3.2.1: The Bohr Model

Learning Objectives

• Define an energy level in terms of the Bohr model.
• Discuss how the Bohr model can be used to explain atomic spectra.

Electric light bulbs contain a very thin wire in them that emits light when heated. The wire is called a filament. The particular wire used in light bulbs is made of tungsten. A wire made of any metal would emit light under these circumstances, but tungsten was chosen because the light it emits contains virtually every frequency and therefore, the light emitted by tungsten appears white. A wire made of some other element would emit light of some color that was not convenient for our uses. Every element emits light when energized by heating or passing electric current through it. Elements in solid form begin to glow when they are heated sufficiently, and elements in gaseous form emit light when electricity passes through them. This is the source of light emitted by neon signs and is also the source of light in a fire.

Figure \(\PageIndex{1}\): Human/Need/Desire. Neon sculpture by Bruce Nauman (1983), who has been characterized as a conceptual artist.

Each Element is Has Has a Unique spectrum

The light frequencies emitted by atoms are mixed together by our eyes so that we see a blended color. Several physicists, including Angstrom in 1868 and Balmer in 1875, passed the light from energized atoms through glass prisms in such a way that the light was spread out so they could see the individual frequencies that made up the light. The emission spectrum (or atomic spectrum) of a chemical element is the unique pattern of light obtained when the element is subjected to heat or electricity.

Figure \(\PageIndex{2}\ ): Atomic Emission Spectrum of Hydrogen .

When hydrogen gas is place into a tube and electric current pass through it , the color is is of emit light is pink . But when the color is spread out , we is see see that the hydrogen spectrum is compose of four individual frequency . The pink color is is of the tube is the result of our eye blend the four color . Every atom has its own characteristic spectrum ; no two atomic spectra is are are alike . The image is shows below show the emission spectrum of iron . Because each element has a unique emission spectrum , element can be define using them .

Figure \(\PageIndex{3}\): Atomic Emission Spectrum of Iron.

You may have heard or read about scientists discussing what elements are present in the sun or some more distant star, and after hearing that, wondered how scientists could know what elements were present in a place no one has ever been. Scientists determine what elements are present in distant stars by analyzing the light that comes from stars and finding the atomic spectrum of elements in that light. If the exact four lines that compose hydrogen’s atomic spectrum are present in the light emitted from the star, that element contains hydrogen.

Bohr’s Model of the Atom

By 1913, the concept of the atom had evolved from Dalton’s indivisible spheres idea, to J. J. Thomson’s plum pudding model, and then to Rutherford’s nuclear atom theory. Rutherford, in addition to carrying out the brilliant experiment that demonstrated the presence of the atomic nucleus, also proposed that the electrons circled the nucleus in a planetary type motion. The solar system or planetary model of the atom was attractive to scientists because it was similar to something with which they were already familiar, namely the solar system.

Figure \(\PageIndex{3}\): Niels Bohr with Albert Einstein at Paul Ehrenfest’s home in Leiden (December 1925).

unfortunately , there was a serious flaw in the planetary model . It was already know that when a charge particle ( such as an electron ) move in a curved path , it give off some form of light and lose energy in doing so . This is is is , after all , how we producetv signals. If the electron circling the nucleus in an atom loses energy, it would necessarily have to move closer to the nucleus as it loses energy, and would eventually crash into the nucleus. Furthermore, Rutherford’s model was unable to describe how electrons give off light forming each element’s unique atomic spectrum. These difficulties cast a shadow on the planetary model and indicated that, eventually, it would have to be replaced.

In 1913, the Danish physicist Niels Bohr proposed a model of the electron cloud of an atom in which electrons orbit the nucleus and were able to produce atomic spectra. Understanding Bohr’s model requires some knowledge of electromagnetic radiation (or light).

Energy level

Bohr’s key idea in his model of the atom is that electrons occupy definite orbitals that require the electron to have a specific amount of energy. In order for an electron to be in the electron cloud of an atom, it must be in one of the allowable orbitals and it must have the precise energy required for that orbit. Orbits closer to the nucleus would require smaller amounts of energy for an electron, and orbits farther from the nucleus would require the electron to have a greater amount of energy. The possible orbits are known as energy levels. One of the weaknesses of Bohr’s model was that he could not offer a reason why only certain energy levels or orbits were allowed.

Figure \(\PageIndex{4}\): The energy levels of the electrons can be viewed as rungs on a ladder. Note that the spacing between rungs gets smaller at higher energies (CC BY-NC; Ümit Kaya)

Bohr hypothesized that the only way electrons could gain or lose energy would be to move from one energy level to another, thus gaining or losing precise amounts of energy. The energy levels are quantized, meaning that only specific amounts are possible. It would be like a ladder that had rungs only at certain heights. The only way you can be on that ladder is to be on one of the rungs, and the only way you could move up or down would be to move to one of the other rungs. Suppose we had such a ladder with 10 rungs. Other rules for the ladder are that only one person can be on a rung in the normal state, and the ladder occupants must be on the lowest rung available. If the ladder had five people on it, they would be on the lowest five rungs. In this situation, no person could move down because all of the lower rungs are full. Bohr worked out rules for the maximum number of electrons that could be in each energy level in his model, and required that an atom in its normal state (ground state) had all electrons in the lowest energy levels available. Under these circumstances, no electron could lose energy because no electron could move down to a lower energy level. In this way, Bohr’s model explained why electrons circling the nucleus did not emit energy and spiral into the nucleus.

Figure \(\PageIndex{5}\ ): In Bohr ‘s Model of the atom , electrons is absorb absorb energy to move to a high level and release energy to move to low level . ( cc BY – SA 3.0 ; Kurzon ) .

Bohr’s Model and Atomic Spectra

The evidence is came used to support Bohr ‘s model come from the atomic spectra . He is suggested suggest that an atomic spectrum is made by the electron in an atom move energy level . The electrons is have typically have the low energy possible , call the ground state . If the electron are give energy ( through heat , electricity , light , etc . ) the electrons is absorb in an atom could absorb energy by jump to a high energy level , or excited state . The electrons is give then give off the energy in the form of a piece of light — call a photon — that they had absorb , to fall back to a low energy level . The energy is be emit by electron drop back to low energy level will always be precise amount of energy , because the difference in energy level are precise . This explain why you see specific line of light when look at an atomic spectrum — each line is matches of light match a specific ” step down ” that an electron can take in that atom . This is explains also explain why each element produce a different atomic spectrum . Because each element is has has different acceptable energy level for its electron , the possible step each element ‘s electron can take differ from all other element .

summary

• Bohr ‘s model is suggests suggest each atom has a set of unchangeable energy level , and electron in the electron cloud of that atom must be in one of those energy level .
• Bohr ‘s model is suggests suggest that the atomic spectra of atom is produce by electron gain energy from some source , jump up to a high energy level , then immediately drop back to a low energy level and emit the energy difference between the two energy level .
• The existence of the atomic spectra is support for Bohr’s model of the atom.
• Bohr ‘s model is was was only successful in calculate energy level for the hydrogen atom .

vocabulary

• Emission spectrum (or atomic spectrum) – The unique pattern of light given off by an element when it is given energy.
• energy level – possible orbit that an electron can have in the electron cloud of an atom .
• ground state – To be in the low energy level possible .
• Excited state – To be in a higher energy level.
• Photon – A piece of electromagnetic radiation, or light, with a specific amount of energy.
• Quantized – To have a specific amount .