Theory:

A molecule is the smallest particle of an element or compound capable of an independent life and exhibiting all of the substance's properties.
 
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In general, a molecule is a set of two or more atoms that are chemically bound or held together by attractive forces.
Atoms from the same or different elements may combine to form a molecule.
Example:
  • To understand it simply, consider a water moleculeH2O, which has two elements hydrogen and oxygen. These two hydrogen and one oxygen atoms combined to form a water molecule.
  • Calcium oxide \(CaO\) contains two elements calcium and oxygen. One atom of calcium and oxygen each combine to form a calcium oxide molecule.
  • Oxygen \(O_2\) where two oxygen atoms make up this molecule.
  • Chlorine \(Cl_2\) where two chlorine atoms make up this molecule.
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Molecules of elements:
The atoms that make up a molecule of elements are of the same nature.
Many elements' molecule are made up of just one electron, such as argon \(Ar\), helium \(He\), neon \(Ne\) and so on.
  • Monoatomic Elements:
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In their molecular form, certain elements are monatomic, meaning they are made up of just one atom.
 
Helium \(He\) is an example of a monoatomic element.
  • Diatomic Elements:
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If the molecule constitutes two atoms, we can classify it as a diatomic molecule. In their molecular shape, other elements have two or more atoms.
 
For example, each molecule of hydrogen \(H_2\), oxygen \(O_2\), and chlorine \(Cl_2\) has two atoms.
  • Triatomic Elements:
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If another atom is combined with O2, it forms ozone (O3), which is a triatomic molecule.
Note:
  • Monoatomic and diatomic molecules are more stable than triatomic molecules.
  • Most non-metals are diatomic molecules.
Molecules of compounds:
Compound molecules contain atoms from two or more separate elements.
 
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Example:
For example, Water \(H_2O\) has three atoms: Two hydrogen (H) atoms and one oxygen (O) atom.
 
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Methane  \(CH_4\) with five atoms: One carbon (C) and four hydrogen (H) atoms respectively.
 
Glucose  \(C_6H_{12}O_6\) contains the elements of carbon \(6\), hydrogen \(12\) and oxygen \(6\) atoms combined to form a glucose molecule.
 
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Sodium chloride \(NaCl\) contains the two elements of hydrogen and oxygen atoms combined to form a sodium chloride molecule.