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Class 9, Science, Chapter-3, Lecture-2, Atoms and Ions (Notes)

ATOM:

The smallest particle of an element which take part in chemical reaction and may or may not exist independently is called an atom.

IONS:

Electrically charged atoms or groups of atoms are called ions

CATIONS:

A positively charged ion is called cation.
Examples: ${\rm{N}}{{\rm{a}}^{\rm{ + }}}{\rm{, C}}{{\rm{a}}^{{\rm{2 + }}}}{\rm{, M}}{{\rm{g}}^{{\rm{2 + }}}}$

  • Cations are formed by the loss of electrons: ${Na{\rm{  }} \to {\rm{  }}N{a^ + }{\rm{  }} + {\rm{  }}{e^ - }}$
  • Cations have smaller size than the atom.

ANIONS:

A negatively charged ion formed by the gain of electron(s) is known as anion.
Examples: $${\rm{C}}{{\rm{l}}^{\rm{ - }}}{\rm{, }}{{\rm{O}}^{{\rm{2 - }}}}{\rm{, }}{\left( {{\rm{P}}{{\rm{O}}_{\rm{4}}}} \right)^{{\rm{3 - }}}}{\rm{, }}{{\rm{N}}^{{\rm{3 - }}}}$$

  • Anions are formed by the gain of electron(s): ${Cl{\rm{  }} + {\rm{  }}{e^ - }{\rm{  }} \to {\rm{  }}C{l^ - }}$
  • Anions have bigger size than the atom.

Differences Between:

Atom

Ion

Atom is the smallest particle of an element which is neutral

Ion is an atom or a group of atoms carrying an electrical charge.

An atom has equal number of negative charges and positive charges.

An Ion has unequal number of negative charges and positive charges.

Atoms may or may not exist independently.

Ions have independent existence in solutions.

Properties of atoms are those of its elements.

Properties of ions are different from those of its elements.

VALENCY:

The number of electrons that can be lost or gained or shared to get stable configurations is called the valency of the atom.

ELECTROVALENCY:

The number of electrons gained or lost by an atom to obtain a stable configuration is called electrovalency.

COVALENCY:

The number of electrons shared by an atom for obtaining a stable configuration is called covalency.

MOLECULE:

The smallest particle of a substance that exists independently is called a molecule.

Types of Molecules:

  • On the basis of nature of atoms present-
  1. Homoatomic molecules:
    Molecules which are made up of the atoms of one element only are called homoatomic molecules.
    e.g.- ${\rm{He,Ne,}}{{\rm{H}}_{\rm{2}}}{\rm{,C}}{{\rm{l}}_{\rm{2}}}{\rm{,}}{{\rm{O}}_{\rm{2}}}{\rm{,}}{{\rm{N}}_{\rm{2}}}{\rm{,}}{{\rm{O}}_{\rm{3}}}{\rm{,}}{{\rm{P}}_{\rm{4}}}{\rm{,}}{{\rm{S}}_{\rm{8}}}$
  2. Heteroatomic molecules:
    Molecules which are made up of atoms of two or more different elements are called heteroatomic molecules.
    e.g.- ${\rm{C}}{{\rm{O}}_{\rm{2}}}{\rm{,}}{{\rm{N}}_{\rm{2}}}{\rm{O,}}{{\rm{H}}_{\rm{2}}}{\rm{O,}}{{\rm{C}}_{{\rm{12}}}}{{\rm{H}}_{{\rm{22}}}}{{\rm{O}}_{{\rm{11}}}}$
  • On the basis of number of atoms present.
  1. Monoatomic - ${\rm{He, Ne}}$
  2. Diatomic - ${H_2},{\rm{ }}{N_2},{\rm{ }}C{l_2},{\rm{ }}CO$
  3. Triatomic - ${\rm{C}}{{\rm{O}}_{\rm{2}}}{\rm{, }}{{\rm{H}}_{\rm{2}}}{\rm{O, }}{{\rm{O}}_{\rm{3}}}$
  4. Tetra atomic -${{\rm{P}}_{\rm{4}}}{\rm{, }}{{\rm{H}}_{\rm{2}}}{{\rm{O}}_{\rm{2}}}$
  5. Polyatomic - ${{\rm{C}}_{12}}{{\rm{H}}_{22}}{{\rm{O}}_{11}}{\rm{,~ }}{{\rm{C}}_4}{{\rm{H}}_{10}}{\rm{,~ }}{{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}$

ATOMICITY:

The number of atoms present in a molecule is called atomicity.
Atomicity of ${\rm{C}}{{\rm{O}}_{\rm{2}}}$ is 3
Atomicity of ${H_2}S{O_4}$ is 7