-
Period: 370 to
History of the Atom
-
415
Democritus Atomic Theory
His theory: Matter could not be divided into smaller and smaller pieces forever, eventually the smallest possible piece would be obtained. This piece would be indivisible. He named the smallest piece of matter “atomos,” meaning “not to be cut.” -
Robert Boyle
Boyle's earliest publication was on the physical properties of air, from which he derived his law that the volume of a given amount of a gas varies inversely with pressure. His work in chemistry was aimed at establishing it as a rational theoretical science on the basis of a mechanistic theory of matter. -
Antoine Lavoisier
He was the first person to succeed in determining the composition of water and in synthesizing the compound from its elements. -
Joesph Proust
(Law of definite proportion or Law of constant composition)
A given compound always contain exactly the same proportion of elements by mass. -
Dalton's Atomic Theory
To Dalton :
All matter is made of atoms. Atoms are indivisible and indestructible.
All atoms of a given element are identical in mass and properties.
Compounds are formed by a combination of two or more different kinds of atoms.
A chemical reaction is a rearrangement of atoms.
Atoms can be neither created nor destroyed. -
Thomson's Atomic Model
"Plum Pudding"
Thomson’s atomic theory suggested that the atom is not indivisible as it was of smaller pieces – electrons and protons.
An atom consists of a sphere of positive charge with negatively charged electron embedded in it. The positive and the negative charges in an atom are equal in magnitude, due to which an atom is electrically neutral. It has no over all negative or positive charge. -
Robert Andrew Millikan
His earliest major success was the accurate determination of the charge carried by an electron, using the elegant "falling-drop method"; he also proved that this quantity was a constant for all electrons, thus demonstrating the atomic structure of electricity. -
Rutherford's Atomic Model
"Planetary Model"
He showed that while the nucleus contains virtually all of the mass of the atom, it only takes up one-billionth of the volume of the atom, an inconceivably tiny amount. Much smaller particles - electrons - orbit the nucleus at a great distance. -
Fredrick Soddy
In 1913 with his formulation of the concept of isotopes, which stated that certain elements exist in two or more forms which have different atomic weights but which are indistinguishable chemically. -
Bohr's Atomic Model
Neils Bohr (1885–1962) refined Rutherford's model in 1913 by proposing that electrons: orbit the nucleus without losing energy; could move only in fixed orbits of specific energies. Electrons with low energy would orbit closer to the nucleus while electrons with high energy orbit further from the nucleus. -
Louis de Broglie
Suggested that, like light, electrons could act as both particles and waves. De Broglie's hypothesis was soon confirmed in experiments that showed electron beams could be diffracted or bent as they passed through a slit much like light could. So, the waves produced by an electron confined in its orbit about the nucleus sets up a standing wave of specific wavelength, energy and frequency (i.e., Bohr's energy levels) much like a guitar string sets up a standing wave when plucked. -
Werner Heisenberg
If an electron traveled as a wave, could you locate the precise position of the electron within the wave? He answered no in what he called the uncertainty principle:
To view an electron in its orbit, you must shine a wavelength of light on it that is smaller than the electron's wavelength.
This small wavelength of light has a high energy. The electron will absorb that energy. The absorbed energy will change the electron's position. -
Erwin Schrödinger
He derived a set of equations or wave functions in 1926 for electrons. According to Schrodinger, electrons confined in their orbits would set up standing waves and you could describe only the probability of where an electron could be. The distributions of these probabilities formed regions of space about the nucleus were called orbitals. Orbitals could be described as electron density clouds. The densest area of the cloud is where you have the greatest probability of finding the electron. -
James Chadwick
In 1932, James Chadwick identified the neutron. The particle proposed by Rutherford as having significant mass and no charge. With the discovery of the neutron three subatomic particles were identified that would help explain observations made at the atomic level.
