Theory:

We know the difference between an atom, an element and a compound. Let us now learn about the term called 'atomicity'. 
In chemistry, atomicity refers to the total number of atoms present in a single molecule of an element, compound, or substance. The number of atoms present in a single molecule is termed its atomicity.
We can distinguish the molecules respective to the number of elements they combined to form a molecule.
Let's look at how to figure out while calculating the atomicity of elements and compounds:
Example:
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\(O + O \rightarrow O_2\)
Here, \(O\) refers to Oxygen atom.
Therefore, to calculate the atomicity, simply add the number of atoms together \(1 + 1 \ = 2\)
Oxygen is a diatomic molecule, meaning that each molecule contains two atoms, giving it an atomicity of two.
 
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\(Cl + Cl \rightarrow Cl_2\)
Here, \(Cl\) refers to Chlorine atom.
\(1 + 1 \ = 2\)
Chlorine is a diatomic molecule. It means that each molecule contains two atoms, giving it an atomicity of two.
 
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 \(Na\)
Here, \(Na\) refers to sodium atom.
\(1\) atom.
Sodium is a monoatomic element. It means that each molecule contains one atom, giving it atomicity of one.
 
For a molecule with multiple elements,
 
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\(O_2 + O \rightarrow O_3\)
Here, \(O\) refers to Oxygen atom, and \(O_2\) refers to oxygen molecule.
\(2 + 1 \ = 3\)
Ozone is a triatomic molecule, meaning that each molecule contains three atoms, giving it an atomicity of three.
 
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 \(C + O_2 \rightarrow CO_2\)
Here, \(C\) refers to Carbon atom, and \(O_2\) refers to oxygen molecule.
\(1 + 2 \ = 3\)
Carbon dioxide is a triatomic compound, meaning that each molecule contains three atoms, giving it an atomicity of three.
 
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\(H_2SO_4\)
Here, \(H_2\) refers to hydrogen, and \(O_4\) refers to oxygen, and \(S\) refers to sulphur.
\(2 + 1 + 4 \ = 7\)
Sulphuric acid is a compound, giving it an atomicity of seven.
 
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\(H_2O\)
Here, \(H_2\) refers to hydrogen, and \(O\) refers to oxygen.
\(2 + 1 \ = 3\)
Water is a compound, giving it an atomicity of three.
 
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\(NaCl\)
Here, \(Na\) refers to sodium atom, and \(Cl\) refers to chlorine atom.
\(1 + 1 \ = 2\)
Sodium Chloride is a compound, giving it an atomicity of two.
Few more examples for elements and their atomicity.
Classification of elements
Name of the elements
Symbol
Atomicity
 Metals
Aluminum
\(Al\)
\(1\)
 
Copper
\(Cu\)
\(1\)
 
Iron
\(Fe\)
\(1\)
Non-metals
Helium
\(He\)
\(1\)
 
Hydrogen
\(H_2\)
\(2\)
 
Nitrogen
\(N_2\)
\(2\)
 
Fluorine 
\(F_2\)
\(2\)
 
Phosphorous
\(P_4\)
\(4\)
Reference: