Page 36 - ICSE Chemistry 6
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ATOMS AND MOLECULES


                 Atom

                 An element is composed of minute par  cles called atoms. An atom is the smallest unit of an element
                 that shows all the proper  es of that element. Some atoms are capable of independent existence while
                 others are found in combina  on with atoms
                                                                     Know Your Scientist
                 of the same element or diff erent elements.
                 For example, an atom of sodium can exist            John Dalton was an English chemist.
                 freely but an atom of hydrogen cannot               He developed the modern atomic
                 exist independently and hence combines              theory from which he concluded
                 with another atom of hydrogen to form a             that elements are made of extremely
                 molecule.                                           small par  cles called atoms.

                 Characteristics of an Atom

                     • The atoms are very reac  ve and therefore exist in combina  on with the atoms of same element or
                        other elements. For example, hydrogen atoms are found in combina  on with atoms of hydrogen
                        or other elements. Noble gases, on the other hand, exist independently.

                     • The atoms of an element are iden  cal and exhibit same proper  es.
                     • The atoms of an element diff er from those of other elements.

                     • The proper  es of an atom are maintained through all physical and chemical changes.
                        For example, a piece of iron when broken down into smaller pieces will retain its property of being
                        a  racted towards a magnet.

                 Molecule

                 A molecule is the smallest par  cle of an element or a compound that exists independently and shows all
                 the proper  es of that substance.

                 Characteristics of a Molecule

                     • The atoms of same element or diff erent elements combine to form molecules. When two or more
                        atoms of the same element combine they form the molecule of an element. When atoms of two
                        or more elements combine they form the molecule of a compound (Table 2.6).
                     • The atoms of diff erent elements are combined in a fi xed whole number ra  o in a molecule.
                     • The atoms in a molecule are held together by chemical bonds. A chemical bond refers to the force
                        of a  rac  on between atoms that enables the forma  on of chemical compounds.


                                             Diagramma  c Representa  on of Some Molecules

                                 Molecules of elements                               Molecules of compounds


                                        N   N                                                H     Cl

                                 Nitrogen molecule (N )                          Hydrogen chloride molecule (HCl)
                                                    2


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