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Introduction
With the rapid development of Chemistry and the discovery of new elements, it was necessary to develop a simple way to study and remember the numerous properties of the elements and their compounds. This gave rise to the necessity of classification of the elements into various groups having similar properties. This classification of elements resulted in the formulation of the Periodic Table, where the elements are arranged according to their properties in a tabular form.
Mendeleev's Periodic Table
In 1869, Mendeleev classified the then known 56 elements on the basis of their physical and chemical properties by giving a law known as the periodic law. The name periodic law stems from the fact that the properties of the elements orderly recur in a cyclic fashion. His periodic law states that "the physical and chemical properties of the elements are periodic function of their atomic masses". This means that when the elements are arranged in the order of their increasing atomic masses, the elements with similar properties recur at regular intervals. Based on this law all the known elements were arranged in the form of a table known as periodic table. Elements with similar properties recur at regular intervals and fall in certain groups or families. The elements in each group were similar to each other in many properties.
Long Form of the Periodic Table or Modern Periodic Table
With developments on the structure of atoms, it was discovered that the atomic number (Z) is the important characteristic of the atom and not the atomic mass. This led to the development of the modern periodic law by Moseley in 1942. The modern periodic law states that "the physical and chemical properties of the elements are periodic function of their atomic numbers". Thus, when the elements were arranged in the order of their increasing atomic numbers, the elements of similar properties recur at regular intervals.
Locating the Position of an Element
Knowing the electronic configuration, one can predict the period, which corresponds to the principal quantum number of the valence shell. The block of the element corresponds to the sub shell, which receives the last electron.
IUPAC Nomenclature for Elements with Z > 100
All elements beyond uranium (Z=92) are synthesized in the laboratories and are known as 'trans uranium elements'. The elements, beyond fermium (Z=100) are known as 'trans fermium elements'. These elements have atomic numbers 101 onwards.
Grouping of Elements
In the long form of the periodic table, elements are grouped into four main blocks, purely on the basis of electronic configurations. Elements are grouped in blocks 's', 'p', 'd' and 'f' depending on the nature of orbital(s) into which the last electron of the atom enters.
Representative or Main Group Elements
These consist of all 's' and 'p' block elements excluding the noble gases (group 18 elements). The chemical properties of the representative elements are determined by the number of valence electrons in their atoms.
Periodic Properties
When the elements are arranged in the order of increasing atomic numbers there is a recurrence of similar properties after certain regular intervals. This regularity is called the periodicity in properties. The distribution of electrons in the various shells determines their physical and chemical character. It has been observed that the properties of the elements depend more on the arrangements of the electrons in the outermost shell (valence shell) and not on the inner shells. This repetition of similar electronic configuration in the outermost or valence shell after certain regular intervals causes periodicity in the properties of elements.
Ionic Radii
These are radii of ions in ionic crystals. Ionic radius may be defined as the effective distance from the center of nucleus of an ion up to which it has an influence on its electron cloud. In ionic compounds the inter nuclear distance may be taken as equal to the sum of the ionic radii of the two ions. The inter nuclear distance in ionic crystals are obtained from X-ray studies.
Ionization Energy
The amount of energy required to remove the most loosely bound electron from an isolated gaseous atom is called ionization energy (IE).
Electron Affinity
Electron affinity is the amount of energy released when an electron is added to an isolated gaseous atom.
Electronegativity
The relative tendency of an atom in a molecule to attract a shared pair of electrons towards itself is termed as Electronegativity. The value of electronegativity of an element describes the ability of its atom to compete for electrons with the other atom to which it is bonded. Electronegativity is however not the property of an isolated atom. Electronegativity is measured on a number of scale levels, the most commonly used are of Pauling or Mulliken.
Valence
The electrons present in the outer most shell are called valence electrons. The valency of an element is defined as its 'combining capacity'.
Electropositive or Metallic Character
The tendency of an element to lose electrons and form positive ions (cations) is called electropositive or metallic character. Elements having low ionization energy are electropositive. For example alkali metals are the most electropositive elements.
Electronegative or Non-metallic Character
The tendency of an element to accept electrons to form an anion is called its non-metallic or electronegative character. Elements that have high electron affinity (or high electronegativity) show greater electronegative or non-metallic character. For example chlorine, oxygen and phosphorous.
Chemical Reactivity and Periodicity in Properties of Compounds
The reactivity of metals decreases from left to right in a period while the reactivity increases down the group. The reactivity of non-metals increases from left to right in a period while the reactivity decreases down the group.
Solubility and Basic Character of Hydroxides
The hydroxides of alkali metals are strongly basic in nature due to their low ionization energy. The basic strength increases down the group due to decrease in ionization energy. The M-OH bond is readily cleaved.
