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)
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