The Periodic Table of Elements

Who Designed the Periodic Table Russian chemist Dimitri Mendeleev (late 1800’s) Originally arranged by increasing atomic mass Later arranged by increasing atomic # (Henry G.J. Moseley)

Columns in the Periodic Table Known as Groups or Families A group or family has similar but not identical properties Example: Group 1 or Alkali Metals Properties – soft, white, shiny metals, highly reactive

Rows in the Periodic Table Known as Periods Are not alike in properties However there is a pattern The first element in a period is almost always an active solid The last element is an inert or inactive gas Each Period tells you how many energy levels each atom has Each energy level can hold a certain number of electrons Energy level 1 - 2 electrons Energy level 2 - 10 electrons Energy level 3 - 18 electrons Energy level 4 - 36 electrons Energy level 5 - 54 electrons Energy level 6 - 86 electrons Energy level 7 - 118 electrons

Why do we care about how many electrons an atom has? The reactivity of an element is due to its electrons specifically the valence electrons Valence electrons are the electrons found in the outer shell of an atom. Orbit or Shell Nucleus Valence Electrons

Lewis Dot Diagrams A diagram to represent electrons in the outer energy level of an atom Uses the atomic symbol and dots Example: Hydrogen H• Beryllium Be: Carbon : C :

Trick to the Lewis Dot Diagrams Some atoms are very easy to figure out how many electrons are in the valence shell Group 1 has 1 Group 2 has 2 Group 13 has 3 Group 14 has 4 Group 15 has 5 Group 16 has 6 Group 17 has 7 Group 18 has 8 Groups 3-12 the number varies