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From Wikipedia

Bohr model

In atomic physics, the Bohr model, devised by Niels Bohr, depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus—similar in structure to the solar system, but with electrostatic forces providing attraction, rather than gravity. This was an improvement on the earlier cubic model (1902), the plum-pudding model (1904), the Saturnian model (1904), and the Rutherford model (1911). Since the Bohr model is a quantum physics-based modification of the Rutherford model, many sources combine the two, referring to the Rutherford–Bohr model.

Introduced by Niels Bohr in 1913, the model's key success lay in explaining the Rydberg formula for the spectral emission lines of atomic hydrogen. While the Rydberg formula had been known experimentally, it did not gain a theoretical underpinning until the Bohr model was introduced. Not only did the Bohr model explain the reason for the structure of the Rydberg formula, it also provided a justification for its empirical results in terms of fundamental physical constants.

The Bohr model is a primitive model of the hydrogen atom. As a theory, it can be derived as a first-order approximation of the hydrogen atom using the broader and much more accurate quantum mechanics, and thus may be considered to be an obsolete scientific theory. However, because of its simplicity, and its correct results for selected systems (see below for application), the Bohr model is still commonly taught to introduce students to quantum mechanics, before moving on to the more accurate but more complex valence shell atom. A related model was originally proposed by Arthur Erich Haas in 1910, but was rejected. The quantum theory of the period between Planck's discovery of the quantum (1900) and the advent of a full-blown quantum mechanics (1925) is often referred to as the old quantum theory.

Origin

In the early 20th century, experiments by Ernest Rutherford established that atoms consisted of a diffuse cloud of negatively charged electrons surrounding a small, dense, positively charged nucleus. Given this experimental data, Rutherford naturally considered a planetary-model atom, the Rutherford model of 1911 – electrons orbiting a solar nucleus – however, said planetary-model atom has a technical difficulty. The laws of classical mechanics (i.e. the Larmor formula), predict that the electron will release electromagnetic radiation while orbiting a nucleus. Because the electron would lose energy, it would gradually spiral inwards, collapsing into the nucleus. This atom model is disastrous, because it predicts that all atoms are unstable.

Also, as the electron spirals inward, the emission would gradually increase in frequency as the orbit got smaller and faster. This would produce a continuous smear, in frequency, of electromagnetic radiation. However, late 19th century experiments with electric discharges through various low-pressure gases in evacuated glass tubes had shown that atoms will only emit light (that is, electromagnetic radiation) at certain discrete frequencies.

To overcome this difficulty, Niels Bohr proposed, in 1913, what is now called the Bohr model of the atom. He suggested that electrons could only have certain classical motions:

  1. The electrons can only travel in special orbits: at a certain discrete set of distances from the nucleus with specific energies.
  2. The electrons of an atom revolve around the nucleus in orbits. These orbits are associated with definite energies and are also called energy shells or energy levels. Thus, the electrons do not continuously lose energy as they travel in a particular orbit. They can only gain and lose energy by jumping from one allowed orbit to another, absorbing or emitting electromagnetic radiation with a frequency ν determined by the energy difference of the levels according to the Planck relation:\Delta{E} = E_2-E_1=h\nu \ , where h is Planck's constant.
  3. The frequency of the radiation emitted at an orbit of period T is as it would be in classical mechanics; it is the reciprocal of the classical orbit period: \nu = {1\over T}

The significance of the Bohr model is that the laws of classical mechanics apply to the motion of the electron about the nucleus only when restricted by a quantum rule. Although rule 3 is not completely well defined for small orbits, because the emission process involves two orbits with two different periods, Bohr could determine the energy spacing between levels using rule 3 and come to an exactly correct quantum rule: the angular momentum L is restricted to be an integer multiple of a fixed unit:

L = n{h \over 2\pi} = n\hbar

where n = 1, 2, 3, ... is called the principal quantum number, and ħ = h/2Ï€. The lowest value of n is 1; this gives a smallest possible orbital radius of 0.0529 nm known as the Bohr radius. Once an electron is in this lowest orbit, it can get no closer to the proton. Starting from the angular momentum quantum rule Bohr was a


From Yahoo Answers

Question:I am doing an assignment that asks for a bohr model of boron.. however, it is an ION of boron, not a regular atom, so I must deal with valence electrons etc. I know that boron has three valence electrons but I want to know how many should be in the other energy levels, since the ion will have 8 electrons not 5, like a usual atom of boron. Many thanks to whoever answers!!

Answers:The regular boron atom has 2 electrons in the first shell and 3 in the next (outer/valence shell). Boron is a nonmetal, so it will always add electrons to become an ion. It will add 5 electrons in the valence shell, so the boron ion has 2 in the inner shell and 8 in the outer shell.

Question:i have to make a bohr model of the element boron for science. any ideas [not 3-D]

Answers:i should be a neucleus in the center with the number of protons and neutrons u have written. then in the first energy level u should have 2 electrons with three in the next energy level after that.

Question:Help!!!!! I dont know where to find this!!! I need to know ASAP. best answer= 10 pts!!! answer like this "1. P=1 N=3 E=9" obviously thats not the right answer.... but it was just an example. PLEASE answer! Its super important D: ALSOOO. IF YOU HAVE A LINK TO A WEBSITE WITH A BOHR MODEL OF ELEMENTS 1-20 THAT WOULD BE AWESOME<3

Answers:Just look at a periodic table The atomic number = number of protons The mass number = number of protons + number of neutrons So number of neutrons = mass number - atomic number and for a neutral atom, number of electrons = number of protons For the Bohr atom stuff... just do what I'd do ... google it

Question:I need to make a bohr model diagram for chemistry. Not sure how to set it up.I know Megnesium has 12 protons, 12 electrons and 12 neutrons. Do i make the nucleus with the 12 protons and neutrons combined?

Answers:yes, protons and neutrons are found in the nucleus, the electrons are in various energy levels outside the nucleus. magnesium has 3 energy levels: 2 electrons in the 1st level, 8 electrons in the 2nd and 2 electrons in the 3rd level. in the nucleus you would show the 12 neutrons and 12 protons Here is a link to a picture of a bohr model of magnesium: http://www.cartage.org.lb/en/themes/sciences/chemistry/generalchemistry/Elements/onlibeinteractive/Magnesium/b0012.gif

From Youtube

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