THE PERIODIC TABLE

Dmitri Mendeleev A Russian scientist who began to classify the 60 known elements of his time Worked with the indivisible model of an atom Atomic masses were known in comparison to hydrogen, which was set as "1"

“I began to look about and write down the elements with their atomic weights and typical properties, analogous elements and like atomic weights on separate cards, and this soon convinced me that the properties of elements are in periodic dependence upon their atomic weights.” --Mendeleev, Principles of Chemistry, 1905, Vol. II

elements were arranged according to periodicity: patterns that repeat at definite intervals - in chemistry periodicity is the occurrence of similar physical and chemical properties of elements at regular intervals

Mendeleev stated that elements arranged in order of increasing atomic mass show a periodic repetition of properties He left blank spaces for where he predicted (based on properties) an element should be –He predicted (accurately!) the properties of yet-undiscovered elements!!!

In the meantime… Chadwick, Rutherford and Thomson discovered the proton, electron, and the neutron

elements were re-arranged by atomic number elements arranged in order of increasing atomic number show a periodic repetition of properties - each column of the periodic table (called a group) has similar properties (periodic trends) Hands-On: elemental samples (Helen)

Chadwick, in Rutherford's lab made another discovery: the neutron! SO… an atom is made of –Positive protons, mass of 1, symbol p + –Uncharged neutrons, mass of 1, symbol n o –Negative electrons, mass of 0, symbol e -

x atomic symbol A mass number (atomic mass) z atomic number

Atomic mass = number of protons + neutrons Atomic number = number of protons So what's the number of electrons? In a neutral atom the number of electrons (negative charges) is equal to the number of protons (positive charges)

Trends in the Periodic Table: Classifying Elements Scientists divide elements into 3 categories –Metals Alkali metals Alkaline earth metals –Metalloids Transition metals –Non-metals Halogens Noble gases

Metals left side and centre of periodic table Solid at room T (except mercury, Hg) Silver (except Cu and Au)

- metals are shiny

- conduct heat

- conduct electricity

malleable: capable of being shaped, the degree to which it can be shaped by pounding with a hammer ductile: physical property of being capable of sustaining large deformations without breaking (for example being drawn into a wire)

Non-metals Right side of periodic table Found in all three states Variety of colours Poor conductors of heat/electricity Usually brittle (having little elasticity: easily cracked or fractured or snapped )

good insulators, ex. tile off space shuttle

Metalloids Along zig-zag line dividing metals and non-metals Metalloids are: Boron, Silicon, Germanium, Arsenic, Antimony, Tellurium, Polonium Have some metallic and non- metallic properties Ex. Si - silicon: shiny, nonmalleable, a semi-conductor

ashes of the alkali plant contain NaOH increases OH- concentrations in water Al Kali is an Arabic word

again, in water will release hydroxide ions

Halogens

I2aawhttp:// I2aaw

salt generators extremely reactive!!!

Three thousand years later...

How is this useful? - meltdown and explosion of the Chernobyl powerplant in Ukraine (1986)

products of nuclear fission were scattered for thousands of miles and even got to Alaska large amounts of radioactive isotopes of strontium, cesium, and iodine were released

alkali metal salts are soluble radioactive cesium will be washed out of the air with rain and dissolve in rivers and streams plus get into ground water it will get into the body's blood stream BUT –can be trapped using positive ions (ex. water softeners) strontium (Sr) in group 2 (alkali earth metals) both Sr and Ca are taken up by bone…bone cancer

Iodine in group 17 (Halogens): soluble and used by thyroid… take extra non- radioactive thyroid to minimize takeup of radioactive iodine