_with_the_Bohr_Model">Problems with the BohrModel

• It violates the Heisenberg Uncertainty Principle because it

considers electrons to have both a known radius and orbit.

• The Bohr Model provides an incorrect value for the ground state

orbital angular momentum.

• It makes poor predictions regarding the spectra of larger

atoms.

• It does not predict the relative intensities of spectral

lines.

• The Bohr Model does not explain fine structure and hyperfine

structure in spectral lines.

• It does not explain the Zeeman Effect.

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