Elements and The Periodic Table

Classification of Matter

Horizontal stripes on nonmetals Vertical Stripes on metalloids

History of the Periodic Table 330 B.C - 4 elements 1800 - 31 elements 1865 - 63 elements Chemists required a systematic method to organize the elements

Johann Dobereiner 1829 Law of Triads Groups of 3 elements with similar properties

John Alexander Newlands 1864 Arranged all known elements in order of increasing atomic mass Observed that every 8th element had similar physical and chemical properties (Law of Octaves) Began to group these elements into “families”

Julius Lothar Meyer 1865 Also arranged the elements in order of increasing atomic mass Found repeating patterns and developed a table of elements

Dimitri Mendeleev 1869 Credited as the Father of the Periodic Table Periodic Law: When arranged by atomic mass, the properties of the elements are in periodic dependence of their atomic mass.

Both Mendeleev and Meyer arranged the elements in order of increasing atomic mass. Both left vacant spaces where unknown elements should fit. So why is Mendeleev called the “father of the modern periodic table” and not Meyer, or both?

Mendeleev... stated that if the atomic weight of an element caused it to be placed in the wrong group, then the weight must be wrong. (He corrected the atomic masses of Be, In, and U) was so confident in his table that he used it to predict the physical properties of three elements that were yet unknown.

After the discovery of these unknown elements between 1874 and 1885, and the fact that Mendeleev’s predictions for Sc, Ga, and Ge were amazingly close to the actual values, his table was generally accepted.

However, in spite of Mendeleev’s great achievement, problems arose when new elements were discovered and more accurate atomic weights determined. By looking at our modern periodic table, can you identify what problems might have caused chemists a headache? Ar and K Co and Ni Te and I Th and Pa

Henry Moseley In 1913, through his work with X-rays, he determined the actual nuclear charge (atomic number) of the elements*. He rearranged the elements in order of increasing atomic number. *“There is in the atom a fundamental quantity which increases by regular steps as we pass from each element to the next. This quantity can only be the charge on the central positive nucleus.” 1887 - 1915

Glenn T. Seaborg After co-discovering 10 new elements, in 1944 he moved 14 elements out of the main body of the periodic table to their current location below the Lanthanide series. These became known as the Actinide series. 1912 - 1999

Glenn T. Seaborg He is the only person to have an element named after him while still alive. "This is the greatest honor ever bestowed upon me - even better, I think, than winning the Nobel Prize." 1912 - 1999

Fig 7 p.17

and the same number of valence electrons

Hydrogen (A class of it’s own) Sometimes it behaves like an alkali metal, sometimes like a halogen, and sometimes in its own unique way.

Group 1: Alkali Metals 1 valence electron (electrons in their outermost shell) Soft, shiny, easily cut with a knife The most reactive metals React violently with water (stored in oil or a vacuum) React with halogens to form salts Never found as free elements in nature, always bonded with other elements Braniac: alkali metals Disposal of sodium 1947

Group 2: Alkaline Earth Metals 2 valence electrons Light, reactive metals Form oxides when exposed to air. React with oxygen to from oxides, react with hydrogen to form hydrides (except beryllium). React with water to liberate hydrogen

Groups 3-12: Transition Metals 1 or 2 valence electrons Strong, hard metals Good conductors of heat and electricity Wide range of chemical and physical properties

Group 17: Halogens 7 valence electrons Extremely reactive nonmetals Not lustrous, nonconductors of electricity

Group 18: Noble Gases Full outer shell Extremely unreactive (inert)

Lanthanides (Rare Earth Metals) Elements 57-70

Actinides Elements 89-102 Transuranic Elements: Synthetic elements