 Dmitri Mendeleev began looking for patterns among the properties of the elements in the 1860’s  What properties did he include? Density, appearance, atomic mass, melting point, etc.  What did he discover? He found that there was a repeating pattern to the properties of the elements when they were arranged according to an increase in the atomic mass

 Mendeleev said that the properties of the elements were periodic  Meaning they had a regular, repeating pattern  What else is periodic? The days of the week The cycle of the moon The number of days in a year  When elements were arranged in order of increasing atomic mass, similar chemical and physical properties were observed every 8 th element

 The periodic table was not complete because all of the elements had not been discovered  However, Mendeleev predicted that when those elements were discovered they would fill in the gaps  But there were still problems  He noticed that some elements were not in their correct place according to their properties

 How did Mendeleev set up the periodic table?  According to atomic mass  How is the periodic table arranged today?  According to atomic number  Henry Moseley was able to determine the atomic number of elements and rearranged the periodic table this way

 Each horizontal row of elements (from left to right) is called a period  How many periods are there total? 7  What period is Li in? 2  Each column of elements (from top to bottom) is called a group  Elements in the same group often have similar chemical and physical properties  Elements in the same group are considered to be in the same family

 Each horizontal row of elements (from left to right) is called a period  How many periods are there total? 7  What period is Li in? 2  Each column of elements (from top to bottom) is called a group  Elements in the same group often have similar chemical and physical properties  Elements in the same group are considered to be in the same family

 Elements in a group (or family) often share similar properties  The properties are similar because the atoms of the elements have the same number of electrons in their outer energy level  How many groups are there?  18

 The most important properties of elements are determined by the number of electrons in the outer energy level  Remember that the outermost electron cloud is called a valence electron cloud  Electrons in the valence electron cloud are called valence electrons  Elements like to have a full valence electron cloud and so they will accept, give away, or share electrons with other elements

 Group 1 – Alkali Metals  1 valence electron  The most reactive metals  Give away their electron very easily  Group 2 – Alkaline Metals  2 valence electrons  Very reactive  Give away their electrons very easily  Groups 3 – 12 – Transition Metals  1 or 2 valence electrons  Do not give away their electrons

 Group 13 – Boron Group  3 valence electrons  They give up their electrons pretty easily  Group 14 – Carbon Group  4 valence electrons  Reactivity varies among the elements in this group  Group 15  5 valence electrons  These elements typically accept electrons from other elements  How many would they want to accept?

 Group 16 – Oxygen Group  6 valence electrons  Reactive nonmetal  Will readily accept how many electrons?  Group 17 – The Halogens  7 valence electrons  Most reactive nonmetals  Will accept how many electrons?  Group 18 – Noble Gases  8 valence electrons  NONREACTIVE  Colorless, odorless gases