History of the atom

A unit of matter, the smallest unit of an element, consisting of a dense, central, positively charged nucleus surrounded by a system of electrons, equal in number to the number of nuclear protons, centimeter and characteristically remaining undivided in chemical reactions except for limited removal, transfer, or exchange of certain electrons.

John Dalton

 From his experiments and observations, he suggested that atoms were like tiny, hard balls. An element is a substance made from only one type of atom. An element cannot be broken down into any simpler substances. Element had its own atoms that differed from others in mass. Dalton believed that atoms were the fundamental building blocks of nature and could not be split. In chemical reactions, the atoms would rearrange themselves and combine with other atoms in new ways.

J.J. Thomson

At the end of the nineteenth century, a scientist called J.J. Thomson discovered the electron. This is a tiny negatively charged particle that is much, much smaller than any atom.
Thomson proposed a different model for the atom. He said that the tiny negatively charged electrons must be embedded in a cloud of positive charge (after all, atoms themselves carry no overall charge, so the charges must balance out). Thomson imagined the electrons as the bits of plum in a plum pudding.

Ernest Rutherford

In 1911, Ernest Rutherford interpreted these results and suggested a new model for the atom. The positive charge must be concentrated in a tiny volume at the centre of the atom; otherwise the heavy alpha particles fired at the foil could never be repelled back towards their source. On this model, the electrons orbited around the dense nucleus.

Niels Bohr

Bohr suggested that the electrons must be orbiting the nucleus in the centre of an atom, containing protons and neutrons. The energy must be given out when 'excited' electrons fall from a high energy level to a low one.

Bohr and Arnold Sommerfeld

Bohr and a German physicist, Arnold Sommerfeld expanded the original Bohr model to explain these variations. According to the Bohr-Sommerfeld model, not only do electrons travel in certain orbits but the orbits have different shapes and the orbits could tilt in the presence of a magnetic field. Orbits can appear circular or elliptical, and they can even swing back and forth through the nucleus in a straight line.

Wolfgang Pauli

Pauli gave a rule governing the behavior of electrons within the atom that agreed with experiment. If an electron has a certain set of quantum numbers, then no other electron in that atom can have the same set of quantum numbers. Physicists call this "Pauli's exclusion principle." It provides an important principle to this day and has even outlived the Bohr-Sommerfeld model that Pauli designed it for.

Louis de Broglie

In 1924 a Frenchman named Louis de Broglie thought about particles of matter. He thought that if light can exist as both particles and waves, why couldn't atom particles also behave like waves? In a few equations derived from Einstein's famous equation, (E=mc2) he showed what matter waves would behave like if they existed at all. (Experiments later proved him correct.)

 Erwin Schrödinger

In 1926 the Austrian physicist, Erwin Schrödinger had an interesting idea:  His theory worked kind of like harmonic theory for a violin string except that the vibrations traveled in circles. The world of the atom, indeed, began to appear very strange. It proved difficult to form an accurate picture of an atom because nothing in our world really compares with it.

Heisenberg

In 1927 Heisenberg formulated an idea, which agreed with tests, that no experiment can measure the position and momentum of a quantum particle simultaneously. Scientists call this the "Heisenberg uncertainty principle." This implies that as one measures the certainty of the position of a particle, the uncertainty in the momentum gets correspondingly larger. Or, with an accurate momentum measurement, the knowledge about the particle's position gets correspondingly less.

James Chadwick

Not until 1932 did the English physicist James Chadwick finally discover the neutron. He found it to measure slightly heavier than the proton with a mass of 1840 electrons and with no charge (neutral). The proton-neutron together, received the name, "nucleon."

Paul Dirac

In 1928, Paul Dirac produced equations which predicted an unthinkable thing at the time- a positive charged electron. He did not accept his own theory at the time. In 1932 in experiments with cosmic rays, Carl Anderson discovered the anti-electron, which proved Dirac's equations. Physicists call it the positron.