What is the Difference Between Atomic Number and Atomicity

The key difference between atomic number and atomicity is that atomic number refers to the number of protons in the nucleus of an atom whereas atomicity refers to the number of atoms that make up a particular molecule.

Atomic number and atomicity are two different terms that describe two different phenomena. Atomic number is the number of protons that exists in the nucleus of an atom, while atomicity is the total number of atoms present in a molecule.

1. Overview and Key Difference
2. What is Atomic Number
3. What is Atomicity
4. Atomic Number vs Atomicity in Tabular Form
5. Summary – Atomic Number vs Atomicity

What is Atomic Number?

Atomic number is the number of protons that exists in the nucleus of an atom. Therefore, it is also known as the proton number. The atomic number is unique to a particular chemical element. This makes it easy to identify a chemical element using the proton number. This value is identical to the charge number of the nucleus. Moreover, if we consider an uncharged atom, the number of electrons is also similar to the atomic number.

The mass number of an atom is similar to the sum of the number of protons and neutrons of an atom. Typically, protons and neutrons have nearly similar masses. The mass defect of nucleon binding is always small when compared to the nucleon mass. Therefore, the atomic mass of any atom occurs within 1% of the whole number that is expressed in unified atomic mass units.

What is Atomicity?

Atomicity is the total number of atoms present in a molecule. The molecules can be monoatomic, diatomic, triatomic, or polyatomic. Monoatomic molecules have only one atom as the molecule. These are mostly the noble gases that have their electron configurations completed. Diatomic molecules have two atoms per molecule. Similarly, triatomic molecules have three atoms per molecule. Moreover, polyatomic molecules have more than three atoms per molecule. The followings are some examples of these different molecule types.

Monoatomic: He, Ne, Ar, Kr, etc.

Diatomic: H2, N2, O2, F2, and Cl2.

Triatomic: O3

Polyatomic: P4, S8

Sometimes, atomicity is used in the same sense as valency. We can use this term to refer to the maximum number of valencies that are observed for an element. Typically, all metals and some other elements, including carbon, have complex structures, where a large, infinite number of atoms are bonded to each other. Therefore, we usually express their atomicity as 1.

In any homonuclear molecule, the atomicity can be determined as a ratio of molecular weight and atomic weight. E.g. molecular weight of an oxygen molecule is approximately 31.999. The atomic weight of one oxygen molecule is approximately 15.999. By dividing 31.999 from 15.999, we get the answer 2, which means the atomicity of the oxygen molecule is 2.

What is the Difference Between Atomic Number and Atomicity?

Atomic number and atomicity are two different terms that describe two different phenomena. The key difference between atomic number and atomicity is that atomic number refers to the number of protons in the nucleus of atom, whereas atomicity refers to the number of atoms that make up a particular molecule.

Below is a summary of the difference between atomic number and atomicity in tabular form for side by side comparison.

Summary – Atomic Number vs Atomicity

Atomic number and atomicity are two different terms that describe two different phenomena. The key difference between atomic number and atomicity is that atomic number refers to the number of protons in the nucleus of atom whereas atomicity refers to the number of atoms that make up a particular molecule.