Earth Science: Matter – Atoms: the Building Blocks of Nature
The first step in the development of modern chemistry was the recognition that atoms existed. Today it is known that every element consists of a specific kind of atom.
At the start of the 19th century, John Dalton. a science teacher from Manchester, England, promoted the theory that all matter is composed of indivisible atoms. The atoms of any element are identical in their mass and chemical makeup. In the years that followed, atoms were assumed by scientists to be round elastic objects that were uniformly filled with matter.
But later in that century this view was definitively refuted when physicist J. J. Thomson showed that negatively charged particles, electrons, can be separated from their atoms. Furthermore, Antoine Henri Becquerel (1852-1908) was actually able to observe natural radioactivity. Through these discoveries physicists began to understand that the atom could be split.
Mainly empty space
At the start of the 20th century, Ernest Rutherford bombarded a thin sheet of gold leaf with alpha particles emitted from a radioactive element. Almost all of the particles penetrated the gold leaf with no deflection, but a few were deflected and some even bounced back. “It was unbelievable, almost as if you were to fire a 38- cm artillery shell at a piece of tissue paper and have it come back and hit you,” wrote Rutherford later.
He concluded that the atoms in the gold leaf consisted mostly of empty space, but at the same time there was a mass in the center of these atoms that could deflect the incoming particles. This led Rutherford to the idea that atoms are constructed of a positively charged core and a negatively charged shell. This idea was further developed by Niels Bohr and others.
According to them, the shell of an atom consists of electrons that circle a nucleus, which is made up of protons and neutrons. Electrons move in fixed orbits, much like the planets. The Bohr atom model is based on a bold hypothesis. From the perspective of classical physics the electrons that orbit the nucleus of the atom would release their energy in the form of radiation.
If this were true the electrons would eventually crash into the nucleus of the atom. The physicist Niels Bohr advanced the proposition that this is exactly what electrons do not do. Starting from this assumption, he developed his own generalized and idealized model of the atom. Using this model, Bohr could explain the wavelength of the light radiated by hydrogen atoms that were in an excited state.
WAVE MECHANICS AND ORBITALS
A problem for physicists from Bohr to Erwin S.Schrddinger was that it seemed that for half the time they would be applying the classical laws of physics, and the other half they would be applying the laws of quantum physics.
Today, quantum wave mechanicsposits that the position and momentum of an electron cannot be simultaneously determined exactly. From this perspective there are no more electron orbits. Instead, the movement of electrons is described and determined by a mathematical function: the wave function.
The wave function value yields the probability of an electron being present within a given small volume. The region in which an electron may be found around an atom has a characteristic shape; this region is called an orbital.