9th science

Atoms and Molecules

Existence of Atoms

Atoms of most of the elements exist in the form of molecule or ion, since they are most reactive. For example, hydrogen, oxygen, chlorine, etc. However, atoms of some elements, which are non-reactive, exist in free-state in nature. For example helium, neon, argon, etc.

Usually atoms exist in following two forms:

  • In the form of molecules
  • In the form of ions

Molecule:

  • Molecule is the smallest particle of a compound.
  • Atoms exist in free states in the form of molecule.
  • Molecule may be formed by the combination of two or more similar atoms of an element, such asoxygen molecule is formed by the combination of two oxygen atoms, molecule of hydrogen which is formed by the combination of two hydrogen atoms.
  • Molecules may be formed by the combination of atoms of two or more different elements. For example molecule of water. It is formed by the combination of two atoms of hydrogen and one atom of oxygen. Molecule of Nitric oxide or nitrogen monoxide. It is formed by the combination of one nitrogen atom and one oxygen atom.
  • A molecule takes part in chemical reaction.
  • Most of the atoms exist in the form of molecule. Molecules are formed by the combination of two or more elements.

Example: Molecule of hydrogen (H2, Molecule of oxygen (O2), Molecule of nitrogen (N2), etc.

Molecules can be of two types:

  • Molecules of elements
  • Molecules of Compounds

Molecules of elements: When two of more atoms of same element combine to form a molecule these are called molecules of element.

Example:

Hydrogen molecule (H2): Molecule of hydrogen is formed by the combination of two hydrogen atoms.

Oxygen molecule (O2): Molecule of oxygen is formed by the combination of two oxygen atoms.

Sulphur molecule (S8): Molecule of sulphur is formed by the combination of eight sulphur atoms.

Phosphoroud molecule (P4): Molecule of phosphorous is formed by the combination of four phosphorous atoms.

Molecules of some non-reactive elements are formed by single atom. For example – helium, neon, argon, etc. molecules: Molecules of metals formed as big cluster of atoms. They are represented by their symbols simply. For example: Iron (Fe), Copper (Cu), Zinc (Zn), etc. These molecules are known as giant molecules.

Carbon is a non-metal, but it also exists as giant molecule and represented by its symbol ‘C’.


Atomicity

Monoatomic: When molecule is formed by single atom only, it is called monoatomic molecule. Generally noble gas forms monoatomic molecules. For example: Helium (He), Neon (Ne), Argon (Ar), Kr (Krypton), Xenon (Xe), Randon (Rn).

Diatomic: When molecule is formed by the cDiatomic: When molecule is formed by the combination of two atoms of it is called diatomic molecule. For example: Hydrogen (H2), Oxygen (O2), Nitrogen (N2), Chlorine (Cl2), etc.

Triatomic: When moleucule is formed by the combination of three atoms it is called triatomic molecule. For example: molecule of ozone (O3

Tetra-atomic: When molecule is formed by the combination of four atoms it is called tetra-atomic molecule. For example: Phosphorous molecule (P4)

Polyatomic:- When molecule is formed by the combination of more than two atoms, it is called polyatomic molecule. For example: Sulphur molecule (S8)

Atomicity of Some Elements
NameAtomicityFormula
ArgonMonoatomicAr
HeliumMonoatomicHe
OxygenDiatomicO2
HydrogenDiatomicH2
NitrogenDiatomicN2
ChlorineDiatomicCl2
PhosphorusTetra-atomicP4
SulphurPoly-atomicS8


Molecules of compounds:When molecule is formed by the combination of two or more atoms of different elements, it is called the molecule of compound.

Example: Molecule of water (H2O) Molecule of water is formed by the combination of two hydrogen and one oxygen atoms.

Molecules of Some Compounds
CompoundCombining Elements
Water (H2)Hydrogen, Oxygen
Ammonia (NH3)Nitrogen, Hydrogen
Carbon dioxide (CO2)Carbon, Oxygen
Hydrogen Chloride (HCl)Hydrogen, Chlorine
Methane (CH4)Carbon, Hydrogen
Ethane (C2H6)Carbon, Hydrogen
Sodium Chloride (NaCl)Sodium, Chlorine
Copper Oxide (CuO)Copper, Oxygen

Ions:

Atoms of several elements exists in the form of ion. Atoms or molecule with negative or positive charge over them are called ions.

Example: Sodium ion (Na+), Potassium ion (K+), Chloride ion (Cl), Fluoride ion (F), etc.

Cations:Ions having positive charge over them are called cations. 

Example: Sodium ion (Na+), Potassium ion (K+), Hydrogen ion (H+), etc.

Anions:Ions having negative charge over them are called anions.

Example: Chloride ion (Cl), Fluoride ion (F), Carbonate ion (CO3– –), etc.


Monoatomic ions:Ions formed by one atom are called monoatomic ions.

Example: Sodium ion (Na+), Potassium ion (K+), Chloride ion (Cl), Fluoride ion (F), etc.

Polyatomic ions:Ions formed by two or more atoms are called polyatomic ions.

Example: Hydroxide ion (OH), Ammonium ion (NH4+), Carbonate ion (CO3– –), etc.



Introduction

Atomic Mass

Existence of Atoms

Mole Concept

In Text Solution

NCERT Solution