### 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:
1. $$O + O \rightarrow O_2$$
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.

2. $$Cl + Cl \rightarrow Cl_2$$
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.

3. $$Na$$
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,

1 $$O_2 + O \rightarrow O_3$$
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.

2 $$C + O_2 \rightarrow CO_2$$
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.

3. $$H_2SO_4$$
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.

4. $$H_2O$$
Here, $$H_2$$ refers to hydrogen, and $$O$$ refers to oxygen.
$$2 + 1 \ = 3$$
Water is a compound, giving it an atomicity of three.

5. $$NaCl$$
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 $$P_4$$ $$4$$
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