Chapter 4: Atomic Structure-A beginning

Electron

This was the first atomic particle discovered by J.J. Thomson in 1897. He characterized the properties of cathode rays, as a stream of negatively charged particles or electrons. Thomson found the particle to be negatively charged. He was also able to measure the charge-to-mass ratio of the cathode rays. The value he obtained was independent of the gas used in the cathode ray tube.

It was not until 12 years later that Robert Millikan was able to determine the charge of an electron. He experimentally measured a value of -1.6022 x 10-19 Coulombs. Using Thomson's charge-to-mass ratio the mass of an electron had a value of 9.109 x 10-31 kg.

Proton

Thomson experimentally determined the existence of positively charged particles in the cathode ray tube, but he was unable to characterize these particles further. In 1919 Ernest Rutherford characterized the proton as a particle with a charge equal in magnitude to that of the electron but with the opposite sign. The mass was measured as 1.673 x 10-27 kg.

Neutron

The neutron was characterized by James Chadwick in 1932. The neutron has almost the same mass as the proton and no charge.

Particle

Charge

Mass

electron

-1.6022 x 10-19 Coulombs

9.109 x 10-31 kilograms

proton

-1.6022 x 10-19 Coulombs

1.673 x 10-27 kilograms

neutron

  

1.675 x 10-27 kilograms

Structure of the Atom

Our current view of the structure of the atom was described as a result of experiments performed under the direction of Ernest Rutherford. In his experiment alpha particles (which he had characterized by 1908) were 'shot' at a thin piece of gold foil. The behavior of the scattered particles lead Rutherford to postulate a new model of the atom. His model, which we currently hold, locates almost all of the mass of the atom in the nucleus with the electron located outside the nucleus.

The Periodic Table

Since Dalton's Atomic Model many elements were being discovered and characterized. By 1789 33 elements had been characterized. This number grew to 54 by 1830. As the number of elements increased efforts were expended to try organize them into some order. Dimitri Mendeleev is credited with the organization of the periodic table. His organization took advantage of the physical and chemical properties of the known elements. While his initial efforts organized the elements into groups on the basis of their properties, the layout depended on the atomic mass of the element. Mendeleev found several instances where conflict between ordering occurred. It was not until the work by H.G.J. Moseley in 1913 that the organization of the periodic table was finalized. Moseley discovered a relationship between the number of protons in the nucleus of an element and the order of the elements as a result of X-rays emitted by metal atoms.

Here are some sample periodic tables.

Here is a WEB page with a discussion of atomic mass units and isotopes.

Important terms