Chem - History & Structure of PT

Dobereiner’s Triad

• Before discovery of protons, electrons, neutrons, Dobereiner found some groups of 3 elements that showed similar properties

• Called these groups ‘Triads’

John Newlands’ Octave

• Arranged the elements on the pt by atomic mass

• Called this the Law of Octaves

• When he did this, he found that each element was similar to the element eight places further on

• E.g. starting at Li, Be is the second element, B is the third and Na is the eighth element.

Newlands’ theory broke down

• His table showed a periodic pattern of properties, but eventually collapsed

• By ordering strictly according to atomic mass, Newlands was forced to put some elements into groups which did not match their chemical properties

• Eg - put iron in same group as oxygen & sulfur (metal & 2 non-metals)

• Because of this, his table was not accepted by other scientists

Dmitri Mendeleev

• 1869, 5 years after Newlands’ Law, Dmitri published a pt

• He also arranged the elements known at the time in order of atomic mass, but did other things that made table much more successful

• Realised the physical & chemical properties of elements were related to their atomic mass in a 'periodic' way, & arranged them so groups of elements with similar properties fell into vertical columns in table

Mendeleev’s Table Gaps

• Mendeleev thought it meant the elements which belonged in the gaps had not yet been discovered

• He was also able to work out atomic mass of missing elements, & so predict their properties, & when they were discovered, he turned out to be right

• One of these was Gallium

Henry Moseley

• Using atomic number instead of atomic mass as organising principle first proposed by British chemist Henry Moseley in 1913, & it solved anomalies like this one

Organisation of Periodic Table

• The elements are listed on PT in order of increasing proton number

• Groups (vertical divisions) of elements have similar chemical properties, also have same no. valence electrons

• Periods (horizontal divisions) have same no. electron shells