The 19 th Century……The Good Life? Typical chemist’s responsibilities during the middle of the 19 th century: Learn the properties of more than 60 elements Learn the properties of all known compounds “Best Guess” atomic mass of the elements Different chemists using different atomic masses for the same element. Not good!

Chapter 6: The Periodic Table Scientist Stanislao Cannizzaro discovered a method for accurately determining the atomic masses of the elements. Russian chemist, Dmitri Mendeleev sought to arrange the elements using both the atomic masses and properties of each. when the elements were arranged in order of increasing atomic mass, certain similarities occurred periodically. his procedure produced the first periodic table of the elements. his periodic table even predicted existence of undiscovered elements and left blanks to account for these elements.

Mendeleev’s Periodic Table Mendeleev arranged elements with similar properties in the same row.

Henry Moseley later recognized that the atomic number, not atomic mass, is the basis for the organization of the periodic table. periodic law: the physical and chemical properties of the elements are periodic functions of their atomic numbers. The Modern Periodic Table the elements are arranged in order of their atomic numbers so that elements with similar properties fall in the same column, or group. the elements are also arranged into periods, or horizontal rows of elements.

Alkali metals Alkaline- Earth metals Transition metals Halogens Noble Gases Li Na K Inner Transition Metals

Blocks (Magic) of the Periodic Table

Periodic Properties 1.Atomic Radius: defined as one-half the distance between the nuclei of two identical atoms chemically bonded together. Atomic Radius Trends: Atomic radius decreases from left to right across a period. Atomic radius decreases from left to right across a period.  Caused by the increasing positive charge of the nucleus at similar energy levels! Caused by the increasing positive charge of the nucleus at similar energy levels! Atomic radius increases down a group. Atomic radius increases down a group.

2. Ionization Energy: the energy required to remove an electron from a neutral atom (producing an ion). Trend: IE increases from left to right across a period.  Due to increasing nuclear charge.  Noble gases do not lose electrons easily (low reactivity). IE generally decreases down the groups.  Electrons are in higher energy levels (further from the nucleus), thus easily removed.

3. Electron Affinity the energy change (released) that occurs when an electron is acquired by a neutral atom. think of EA as the “desire” or “liking” of electrons. EA Trends: EA increases left to right across a period. EA decreases from top to bottom in a group. 4. Ionic Radius when an atom gains/loses electrons it becomes an ion. Cation: positive ion (losing electrons) Anion: negative ion (gaining electrons)

5. Electronegativity: a measure of the ability of an atom in a chemical compound to attract electrons. Fluorine is the most electronegative element. Trend: Electronegativity increases from left to right within a period. Electronegativity decreases from top to bottom within a group. Practice Time!