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Unit 8 Periodic Table SUMMARY HISTORY/DEVELOPMENT Dobereiner – 1817, organized the known elements into triads (groups of 3) based on appearance and reactions Newlands – 1864, organized the known elements into octaves (groups of 8) based on properties Mendeleev – 1869, considered the father of the modern periodic table; organized the known elements into columns and rows based on increasing (relative) atomic mass so that properties repeated themselves; left holes in his periodic table for yet undiscovered elements and predicted the properties and masses of these unknown elements (his predictions were correct!) Moseley – 1913, organized the known elements by atomic number (number of protons); this only moved around a few elements from Mendeleev’s table Seaborg – 1930s, formulated the Actinide concept of heavy elements and how they connected to the existing periodic table LAYOUT Groups – vertical (up and down) columns, 1-18, atomic number increases top to bottom, elements in same group share similar physical and chemical properties Periods – horizontal (across) rows, 1-7, atomic number increases from left to right alkali metals – group 1 alkaline earth metals – group 2 halogens – group 17 noble gases – group 18 transition metals – groups 3-12 lanthanide and actinide series (also called inner transition metals) – bottom 2 periods metalloids – along the bolded staircase, excluding Aluminum (Al) and Astatine (At) metals – left of metalloids, except for Hydrogen (H) nonmetals – right of metalloids, plus Hydrogen (H) PROPERTIES OF ELEMENTS Effective nuclear charge (Zeff) – attraction of the + nucleus for the valence electrons in an atom; Zeff = # protons - # core electrons (this is only used to compare elements in the same period) Atomic radius – describes the size of an atom; half the distance between adjacent nuclei; measured in Angstroms (0.1 nm or 1 x 10-10 m) Ionic radius – describes the size of an ion; half the distance between adjacent nuclei of an ion; measured in Angstroms (0.1 nm or 1 x 10-10 m) Electronegativity – a chemical property that describes how strongly an atom attracts bonding electrons towards itself; no units because it is relative Ionization energy – a physical property that describes the energy required to remove an electron from an atom; typically measured in kJ/mole; 1st ionization energy is energy required to remove the 1st valence electron from an atom (chemical) Reactivity – depends upon the relative attraction between valence electrons of one element and the nucleus of another element TRENDS Effective nuclear charge (Zeff) – increases from left to right across a period (because the # of protons increases but the number of shielding electrons doesn’t change) and decreases within a group (because the distance between the + nucleus and valence electrons increases) Atomic radius – increases from right to left across a period and from top to bottom in a group; Cesium (Cs) has the largest atomic radius (because Fr is not stable enough to get accurate measurements) Ionic radius metals form cations (+ charged ions); cations are smaller than the atom from which they are formed because the cation has lost valence electrons; this results in an increased P:E ratio when compared to the atom nonmetals form anions (- charged ions); anions are larger than the atom from which they are formed because the anion has gained valence electrons; this results in a decreased P:E ratio when compared to the atom Electronegativity – increases from left to right across a period (but DOES NOT include the noble gases group, because they do not form bonds) and from bottom to top in a group; Fluorine (F) has the highest electronegativity value Ionization energy – increases from left to right across a period and from bottom to top in a group; Helium (He) has the highest ionization energy (chemical) reactivity – for metals, increases from right to left across a period and from top to bottom in a group; Francium (Fr) is the most reactive metal; for nonmetals, increases from left to right across a period (excluding the noble gases) and from bottom to top in a group, Fluorine (F) is the most reactive nonmetal