Why, in an atom, does the negatively charged electron not collapse into the positively charged nucleus? Is this in any way similar to the reason why large systems like stars and planets do not collapse into each other under the pull of gravity?
鈥� When Ernest Rutherford, the New Zealand founder of nuclear physics, first discovered the atomic nucleus, he did indeed propose that electrons did not fall toward the nucleus of the atom because the attractive forces of the nucleus were being balanced by the orbital velocity of the electron in much the same way as a planet orbiting a star.
However, the Danish physicist Niels Bohr modified this theory after Albert Einstein and Max Planck found that energy could only exist in certain discrete amounts, or quanta. This meant that electrons could be seen to have both wave and particle properties, and required that the circumference of the orbit of an electron could not be zero. This means, of course, it could never reach the nucleus.
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We have since adopted the model proposed by the Austrian theoretical physicist Erwin Schr枚dinger. Instead of orbiting the nucleus like planets, his model has electrons occupying 鈥渃louds鈥� where it is statistically probable that they will exist, although we may never determine an electron鈥檚 position and velocity at the same time.
鈥淚nstead of orbiting the nucleus like planets, Schr枚dinger鈥檚 model has electrons occupying clouds where it is probable that they will exist鈥�
Michael Ludlam, Sheffield, South Yorkshire, UK
鈥� Niels Bohr asked this very question in 1913. The atom was known to have a small heavy nucleus, and the much lighter electrons were thought to orbit like planets around the sun. As long as a planet does not lose energy, it can continue its orbit indefinitely.
According to the laws of electromagnetism, charged particles moving in a circle ought to radiate energy as waves. Bohr calculated that a hydrogen atom should collapse with a flash of light in a matter of femtoseconds. Because this does not happen, he proposed what has become known as the 鈥渙ld鈥� quantum mechanics. It asserted that the electron鈥檚 angular momentum had to be a multiple of Planck鈥檚 constant.
鈥淚n modern quantum theory, an electron has a wave character, and a stable atom can be thought of as a box confining the wave鈥�
The rule meant that electrons could only occupy particular orbits, and there was a minimum size of orbit. Using this, Bohr was able to predict the entire spectrum of excited states of hydrogen, which was a quite astounding achievement.
But Bohr鈥檚 theory was hard to apply to more complex atoms and was superseded by Erwin Schr枚dinger鈥檚 wave mechanics in 1927, which is the start of modern quantum theory.
Schr枚dinger鈥檚 formulation shows an electron has a wave character, and a stable atom can be thought of as a box confining the wave. An electron has a wavelength equal to Planck鈥檚 constant divided by its momentum, so the faster an electron moves, the shorter its wavelength. To confine the electron near the nucleus the electron must move very quickly.
But conversely a fast-moving electron can escape the pull of the nucleus. So you can think of the size of an atom as resulting from a compromise between the electrons having enough kinetic energy so their waves will fit in the box, but not so much that they can escape.
David Barnett, Institute for Advanced Physics London, UK
鈥� As Niels Bohr realised in 1913, the electron just doesn鈥檛.
And no, large solar systems don鈥檛 not collapse for quantum-mechanical reasons. They don鈥檛 collapse because the planets鈥� velocities keep them in free fall.
Jay M. Pasachoff, Field Memorial Professor of Astronomy, Williams College, Williamstown, Massachusetts, US