 Dmitri Mendeleev (Дми́трий Менделе́ев) ◦ First devised a periodic chart ◦ Ordered elements by atomic mass ◦ Some inconsistencies  Henry Moseley ◦ Revised the periodic chart ◦ Ordered elements by atomic number

 Many of the physical and chemical properties of the elements tend to recur in a systematic manner with increasing atomic number

 Periods or series ◦ Horizontal rows ◦ Same number of energy levels  Groups or families ◦ Vertical columns ◦ Similar reactivity ◦ Same number of valence electrons

 Alkali metals ◦ Group 1 ◦ Most active metals ◦ 1 valence electron  Alkaline Earth metals ◦ Group 2 ◦ 2 valence electrons ◦ Still rather active

 Transition metals ◦ Most stable metals ◦ Groups 3-12 ◦ Variable charges ◦ Colored ions  Rare earth metals ◦ Lanthanides and actinides ◦ f block ◦ Not necessarily rare, just historically rare uses ◦ From the earth

 Pnictogens ◦ Group 15  Chalcogens ◦ Group 16  Halogens ◦ Group 17 ◦ Most reactive nonmetals

 Noble gases ◦ Group 18 ◦ Full outer shells ◦ Essentially nonreactive

 Metals ◦ Ductile ◦ Malleable ◦ Conductive ◦ lustrous  Metalloids  Nonmetals ◦ brittle

 Gas: H, N, O, F, Cl, Noble gases  Liquid: Br, Hg  Solid: everything else

 Half the average distance between nuclei of adjacent atoms of the same element.  Measure of atomic size  Atomic radius is the distance from the nucleus to the outer electron shell

 The energy required to pull a valence electron off of an atom.  This can be thought of as the negative of the energy of the highest occupied atomic orbital.  Remember, the more tightly a nucleus holds it electrons, the higher the ionization energy.  More relevant for metals.

A measure of how greedy for electrons an atom will be in chemical interactions with other elements. The small 'tight' atoms have large electronegativity and the big 'squishy' atoms have low electronegativity We will use the electronegativity in our discussion of how polar a chemical bond is between unlike elements because it is a good measure of how unequally bonding electrons will be shared between atoms. More relevant for nonmetals.

 Metals ◦ Loses valence electrons ◦ Radius decreases  Nonmetals ◦ Gains valence electrons ◦ More electrons increases repulsion among them ◦ Valence shell expands ◦ Radius increases