Existence of Atoms
Atoms of most of the elements exist in the form of molecule or ion, since they are generally 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.
Atoms generally exist in following two forms:
- In the form of molecules
- In the form of ions
Quiz
See Answer
1: (b) Two, 2: (b) Two, 3: (c) 8, 4: (d) Four, 5: (a) Neon, 6: (b) Diatomic, 7: (d) Ozone, 8: (a) HCl, 9: (c) NH3, 10: (d) CaCO3
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. Examples: molecuels of O2 and H2
- Molecules may also be formed by combination of atoms of two or more different elements. Examples: molecules of H2O and NH3
- A molecule takes part in chemical reaction.
- Most of the atoms exist in the form of molecule.
Molecules can be of two types:
- Molecules of elements
- Molecules of Compounds
Molecules of elements
When two of more atoms of the same element combine to form a molecule these are called molecules of element.
Examples:
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 a 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 a 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 a moleucule is formed by the combination of three atoms it is called triatomic molecule. For example: molecule of ozone (O3
Tetra-atomic
When a molecule is formed by the combination of four atoms it is called tetra-atomic molecule. For example: Phosphorous molecule (P4)
Polyatomic
When a 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 |
Molecules of compounds
When a 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 |
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.