Modern Periodic Table (Section 5.2)

The PERIODIC TABLE arranges elements in columns, based on sets of properties that repeat from row to row Elements are arranged by increasing atomic number

Periods A PERIOD is a row in the table of elements The number of elements per period varies because the number of available orbitals increases from one energy level to the next

Groups A GROUP is a column on the periodic table Properties repeat in a predictable way when atomic numbers are used to arrange elements into groups Elements in groups have similar electron configurations and chemical properties PERIODIC LAW is the pattern of repeating properties

Atomic Mass Depends on the distribution of an element’s isotopes in nature and the masses of those isotopes ATOMIC MASS UNIT (amu) is defined as 1/12 the mass of a Carbon-12 atom Isotopes of chlorine Chlorine-35 occurs 75.78% naturally Chlorine-37 occurs only 24.22% naturally Therefore the atomic mass of chlorine is is the WEIGHTED AVERAGE (35.453 and not 35) Weighted averages Because one isotope occurs more than another you can’t just divide the sum of both masses by 2 For example chlorine-35 occurs more than chlorine-37 which has more mass so the atomic mass of chlorine is 35.453 (a weighted average) instead of 35.967 (what you would get if you took both masses and divided by 2)

Classes of Elements SOLID, LIQUID, or GAS (based on their state at room temperature) NATURALLY OCCURING vs. NOT NATURALLY OCCURING METAL, NONMETAL, METALLOID

Metals Good conductors of electric current and heat Solid at room temperature (except mercury) Malleable (can be pounded into flat sheets) and ductile (drawn into thin wires) Some are extremely reactive and some do not react easily

Transition Metals Elements that form a bridge between the elements on the left and right sides of the table Form compounds with distinctive colors

Nonmetals Poor conductors of heat and electric current Low boiling points, many are gases at room temperature All the gases in the periodic table are nonmetals The ones that are solid at room temperature tend to be brittle Some are extremely reactive, some hardly react at all, and some fall in between Fluorine is the most reactive nonmetal

Metalloids Properties that fall between those of metals and nonmetals Ability to conduct electricity varies with temperature Silicon and germanium are good insulators at room temperature and good conductors at high temperatures

Variation Across a Period Across a period from left to right, the elements become less metallic and more nonmetallic in their properties