Questions of Science

Intro: 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?

When Ernest Rutherford, the New Zealand-born 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.

We have since adopted the model proposed by the Austrian theoretical physicist Erwin Schrödinger. Instead of orbiting the nucleus like planets, his model has electrons occupying ‘clouds’ where it is statistically probable that they will exist, although we may never determine an electron’s position and velocity at the same time.

Niels Bohr’s questioning in 1913 deserves further explanation. The atom was known to have a small heavy nucleus, and the much lighter electrons were thought to orbit it 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 ‘old’ quantum mechanics. It asserted that the electron’s angular momentum had to be a multiple of Planck’s constant.

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But Bohr’s theory was hard to apply to more complex atoms and was superseded by Erwin Schrödinger’s wave mechanics in 1927, which is the start of modern quantum theory.

Schrödinger’s formulation shows that 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’s 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.

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 for their waves to fit in the box, but not so much that they can escape.

Large solar systems don’t collapse for quantum-mechanical reasons. They don’t collapse because the planets’ velocities keep them in freefall.

Science-in-motion: a series of short articles following topics in science.

. Atomic structure

Atoms consist of a tiny dense nucleus containing positively charged protons and uncharged neutrons, surrounded by clouds of electrons. Because protons and neutrons are much heavier than electrons, most of an atom’s mass resides in the central nucleus.

Each chemical element has a unique number of protons in its nucleus, its ‘atomic number’. For instance, the element carbon with six protons has the atomic number six. However, single elements can have different numbers of neutrons in the nucleus. For example, carbon has three naturally occurring ‘isotopes’ with six, seven or eight neutrons. The sum of protons and neutrons in an atom’s nucleus is called the atomic mass number.

Normally, the net electric charge of an atom is zero because the number of electrons is the same as the number of protons, and their equal and opposite electric charges cancel out. However, it’s possible to knock electrons out of atoms or add extra ones to create positively or negatively charged particles known as ‘ions’.

. Negatively charged electrons in orbit around the nucleus . Nucleus containing positively charged protons and neutrons