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-
- Homoatomic molecules:
Molecules which are made up of the atoms of one element only are called homoatomic molecules.
e.g.- ${\rm{He,Ne,}}{{\rm{H}}_{\rm{2}}}{\rm{,C}}{{\rm{l}}_{\rm{2}}}{\rm{,}}{{\rm{O}}_{\rm{2}}}{\rm{,}}{{\rm{N}}_{\rm{2}}}{\rm{,}}{{\rm{O}}_{\rm{3}}}{\rm{,}}{{\rm{P}}_{\rm{4}}}{\rm{,}}{{\rm{S}}_{\rm{8}}}$ - 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.
- Monoatomic - ${\rm{He, Ne}}$
- Diatomic - ${H_2},{\rm{ }}{N_2},{\rm{ }}C{l_2},{\rm{ }}CO$
- Triatomic - ${\rm{C}}{{\rm{O}}_{\rm{2}}}{\rm{, }}{{\rm{H}}_{\rm{2}}}{\rm{O, }}{{\rm{O}}_{\rm{3}}}$
- Tetra atomic -${{\rm{P}}_{\rm{4}}}{\rm{, }}{{\rm{H}}_{\rm{2}}}{{\rm{O}}_{\rm{2}}}$
- 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