The Periodic Table Ch 6

History of the Periodic Table Only 13 elements had been discovered by 1700 As time went on and more elements were discovered (5 in the decade of ) a method to organize the elements was needed. Around the world chemists used different mass values for the elements which made it hard to reproduce experiments.

(continued) In 1869 Russian scientist Dimitri Mendeleev, demonstrated a connection between atomic mass and elemental properties. Using element cards, it took him 18 months to set it up.

(continued) Mendeleev developed his table while making a textbook for his students. He wrote each of the about 60 elements each on a note card and arranged them in order of increasing atomic mass. He left blanks in his table for elements that weren’t discovered yet.

Mendeleev’s periodic table

Discovery After the publication of his Periodic Table missing elements (Ga, Ge, Sc) were found with masses and properties very close to what Mendeleev predicted.

(continued) Mendeleev noticed that some elements seemed out of place in his table. He thought the masses (from other scientists) were wrong. In 1913, Moseley determined the atomic number for each element. Elements are now arranged by atomic number, not mass, in the modern periodic table.

Modern Periodic Table

Organized into columns called groups and rows called periods Three main classes of elements: 1. metals 2. nonmetals 3. metalloids

Periodic Table Elements in any group have similar physical and chemical properties Properties of elements in periods change from group to group Symbol placed in a square Atomic number above the symbol Atomic mass below the symbol

Metals Most elements are metals. Properties: 1. Good conductors of heat and electricity 2. Solid at room temperature (except Mercury) 3. Reflect light (shiny)

Nonmetals Located in the upper right corner of PT Greater variation among these than metals. Most are gases at room temperature. A few are solids (Sulfur & Phosphorus) and one is a liquid (Bromine) Tend to have properties opposite of metals.

Metalloids Generally have properties similar to metals and nonmetals. An element in this group may behave like a metal under certain conditions, and then behave like a nonmetal under different conditions. For example, the metalloid Silicon is a poor conductor of electricity, but it becomes a good conductor when it is mixed with another metalloid, Boron.

Main groups Alkali metals = Group 1A Alkaline earth metals = Group 2A Halogens = Group 7A (not incl. At) Noble gases = Group 8A Transition metals (B-groups)

Other groups The elements in groups 1, 2 and 13 – 18 (groups 1A – 8A) are called representative elements. The elements in group 3 – 12 (B groups) are known as transition elements / metals.