std xi-chem-ch1-concepts: elements-atoms-molecules

Standard/ Class/ Grade XI ChemistryChapter 1 Basic ConceptsGurudatta K Wagh, gkwagh@gmail.com

Elements, Atoms and Molecules Elements 118 (natural + synthesised/ man made) Atom• Smallest indivisible particle of an element• May or may not have free existence• Definite mass 10-26 kg• Spherical shape, radius 10-15 m• Smallest atom – hydrogen, mass 1.667 X 10-26 kg• Atoms react to form compounds

Compound• Composed of atoms of two or more elements in a fixed

proportion• Properties of constituent atoms and the compound formed are

different e.g. Carbon burns in oxygen to form carbon dioxide. Carbon is combustible, oxygen helps combustion but the product carbon dioxide extinguishes fire. Similarly, hydrogen is inflammable and catches fire whereas oxygen helps combustion but the compound water helps to put off fire.

Molecule• Smallest unit of a compound• Has all the properties of original compound• Cannot be divided into constituent by simple methods• Is an aggregate of two or more atoms of definite composition

which are held together by chemical bonds• May contain atoms of same elements or different elements