# Bohr's Atomic Model

1. Oct 14, 2005

### tiagobt

My physics teacher told me that one of the innacuracies of Bohr's Atomic Model is that the nucleus (positive) would attract the electron (negative) making it move as a spiral until it collapses with the nucleus. What I don't get is why the same thing doesn't happen with a planetary system. How different is magnetic force from gravitational force?

2. Oct 14, 2005

### Staff: Mentor

The reason has to do with the fact that the electron has a charge, and that--according to classical physics--accelerating charges radiate energy. Thus the orbiting electron would quickly lose its energy and spiral into the nucleus.

That doesn't happen to planets.

3. Oct 14, 2005

### pervect

Staff Emeritus
The "spiraling" mechanism involves the emission of electromagnetic waves (for charges), or gravitational waves (for masses) which are necessary to maintain the conservation of momentum.

In theory, the Earth does spiral into the sun via the emission of gravitational radiation - it is just that this is a very weak effect. (So weak that it cannot be directly observed).

Gravity is universally attractive, so there can be no dipole terms to generate gravitational radiation. The first terms which can generate gravitational radiation are quadropole terms. This means that gravitational radiation is surpressed even more than electromagnetic radiation.

See for instance the sci.physics.faq
http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html

4. Oct 15, 2005

### rbj

what about gravitational radiation? isn't there some energy radiated as gravity waves because of the accelerated masses of the planets? (admittedly, the acceleration is far less than that in the Bohr model.)

5. Oct 15, 2005

### Staff: Mentor

Absolutely, as pervect explained. (I ignored such small effects in my response; pervect's post is more complete and accurate.)

6. Oct 17, 2005

### Staff: Mentor

Actually, this is a problem with a predecessor to Bohr's model, not with Bohr's model itself. Before Bohr entered the picture, physicists considered what one might call the "naïve planetary model" of the atom, in which the electrons are classical particles orbiting the nucleus under the influence of the electric force. This model was quickly seen to have the "radiation problem" discussed in this thread.

Bohr's model specifically postulated that atomic electrons are restricted to certain discrete orbits via quantization of orbital angular momentum, and do not undergo the classical electromagnetic radiation process while they are in these orbits.

Bohr's model does make some incorrect predictions, although it gets the basic energy levels of hydrogen right; and it couldn't be extended to multi-electron atoms. People tried to fix it by introducing elliptical orbits and relativistic corrections (try a Google search on "bohr sommerfeld atomic model"), but in the end it was superseded by Schrödinger's quantum mechanics, plus the concept of "spin".

7. Oct 17, 2005

### Mk

The electron is not changing its direction of movement?

8. Oct 17, 2005

### Staff: Mentor

Says who? In the classical picture of an orbiting electron, the electron's direction of motion is constantly changing.