Physical Science: Ch 5 Notes A. Organizing the Elements Dmitri Mendeleev was a Russian chemist and teacher who organized the elements in the 1860s. Periodic table – an arrangement of elements in columns, based on a set of properties that repeat from row to row Period: each row in the table of elements is a different period Group: each column on the periodic table is a different group Atomic mass units (amu) – the units used to report atomic masses since the mass of an atom is so small in grams Atomic Mass = the sum of protons and neutrons in an atom

Physical Science: Ch 5 Notes B. Classifying the Elements 1. Classify by state of matter (at room temperature) Liquids: Mercury and Bromine Gases: Hydrogen, Nitrogen, Oxygen, Fluorine, Chlorine, Helium, Neon, Argon, Krypton, Xenon, Radon Solids: all the rest! 2. Classify by general properties Metals: majority of elements (left side and middle of periodic table) Metalloids: Boron, Silicon, Germanium, Arsenic, Antimony, Tellurium, Polonium, Astatine Nonmetals: Elements at the top, right of the periodic table and Hydrogen

Physical Science: Ch 5 Notes classifying continued… 3. Elements can also be divided into those that occur naturally and those that do not Elements with atomic numbers 43 and 61 do not occur naturally Elements with atomic numbers 93 and above are also not naturally occurring. C. Element Characteristics Metals – elements that are good conductors of electric current and heat (malleable and shiny too!) Ductile – can be drawn into thin wires Nonmetals – elements that are poor conductors of heat and electric current (most are gases, some are dull, brittle solids) Metalloids – elements with properties that fall between those of metals and nonmetals

Physical Science: Ch 5 Notes D. Electron Trends Valence electron – an electron that is in the highest occupied energy level of an atom These electrons play a key role in chemical reactions – atoms want to have a full outer energy level so they will gain or lose electrons to make this happen Properties vary across a period because the number of valence electrons increases from left to right Elements in a group have similar properties because they have the same number of valence electrons.