Periodicity Chapter 5

The History of the Development of the Periodic Table In the 1790’s Antoine Lavoisier (French Chemist) compiled a list of 23 elements. Some of these were gold, silver, carbon, oxygen. In 1860 Stanislao Cannizzaro (Italian chemist) presented a method for accurately measuring the relative masses of atoms. In 1864 John Newlands(English Chemist) noticed that when elements were arranged by increasing atomic mass, their properties repeated every eighth element. He called this the Law of Octaves

Lothar Meyer and Dimitri Mendeleleev In 1869 Lothar Meyer (German chemist) and Dmitri Mendeleev (Russian chemist) each demonstrated a connection between atomic mass and elemental properties. However, Mendeleev produced the first useful and widely accepted periodic table arranged in order of increasing atomic mass. An important facet of his table was that he left blank spaces in the table where he thought undiscovered elements should go. Mendeleev is considered the “father” of the periodic table.

Properties of some elements predicted by Mendeleev Predicted elements Element & year discovered Properties Predicted properties Observed properties Ekaaluminum Gallium 1875 Density of metal 6.0 g/ml 5.96 g/ml Melting point Low 30 °C Oxide formula Ea2O3 Ga2O3 Ekaboron Scandium 1877 3.5 g/ml 3.86 g/ml Eb2O3 Sb2O3 Solubility of oxide Dissolves in acid Ekasilicon Germanium 1886 High 900°C 5.5 g/ml 5.47 g/ml Color metal Dark gray Grayish white EsO2 GeO2 Density of oxide 4.7 g/ml 4.70 g/ml Chloride formula EsCl4 GeCl4

Henry Moseley In 1913 Henry Moseley(English chemist) discovered that atoms of each element contain a unique number of protons in their nuclei—the number of protons being equal to the atom’s atomic number. He proposed the Periodic Law, which states that there is a periodic repetition of chemical and physical properties of the elements when they are arranged by increasing atomic number. His arrangement of elements by atomic number resulted in a clear periodic pattern of properties. This is the basis of the Modern Periodic Table which we are using now.

Periodic Law: The physical and chemical properties of the elements are periodic functions of their atomic numbers. This periodicity in properties of the elements is a function of the arrangement of electrons around the nucleus of an atom (remember that the # of electrons = the # of protons in a neutral atom)   Valence electrons: The electrons available to be lost, gained, or shared in the formation of chemical compounds; i.e. the “outer shell” electrons.

Periodic trends Atomic radii: one half the distance between the nuclei of identical atoms that are bonded together.

Atomic radii vs. atomic number

Atomic Radii vs. atomic number

Cations

Anions

Ionization energy The energy required to remove one electron from a neutral atom or an element. (kJ/mol) A + energy  A+ + e-

A + e-  A- + energy A + e- + energy  A- Electron Affinity The energy change that occurs when an electron is acquired by a neutral atom. Most atoms RELEASE energy when they acquire an electron. A + e-  A- + energy Some atoms must be “forced” to gain an electron by the addition of energy. A + e- + energy  A-

Electron affinity vs. atomic number

a measure of the ability of an atom in a chemical compound to attract Electronegativity a measure of the ability of an atom in a chemical compound to attract electrons. Highest value is arbitrarily given to Fluorine of 4.0.

Electronegativity

Electronegativity vs. atomic number

General reactivity trends