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Introduction
The physical and the chemical properties of elements are periodic functions of their atomic numbers.
Main Features of Modern Periodic Table
1. There are 18 vertical columns or groups. These are further sub-divided into A and B (groups I to VII), VIII group and zero group.
2. Member of the same group have similar electronic configuration of the valence shell and thus show same valency.
3. Elements of groups IA to VIIA are called groups of typical elements, representative elements or normal elements.
Periodicity of Elements
The recurrence of similar properties of the elements when they are arranged in the order of increasing atomic number, after certain regular intervals, is called periodicity.
Variation of Properties
The periodic trends of the following properties will be studied here.
* Atomic size
* Metallic and Non-metallic character
* Ionization potential
Atomic Size and Atomic Radius
The distance from the centre of the nucleus to the outermost shell of an atom is called the atomic radius of that atom.
Metallic and Non-metallic Character
The tendency of an element to lose electrons and form positive ions (cations) is called electropositive or metallic character. For e.g., alkali metals are the most electropositive elements.
Ionization Potential
Ionization potential (or ionization energy) is the amount of energy required to remove one or more electrons from the outermost shell of an isolated atom in the gaseous state.
Atom(g) + IE 
Positive ion(g) + electron(g)
Electron Affinity
Electron affinity is the amount of energy released when an electron is added to an isolated gaseous atom.
Electronegativity
Electronegativity is the tendency of an atom to attract electrons towards itself in a molecule of a compound. 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.
Relation Between Atomic Number and Atomic Mass
While studying the periodic table, it will be noticed that with the increase in the atomic number, there is an increase in atomic mass as well (with some exceptions). Usually an element with high atomic number has high atomic mass as well.
Summary
Periodic table is an arrangement of elements in the increasing order of atomic number such that elements with similar properties fall under same vertical column.
