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The periodic table is arranged in the order of increasing atomic numbers of the elements. It starts from an alkali metal and end at an inert gas. The next element is again an alkali metal and is placed just below the previous alkali metal. Sodium (Na), Potassium (K), etc are examples of alkali metals and Helium (He), Krypton (Kr), etc are examples of inert gases. 
The horizontal lines of elements form a period and the vertical ones form a group. Elements falling under the same group show similar chemical properties. 
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