5.1 – NOTES History of the Periodic Table

I. Development of the Modern Periodic Table A. History of the Periodic Table’s Development – - 1790’s, Lavoisier compiled a list of 23 known elements - Au, Ag known since prehistoric times - discovery of electricity allowed for the electrolysis of several compounds, led to “new” elements; as did industrial revolution - by 1870 over 70 elements were known;

1. Dobereiner – 1829 - found triads of elements - trios of elements with similar properties. Li, Na, K Ca, Sr, Ba Cl, Br, I were some of his triads

2. John Newlands – 1865 - Arranged elements by atomic weight and found that similar elements appeared to be 7 elements apart; - “Law of Octaves”. He went no further in his classification of the elements. - When a pattern repeats in a specific manner, said to be periodic; - wasn’t consistent (didn’t apply to all elements); system failed; - **In the 1860’s a new system for calculating mass was discovered

3. Meyer, 1869 - Organized the elements, but made no attempt at predictions. - Demonstrated connection b/t mass and properties like Mendeleev - also published shortly after Mendeleev;

4. Mendeleev, 1869 – is given credit for organizing the periodic table - He arranged elements in order of increasing atomic mass. - Similar elements were placed next to each other in horizontal rows. - Furthermore, he predicted the existence of 3 missing elements, Sc, Ga, and Ge, and predicted their properties very accurately. - The family of elements that was completely missing was the noble gases, which had not yet been discovered. Colorless/odorless/tasteless  no evidence of their presence

5. Moseley – 1913-15 - Determined the atomic numbers of the known elements and rearranged the periodic table. - As new elements were discovered, variation in properties was noticed, no longer matched up; - identification of atomic # fixed the problem and the periodic table was rearranged according to increasing atomic number

B. The modern periodic table - Periodic Law – elements are arranged by increasing atomic #; vertical columns = families/groups - similar properties/rxns due to similar e- configurations - Groups 1A – 8A known as the representative elements b/c possess a wide range of chemical/physical properties; - horizontal rows = series/periods; demonstrate a slow pattern of change called trends