Bohr Atomo

‪Build an Atom‬ - PhET Interactive Simulations.

In 1913 Neils Bohr proposed his model of atom which superceded Rutherford's atomic model. Though the planetary model proposed by Rutherford was widely accepted, it fell short on many counts. The nuclear atom proposed by Rutherford was unstable. According to classical theories this atom should collapse.
Bohr agreed with Rutherford’s proposal that in the atom the electrons revolve around a central positively charged nucleus that is responsible for most of the weight of the atom. But from his special evidence, he concluded that electrons are found at only certain distances from the.

In 1913, Neils Bohr built on the work of Max Planck and Albert Einstein and proposed that the movement of electrons within an atom was quantized. In other words, Bohr’s work suggested that electrons do not move freely around the atom, but can only occupy specific energy levels within the atom. Evidence that led to this proposal was the observation of line spectra – distinct bands of light emitted by atoms after they were excited by heat or electricity. Bohr’s theory explained that those spectral emission lines were due to the energy released by the relaxation of excited electrons between specific orbitals. This animation allows you to add different amounts of energy to see how the electron in a hydrogen atom responds, and then manually allow that electron to relax to specific orbitals within the atom (note that in nature electrons relax spontaneously and instantaneously). The animation provides data on the energy added and released and the resulting wavelengths of light emitted. The complete line spectra for the transitions represented in this animation are shown below (note, a hydrogen atom has additional lines related to transitions not shown here).

The Bohr
Model

The most important properties of atomic and molecular structure may beexemplified using a simplified picture of an atom that is called the BohrModel. This model was proposed by Niels Bohr in 1915; itis not completely correct, butit has many features that are approximately correctand it is sufficient for much of our discussion. The correct theory of theatom is called quantum mechanics; the Bohr Model is an approximationto quantum mechanics that has the virtue of being much simpler.(Here is a more realistic discussion of what atomic orbitals look like in quantummechanics.)

A Planetary Model of the Atom

The Bohr atom

The Bohr Model is probably familar as the 'planetary model' of the atom illustrated in the adjacent figure that,for example, is used as a symbol for atomic energy (a bit of a misnomer, sincethe energy in 'atomic energy' is actually the energy of the nucleus, ratherthan the entire atom). In the Bohr Model the neutrons and protons (symbolized by red and blue balls in the adjacent image) occupy adense central region called the nucleus, and the electrons orbit the nucleusmuch like planets orbiting the Sun (but the orbits are not confined to a planeas is approximately true in the Solar System). The adjacent image is not toscale since in the realistic casethe radius of the nucleus is about 100,000 times smaller than theradius of the entire atom, and as far as we can tell electrons are pointparticles without a physical extent.

This similarity between a planetary model and the Bohr Model of the atomultimately arises because the attractivegravitational force in a solar systemand the attractiveCoulomb (electrical) force between the positively charged nucleus andthe negatively charged electrons in an atom are mathematically of the same form.(The form is the same, but the intrinsicstrength of the Coulombinteraction is much larger than that of the gravitational interaction; inaddition, there are positive and negative electrical charges so the Coulombinteraction can be either attractive or repulsive, but gravitation is alwaysattractive in our present Universe.)

But the Orbits Are Quantized

Quantized energy levels in hydrogen

Bohr Atom Of Vanadium

The basic feature of quantum mechanics that is incorporated in the Bohr Modeland that is completely different from the analogous planetary model is that theenergy of the particles in the Bohr atom is restricted to certain discretevalues. One says that the energy is quantized. This means that onlycertain orbits with certain radii are allowed; orbits in between simply don'texist.

The adjacent figure shows such quantized energy levels for the hydrogen atom.These levels are labeled by an integer n that is called a quantum number.The lowest energy state is generally termed the ground state. The stateswith successively more energy than the ground state are called the first excitedstate, the second excited state, and so on. Beyond an energycalled the ionization potential the single electron of the hydrogen atom is nolonger bound to the atom. Then the energy levels form a continuum. In the case ofhydrogen, this continuum starts at 13.6 eV above the ground state ('eV' standsfor 'electron-Volt', a common unit of energy in atomic physics).

Although this behavior may seem strange to our minds that are trainedfrom birth bywatchingphenomena in the macroscopic world, this is the way things behave in thestrange world of the quantum that holds sway at the atomic level.

Atomic Excitation and De-excitation

Atoms can make transitions between the orbits allowed by quantum mechanics byabsorbing or emitting exactly the energy difference between the orbits. Thefollowing figure shows an atomic excitation cause by absorption of a photon andan atomic de-excitation caused by emission of a photon.

Bohr Atom Oxygen

Excitation by absorption of light and de-excitation by emission of light

In each case thewavelength of the emitted or absorbed light is exactly such that the photoncarries the energy difference between the two orbits. This energy may be calculated bydividing the product of the Planck constant and the speed of lighthc by the wavelength of the light). Thus, an atom can absorbor emit only certain discrete wavelengths (or equivalently, frequencies orenergies).