Class 9 Science
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:
Example: Molecule of hydrogen (H2, Molecule of oxygen (O2), Molecule of nitrogen (N2), etc.
Molecules can be of two types:
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’.
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 | ||
|---|---|---|
| Name | Atomicity | Formula |
| Argon | Monoatomic | Ar |
| Helium | Monoatomic | He |
| Oxygen | Diatomic | O2 |
| Hydrogen | Diatomic | H2 |
| Nitrogen | Diatomic | N2 |
| Chlorine | Diatomic | Cl2 |
| Phosphorus | Tetra-atomic | P4 |
| Sulphur | Poly-atomic | S8 |
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 | |
|---|---|
| Compound | Combining 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 |
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.
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