Mass Number and Atomic Mass are both atomic properties relevant to the discussion revolving around every element that scientists have discovered to date. These properties have everything to do with the rudimentary atomic particles that make up all matter: electrons, protons, neutrons, and other particles beyond the scope of this article.
Mass Number vs Atomic Mass
The main difference between the two terms Mass Number and Atomic Mass is that Mass Number is an integer stating the total quantity of nucleons in an atomic nucleus. On the other hand, Atomic Mass is the average of nucleons in all the natural isotopes of an element. These are two of the most fundamental properties of every element represented in the periodic table of elements.
The concept of Mass Numbers is relatively easy to grasp. It is simply that property of any element in chemistry which denotes the exact number of nucleons existing in the atom of an element. When paired with atomic numbers, they can give the precise number of electrons, protons, and neutrons that comprise any element’s atom.
Atomic Mass is slightly more complex. It enumerates the mass of the atom of an element by analysing the number of protons and neutrons in the naturally occurring isotopes of any specific element of the periodic table. It also has a close association with the frequency of occurrence of each particular isotope.
Comparison Table Between Mass Number and Atomic Mass
What Is Mass Number?
The mass number is the sum of nucleons (protons, together with neutrons) located in an atomic nucleus.
Mass Number (A) = neutrons (no) + protons (p+)
Case in point, a carbon atom has 6 protons and six neutrons. Consequently, the Mass Number for Carbon is 6+6 = 12.
The mass number is also titled Atomic Mass Number, which leads to further confusion between the two. Since Mass Number is a tally, the result is always an integer. In case one is aware of an element’s atomic number (which denotes the electron number of an atom), they can calculate the neutron number by deduction of the former from the latter.
Neutrons (no) = Mass Number (A) – Atomic Number (Z)
The number of neutrons sometimes vary in different atoms of the same element. However, since these atoms belong to one element, the proton number and electron number are unaltered. Therefore, the Mass Number varies among them. These atoms with different Mass Numbers but the same atomic numbers are called isotopes.
To cite an example, the number of naturally occurring isotopes of the element Hydrogen is three. 1H, 2H, and 3H. The particle distribution in the three is as follows:
Therefore we can see that the isotopes have dissimilar Mass Numbers.
What is Atomic Mass?
As the name suggests, the Atomic Mass (or Atomic Weight) is the complete mass of an atom. An atom comprises electrons, protons, and neutrons. Considering this, the Atomic Mass ought to merely be the totting up of the masses of these three particles. However, Rutherford proved by extensive scientific experiments that the nucleus is where the vast majority of an element’s mass is concentrated. As the nucleus is composed of neutrons and protons, the element’s Atomic Mass is determined by the masses of these nucleonic particles. The mass of electrons is a small value and does not factor in. Instead, it is calculated by averaging the neutron and proton numbers of an element’s natural isotopes. In other words, it is a relative mass, i.e. ratio. The value of Atomic Mass is generally in decimals. Numerically, Mass Number and Atomic Mass are approximately equal. Atomic Mass can also be said to be a multiple of 1/12th of the Atomic Mass of Carbon, i.e. 1/12th of 12 units.
We cite the same example cited for Mass Number: hydrogen with its three isotopes 1H, 2H, and 3H. 99.98% of all the naturally occurring hydrogen on earth is 1H. 1H, 2H, and 3H are combined to procure the Atomic Mass of hydrogen, whose value comes to 1.00784 g/mol.
Main Differences Between Mass Number and Atomic Mass
1. The main difference between Mass Number and Atomic Mass is that the Mass Number is solely the result of neutrons plus protons in a nucleus, and the mass of the atom is called Atomic Mass.
2. The Mass Number is numerically a whole number or integer because it’s a summation result of countable particles. However, since the mass of a particle is rarely a whole number, unsurprisingly, Atomic Mass generally has a decimal value.
3. Mass Numbers don’t have any units. The unit following the value of Atomic Mass may be dalton (Da). It might also be unified mass (u), or sometimes Atomic Mass Unit (AMU).
4. Once an element has been discovered and its properties defined, the Mass Number can never change. However, if and when scientists revise the abundance of the naturally arising isotopes of elements, the Atomic Mass might be recalculated, and the value may alter.
5. The purpose of Mass Number is to state the number of nucleons located in an atom. Atomic Mass, on the other hand, denotes the mass of the entirety of an elemental atom.
6. The reason why the Mass Numbers of the isotopes of a single element differ is that the neutron number varies from isotope to isotope. The Atomic Mass is identical in the case of all the isotopes of an element.
Conclusion
The Mass Number and Atomic Mass are oft-confused properties. Both of these are determined by the neutrons and protons present in the atom of the elements. Since these nucleonic particles influence and sometimes determine an element’s characteristic physical and chemical properties, it is construed that if one knows the Mass Number and Atomic Mass of an element, one can form an idea about the element’s properties. The most fundamental difference between Mass Numbers and Atomic Masses of elements is in the way they are calculated. The mass number is a countable value calculated through the summation of whole numbers and, therefore, is a positive integer. Atomic Mass is a measurable value calculated through the average abundance of the isotopes of an element and therefore is a decimal value.
References
- https://cdnsciencepub.com/doi/abs/10.1139/p57-114
- https://www.sciencedirect.com/science/article/abs/pii/S0375947403018086