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The atomicity of a substance is the number of atoms that each molecule of the substance has.

To illustrate this with an example, consider the reaction between hydrogen and oxygen to form water.

2H2 + O2 --> 2H2O

Two molecule of hydrogen react with one molecule of oxygen to form...

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The atomicity of a substance is the number of atoms that each molecule of the substance has.

To illustrate this with an example, consider the reaction between hydrogen and oxygen to form water.

2H2 + O2 --> 2H2O

Two molecule of hydrogen react with one molecule of oxygen to form two molecules of water. Each molecule of hydrogen has two atoms of hydrogen, the atomicity of hydrogen is two. Similarly each molecule of oxygen contains two atoms of oxygen, the atomicity of oxygen is two. One molecule of water contains two atoms of hydrogen and one atom of oxygen, the atomicity of water is three.

In gases, mono-atomic gases have molecules that are made of only one atom, this is shown by the inert gases. Diatomic gases have two atoms in each molecule, examples are hydrogen, carbon monoxide, chlorine, etc. Triatomic gases have three atoms in a molecule, examples are ozone, nitrogen dioxide, carbon dioxide, etc.

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Atomicity can be defined as the number of atoms in a molecule. Atomicity can be defined for both elements as well as other substances. For example, nitrogen gas is made up of two individual atoms of the element nitrogen and has a chemical formula of N2. That is, each molecule of nitrogen gas contains 2 atoms and thus, nitrogen gas has an atomicity of 2. We can similarly calculate the atomicity of other elements. 

Water is made up of atoms of oxygen and hydrogen, mixed in a fixed 1:2 ratio (1 atom of oxygen and 2 atoms of hydrogen). It has a chemical formula of H2O and since it contains 2 atoms of hydrogen and 1 atom of oxygen, each water molecule has an atomicity of 3. We can similarly calculate the atomicity of other substance.

Hope this helps. 

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