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Bohr model
Excerpted from Wikipedia, on July 8, 2010
The Bohr model of the hydrogen atom (Z = 1) or
a hydrogen-like ion (Z > 1), where the negatively
charged electron confined to an atomic shell
encircles a small, positively charged atomic
nucleus and where an electron jump between
orbits is accompanied by an emitted or absorbed
amount of electromagnetic energy (hν). The
orbits in which the electron may travel are shown
as grey circles; their radius increases as n2, where
n is the principal quantum number.
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.
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. It predicts 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 do not continuously lose energy as they travel. They can only gain
and lose energy by jumping from one allowed orbit to another, absorbing or
emitting electromagnetic radiation.
Other points are:
1. Like Einstein's theory of the Photoelectric effect, Bohr's theory assumes that
during a quantum jump a discrete amount of energy is radiated.
2. The Bohr model gives almost exact results only for a system where two charged
points orbit each other at speeds much less than that of light. This includes oneelectron systems such as the hydrogen atom, singly-ionized helium, doubly
ionized lithium, Classical mechanics
Shell model of the atom
Bohr extended the model of Hydrogen to give an approximate model for heavier atoms.
This gave a physical picture which reproduced many known atomic properties for the
first time.
Heavier atoms have more protons in the nucleus, and more electrons to cancel the charge.
Bohr's idea was that each discrete orbit could only hold a certain number of electrons.
After that orbit is full, the next level would have to be used. This gives the atom a shell
structure, in which each shell corresponds to a Bohr orbit.
This model is even more approximate than the model of hydrogen, because it treats the
electrons in each shell as non-interacting. But the repulsions of electrons are taken into
account somewhat by the phenomenon of screening. The electrons in outer orbits do not
only orbit the nucleus, but they also orbit the inner electrons, so the effective charge Z
that they feel is reduced by the number of the electrons in the inner orbit.
The shell model was able to qualitatively explain many of the mysterious properties of
atoms which became codified in the late 19th century in the periodic table of the
elements. One property was the size of atoms, which could be determined approximately
by measuring the viscosity of gases and density of pure crystalline solids. Atoms tend to
get smaller toward the right in the periodic table, and become much larger at the next line
of the table. Atoms to the right of the table tend to gain electrons, while atoms to the left
tend to lose them. Every element on the last column of the table is chemically inert (noble
gas).