Just over one hundred different kinds of atoms are known.
Ninety two of these elements are naturally occurring (i.e. they are found on the earth), and the remainder have been made artificially in nuclear reactors.
One amu. is 1.66*10-27kg.
This indicates that chlorine is a mixture of these two isotopes, and that the isotope with mass number 35 is the more abundant atom present.
Indeed, the isotope of mass number 35 is approximately three times more abundant than the isotope of mass number 37.
To explain why the electrons do not loose energy continuously Niels Bohr suggested that the electrons are confined to specific shells which have fixed energy levels. However, because of the Heisenberg Uncertainty Principle, the locations of these shells about a nucleus cannot be rigidly defined. Thus, the atomic orbitals are diffuse regions in space around the nucleus of an atom in which the electrons are most likely to be found.
The exact nature of each atomic orbital is described in detail its set of Quantum Numbers, each of which describes a different aspect of the shell.
It is now known that these atomic particles are themselves composed of sub-atomic particles (i.e. the quarks and leptons). These sub-atomic particles are also called the fundamental particles or elementary particles.
However, this further detail of the structure of the nucleus of the atom has no chemical consequences and will not be considered further in this text.
When an electron drops from an orbital of high energy to one of lower energy, a photon (i.e. a packet of light) is emitted corresponding to the energy difference between the orbitals.
In 1803AD, John Dalton proposed his atomic theory in which he held that the atom was simple, solid and impenetrable. All elements were made up of these minute particles
Ninety years later, Pierre and Marie Curie were to discover and isolate radium, a new element which spontaneously disintegrated into other elements. This proved that the atoms of one element at least were not indivisible.
From about 1850AD onwards, the study of electric discharge in high vacuum by Crookes, Goldstein and others, which was made possible by Callan's work at Maynooth, led to the discovery of the particles which Stoney named Electrons. These electrons emanated from any metal used as a cathode in a discharge tube and for this reason were called rays. Thus, the electron was established as a fundamental particle of matter.
By convention the charge on the electron was considered negative and in 1909AD,R A Millikan showed by his classic oil-drop experiment that the electron possessed definite charge and mass.
Since the atom was electrically neutral, the existence of negative charge on the electron mandated the existence of an equal amount of positive charge. The problem arose as to the distribution of charge throughout the atom. J JThompson had proposed that the negative electrons were embedded in the atom at random.
In 1911AD, Sir Ernst Rutherford used a radioactive source to bombard a thin gold leaf with alpha particles, which are positively charged (i.e. helium atoms without electrons). Most of these alpha-particles passed through the thin gold leaf undeflected, some were deflected outward and a few were deflected back along their own path. He concluded that the positive charge was concentrated in a small region, called the nucleus, round which the electrons revolved. Those alpha particles which were deflected through large angles had come close to nucleus and were repelled by its concentrated positive charge. Rutherford's model of the atom contained a revolving electron which according to classical physics should lose energy continuously and spiral into the nucleus.
Neils Bohr discovered the way in which the electrons travel about the nucleus. He suggested that an electron could revolve in only certain allowed orbits. As long as an electron remained in an allowed orbit, it does not lose or gain energy. The Bohr model was satisfactory in explaining the behaviour of the electron in the hydrogen atom, but failed with multi-electron elements.
Rutherford suggested that the proton was a second fundamental sub-atomic particle in the nucleus, and he hinted that a neutral particle might also exist.
In 1932AD, Chadwick discovered that second kind of particle exists in the nucleus, which has no electrical charge. He named these particles Neutrons.