When electricity is applied to a sealed glass tube containing a gas at low pressure the tubes appears to glow.

1870s, William Crookes observed that cathode rays are attracted by a magnetic field.

1897, Thomson demonstrated that cathode rays are deflected by an electric field as well.

1886, Goldstein discovered positive rays using cathode-ray tubes with small hold in the cathode.

Thomson Model of the Atom

In 1903, J. J. Thomson proposed a subatomic model of the atom. The model pictured a positively-charged atom containing negatively-charged electrons. Thomson visualized electrons in homogeneous spheres of positive chare in a way that was analogous to raisins in English plum pudding Thus, the Thomson proposal became popularly know as the plum pudding model of the atom.

This model was not correct, but it was consistent with the evidence at the time.

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Rutherford Model of the Atom

In 1911, Rutherford proposed a new model of the atom. He suggested that negatively-charged electrons were distributed about a positively-charged nucleus. Rutherford was able to estimate the size of the atom and its nucleus. He calculated that an atom has a diameter of about 1x 10^-8 cm, and the nucleus has a diameter of 1x10^-13 cm.

 

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Atomic Mass of an Element

The atomic mass of an element is the weighted average mass of all naturally occurring isotopes.

Isotope Mass Abundance

12C 12.000 amu 98.89%

13C 13.003 amu 1.11%

12C 12.000 amu x 0.9889 = 11.87 amu

13C 13.003 amu x 0.0111 = 0.144 amu

1900, Max Planck proposed that radiant energy is not continuous, but rather, the radiation is emitted in small bundles.

The idea that energy is released in discrete unit is referred to as the quantum concept.

1913, Bohr speculated that electrons orbit around the atomic nucleus just as planets circle around the Sun.

He suggested that the electron orbits were at a fixed distance from the nucleus and had a definite energy.

The electron was said to travel in a fixed-energy orbit that was referred to as an energy level.

The collection of narrow bands of light energy is referred to as an emission line spectrum, and the individual bands of light are called spectral lines.

The concept of electron energy levels is supported by spectral lines.

The study of emission spectra revealed that each element produced a unique set of spectral lines.

This observation indicated that the energy levels must be unique for atoms of each element. A line spectrum is used as an "atomic fingerprint."

The model proposed by Niels Bohr was supported experimentally by the emission spectrum of hydrogen.

The emission spectrum of other elements besides hydrogen had far too many lines to interpret.

The model that eventually emerged had electrons occupying a energy sub-level with a main energy level. These energy sub-levels were designated s, p, d, and f in reference to the sharp, principal, diffuse, and fine lines in the emission spectra of the elements.

S sublevel = 2 e-

p sublevel = 6 e-

d sublevel = 10 e-

In 1920 Werner Heisenberg suggested the uncertainty principle; that is, it is impossible to simultaneously know the precise location and energy of an electron. Instead, the energy of an electron can only be known in terms of its probability of being located somewhere within the atom. This description gave rise to the quantum mechanical atom. A location within the atom where there is a high probability of finding a electron having a certain energy is called an orbital.