Periodic Properties of the Elements

Development of the Periodic Table

The most significant tool for organizing and remembering chemical facts is the periodic table

• Based on the periodic nature of electron configurations
• Elements in the same column have the same number of valence electrons
• Similarities in chemical nature due to similarities in valence electron configuration

• Certain elements, such as gold and silver, can be found naturally in their elemental form and were discovered thousands of years ago.
• Some radioactive elements are quite unstable and their isolation is dependent upon modern technology
• The majority of elements are stable, but commonly present in compound form with other elements

In the 1800's methods were developed to isolate various elements from compound form

• 1800: 31 elements identified
• 1865: 63 elements identified

1869: Dmitri Mendeleev and Lothar Meyer published schemes for classifying elements

Mendeleev's insistence on ordering elements by atomic weight, and grouping them by characteristics resulted in several "gaps" in the table

• Both Gallium (Ga) and Germanium (Ge) were unknown at the time
• Thus there was a gap under Aluminum (Al) and a gap under Silicon (Si)
• Mendeleev concluded therefore that there must be two elements, which he called "eka-Aluminum" and "eka-Silicon" which must fill these gaps

Mendeleev predicted not only that Ga and Ge must exist, but also described some of their general physical properties

• Their approximate atomic weight
• The stoichiometric relationship for compounds involving oxygen and chlorine

Ga and Ge were discovered decades later, but their physical and chemical characteristics as predicted by Mendeleev were correct

1911 Rutherford model of the atom:

• most of the mass of the atom was located in a dense nucleus
• the nucleus had a net positive charge
• beyond the nucleus was a mostly empty space which contained electrons with a net negative charge

1913 Henry Moseley (killed at Gallipoli at age 28)

• investigated the characteristic frequencies of X-rays produced by bombarding each of the elements in turn by high energy cathode rays (electrons).
• He discovered a mathematical relationship between the frequency and the atomic number (the "serial number" in the periodic table).
• This must mean that the atomic number is more than a serial number; that it has some physical basis.
• Moseley proposed that the atomic number was the number of electrons in the atom of the specific element.
• This also means that the atomic number is the number of positive charges carried by the nucleus.

Moseley's experiments:

Moseley was using various elements (metals) as targets in cathode ray tubes. He noticed that when struck by the cathode rays, different metals gave off x-rays with distinct wavelengths. Essentially what was happening was that the cathode rays (high energy electrons) were knocking out the inner-most electrons of the metal targets. X-rays were emitted when an outer electron "fell" down into this inner shell. Since the inner-most electrons are not shielded by other electrons, the energy required to knock them out is dependent upon the number of protons in the nucleus. Thus, the emitted x-ray frequency associated with an outer electron falling down into this shell is related to the number of protons in the nucleus. Moseley realized that the atomic numbers were not just a convenient numbering scheme for the elements, but had a real physical meaning - ultimately realized as being the number of protons (and electrons) in a (neutral) element.