Vocabulary Unit 2b periodic table - a chart of the chemical elements that displays them in rows horizontally in order of increasing atomic number and vertically according to similar chemical properties of their atoms trend - periodic trends, arising from the arrangement of the periodic table, provide chemists with an invaluable tool to quickly predict an element's properties period - row in the periodic table that determine the number of energy levels group - column in the periodic table metals - any of several chemical elements that are usually shiny solids that conduct heat or electricity and can be formed into sheets nonmetals - any of five related nonmetallic elements (fluorine or chlorine or bromine or iodine or astatine) metalloid - of or being a nonmetallic element that has some of the properties of metal representative elements - those elements within the first two groups on the far left and the last six groups on the far right of the Periodic Table transition elements - any of the set of metallic elements occupying a central block that show variable valence and a strong tendency to form coordination compounds noble gases - any of the gaseous elements helium, neon, argon, krypton, xenon, and radon, occupying Group (18) of the periodic table. halogen - reactive nonmetallic elements that form strongly acidic compounds with hydrogen, from which simple salts can be made alkali metals - very reactive, electropositive, monovalent metals forming strongly alkaline hydroxides alkaline earth metal - six chemical elements in group 2 of the periodic table (beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra)) energy level - the specific energies that electrons can have when occupying orbitals in the electron cloud valence electron - electrons that occupy that outer shell (energy level) of the atom and determine reactivity reactivity - the sharing of valence electrons