Chapter 5 The Periodic Law Back |
The Periodic Law History of the Periodic Table Mendeleev and Chemical Periodicity Used values of atomic masses established by Cannizzaro. Wished to organize elements by their properties. Looked for trends or patterns in properties. Found repetitive properties---periodicity---when elements were arranged according to mass. Grouped elements with similar properties together. Mendeleev and Chemical Periodicity Created the first periodic table. Left open spaces for predicted elements. Note: The elements predicted by Mendeleev in 1869 were all found by 1886 and their properties were almost exactly those predicted by Mendeleev years earlier. Properties of Some Elements Predicted by Mendeleev Moseley and the Periodic Law Organized elements by increasing nuclear charge. Led to the atomic number concept. Reorganized the periodic table in order of atomic number. Supported Mendeleev’s periodic law which stated “The physical and chemical properties of the elements are periodic functions of their atomic numbers.” The Modern Periodic Table It is an arrangement of the elements in order of their atomic numbers so that elements with similar properties fall in the same column or group. The most significant additions were the noble gases. Helium was discovered in 1868. Argon was discovered by Ramsey in 1894. He also discovered krypton and xenon in 1898. Dorn discovered radon in 1900. The Modern Periodic Table The noble gases are known by this name because they are nonreactive or inert. The lanthanides are elements 58 to 71. The actinides are elements 90 to 103. The Periodic Law Electron Configurations and the Periodic Table Periods and Blocks of the Periodic Table In the first period, the 1s sublevel is filled. Since it holds only 2 electrons, the first period must fill with 2 electrons. In the second period, there are 2 sublevels, s and p. In the third period, there are 3 sublevels, s, p, and d. Periods and Blocks of the Periodic Table The period of an element can be found by its configuration. Arsenic, for example, is [Ar]4s23d104p3 which means that it is in the fourth period because the highest occupied main level is n = 4. Based on the electron configurations of the elements, the periodic table can be divided into four blocks, the s, p, d, and f blocks. Blocks of the Periodic Table The s-Block Elements: Groups 1 and 2 These are chemically reactive metals. The Group 1 (ns1) metals are more reactive than the Group 2 (ns2) metals. Group 1 elements contain 1 valence electron(s) and are known as the alkali metals. Group 2 elements contain 2 valence electron(s) and are known as the alkaline earth metals. Hydrogen and Helium are in the s-block but their properties are not like the others in the block. They are unique. The d-Block Elements: Groups 3-12 The Transition Metals The d-sublevel first appears in the 3rd main level. The d-sublevels are slightly higher in energy than the previous s-sublevel. This is where the main energy levels begin to overlap. There are exceptions in this block. Some elements fill electrons to achieve the lowest possible energy state. Two exceptions in the 4th period are chromium and copper. Experimentally it was found that sublevels prefer to be half-filled or completely
filled if they are close. The d-Block Elements: Groups 3-12 Chromium has the configuration 1s22s22p63s23p64s13d5. Copper has the configuration 1s22s22p63s23p64s13d10. Palladium, platinum, molybdenum, silver and many others are also exceptions to the normal filling pattern. In each case the abnormal filling occurs to provide the lowest possible energy state for the atom. These elements are not as predictable as the s-block or the p-block. They usually have more than one oxidation state. The p-Block Elements: Groups 13-18 These are atoms which add an electron to the p-sublevel. These are main-group elements or representative elements. All of the nonmetals and metalloids are included in the p-block. There are 8 metals here also. The pnictogens(Gp 15) are relatively inactive. The chalcogens(Gp 16) are somewhat active. The halogens(Gp 17) are the most reactive nonmetals. Fluorine is the most reactive nonmetal on the periodic table. The f-Block Elements: Lanthanides and Actinides These are located on the bottom of the periodic table but are really placed in the sixth and seventh periods. The first main level that has an f-sublevel is the ___ level. There are 14 elements on each row. They are similar in reactivity to Group 2 elements. Main Group (Representative) Elements Group 1A – Alkali metals – very reactive metals – never found freely in nature Group 2A – Alkaline earth metals – reactive metals – never found freely in nature Group 3A(13) – Boron group – less reactive – contains a metalloid and metals Group 4A(14) – Carbon group – most diverse group Group 5A(15) –Pnictogens Group 6A(16) – Chalcogens Group 7A(17) – Halogens – most reactive nonmetals Group 8A(18) – Noble gases – most stable Relationship between Periodicity and Electron Configuration Trends Atomic radii As one goes down a group, the atomic radii increase. As one goes across a period, the atomic radii decrease. Ionic
radii As one goes down a group, the ionic radii increase. As one goes across a period, the ionic radii go down until you get to the nonmetals. The most negative ion in a series is the largest.
--As one goes from the most positive ion to the most negative ion, size increases. Trends in Atomic Radii First ionization energy As one goes down a group, the ionization energy goes down As one goes across a period, the ionization energy increases. Second ionization energy It is approximately twice the first ionization energy. Ionization energy jumps up dramatically when you try to remove an electron from a filled shell. Trends in Ionization Energies Successive Values for Ionization Energies Trends Electronegativity Electronegativity As one goes down a group, the electronegativity goes down. As one goes across a period, the electronegativity increases Electronegativity Trends Electron Affinity Electron affinity As one goes down a group, the electron affinity gets more positive. As one goes across a period, the electron affinity gets more negative. Density As one goes down a group, the density increases. Melting point As one goes down a group, the melting point decreases in metals and increases in nonmetals. Electron Affinity Trends |