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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:

Answers:The Bohr model puts electrons in circular orbits around the nucleus that correspond only to the principle quantum #. It does not distinguish the subshells (s, p, d, etc). Carbon is 1s2 2s2 2p2 So in the Bohr model there would be two concentric circular orbits for n = 1 and n = 2 shells. The smaller n = 1 orbit would have two electrons and the larger n = 2 orbit 4 electrons. oxygen is 1s2 2s2 2p4 again two orbits; n = 1 with 2 electrons and n = 2 with 6 Oxygen has the greater electronegativity because it has a greater effective nuclear charge (Zeff)

Question:I need to draw an atom (bohr model) of oxygen

Answers:On this website: http://library.thinkquest.org/10676/Period/o.html You will find this photo labled Bohr Model of Oxygen: http://library.thinkquest.org/10676/Period/b0008.gif Here is another reference: http://www.chemicalelements.com/elements/o.html I hope that helps.

Question:What are Neils Bohr's theoretical connections between the energy level transitions of an electron and the specific color of a photon in the hydrogen line spectrum? Please explain the advantages and disadvantages in the modern quantum mechanical model over the Bohr model. Use the following concepts in your answer: Heisenberg Uncertainty Principle, quantum numbers, orbital, sublevel, Hund's Rule, Pauli Exclusion Principle. There is a greater variation between the properties (both chemical and physical) of the first and second of a group or family in the periodic table than between the properties of the second and third members of the group. Consider as examples either the group containing nitrogen or the one containing oxygen. Select three properties and discuss the variation of these properties to illustrate the generalization expressed in the first sentence of the question.

Answers:I will give concepts in outline form. Anything more would be doing your homework (or taking an exam) for you. Dalton's model had atoms like little balls. Every element had a different kind of ball (atom). Neils Bohr's experiments with cathode rays showed that atoms had parts. He used the planetary system as a model. Heavy nucleus in the center and light electrons in orbits around this nucleus. We still call the energy levels orbits (although some use the term shell instead). For Hydrogen, a single proton is the nucleus, orbited by a single electron. If the Hydrogen atom is not interacting with the environment, the electron is in the lowest possible energy level, called the ground state. The Hydrogen atom can only absorb specific amounts of energy. This energy excites the electron into a higher or more energetic level. This is called the excited state. The excited electron can only emit specific amounts of energy to return to the ground state. The energy of a photon is related to the wavelength of light: E = hc / lambda where lambda is the wavelength. Different wavelengths of light appear as different colors. Thus specific amounts of energy means specific wavelengths of light which means specific colors in the spectrum. Your second question requires an entire text book chapter to answer. The Bohr model is too simplistic (unrealistic) to be able to predict the behavior of subatomic pieces (they are not classical particles). Electrons are not like little round balls. They are clouds of location probability (the Heisenberg Uncertainty fits in here). The shapes of the clouds are determined mathematically by considering the quantum numbers associated with the specific electrons involved. The advantages include a better predictive model and a clean transition into chemical bonding based upon electron behavior. The disadvantages include mathematics beyond most humans, even with assumptions, The model also leaves common sense behind (how can a p-orbital electron be on one side of the nucleus or the other without ever passing through the nucleus?). The principal quantum number determines the shell (energy level) and limits the number of subshells (orbitals) involved. How these subshells are populated is called the Aufbau rules which includes Hund's Rule and the Pauli Exclusion Principle (see the second site below). The last part of your question is beyond the scope of this format. What you need to do is work on your answer and then post more specific questions on the details. I can tell you that part of the answer is determined by relative size and energy of the shells involved. In your examples, Nitrogen and Oxygen are diatomic gases, while Phosphorus & Arsenic and Sulphur & Selenium are all solids.

Question:Is a Bohr model simply two dimensional like this? http://www.cartage.org.lb/en/themes/sciences/chemistry/generalchemistry/elements/onlibeinteractive/Oxygen/b0008.gif Or is a Bohr model like this. Or is this a Sommerfeld model? http://www.humanthermodynamics.com/Atom_diagram.jpg I need help on which one to do for a Chemistry project I wanted to do a 3D model like the second one but my write up has to be on Bohr I was just confused on if the second model was a Bohr model or a Sommerfeld model. Please Help!!!!!

Answers:difference between Bohr orbit and Sommerfeld is the shape of the orbits, Bohr's is circular and sommerfeld elliptical.. the second pic is the rutherford model.. Bohr atom looks like the one in 1st pic.. but it's suppose to be in 3D.

From Youtube

BOHRS MODEL FOR HYDROGEN ATOM :Description of Bohr's model of Hydrogen Atom

Bohr's Model of Atom :Check us out at www.tutorvista.com 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 RutherfordBohr 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 ...