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MASS NUMBER- Definition, Depiction, Usage, Practice Problems and Frequently Asked Questions.
We have often seen vegetable vendors use weighing stones to measure a fruit or vegetable of choice. Hence we often say, “two kilos of mangoes” or “five kg of potatoes”. This can be understood as a certain number multiplied by our reference weight measure, which is one kilogram. This weight measure is standardised not only according to the international standards, but also our convenience.
In a similar fashion, when we consider atoms, the weighing stone is often a nucleon. Is it, “a proton or neutron?”, will be your next question. But remember that there is a very small difference between their masses, so we use the mass of proton as our weighing stone or reference. This means that if we want to talk about the nitrogen atom, we can say it has 7 protons and 7 neutrons. It is helpful to know that for most calculations, it is okay to ignore the mass of electrons as they have negligible mass and most of the mass of an atom is in the nucleus. Hence, I can talk about its mass as equal to that of 7 + 7 = 14 protons. This system is way more convenient than saying 2.32 x 10-23 g is the mass of one nitrogen atom. In fact this system is the skeleton of interpretation behind the idea of “mass number”. Let us explore this concept in detail.
TABLE OF CONTENTS:
DEFINITION
The total number of protons and neutrons in an atomic nucleus is known as the mass number. It is often known as the atomic mass number or nucleon number. When expressed in terms of atomic mass units, it is roughly equivalent to the atom's atomic mass and always a whole number.
Example: Most chlorine atoms on Earth have 17 protons and 18 neutrons. Hence its mass number is 17 + 18 = 35.
Mass number is a dimensionless quantity. As the name suggests, it is just a “number”. It is also different for different nuclides.
DEPICTION
Generally atoms are depicted in multiple ways. If we want to continue talking about the nuclide taken above, we can write it as Chlorine-35, Cl-35 or even 
As already discussed, 17 is the atomic number or proton number, commonly represented by Z. 35 is the mass number, generally denoted by A.
- Since electrons are much lighter than protons and neutrons, they are not included in the mass calculation.
- Mass number is always a positive integral value.
USAGE
- Using mass number simplifies all stoichiometric calculations in theoretical chemistry.
- It helps us pinpoint the exact isotope under discussion.
PRACTICE PROBLEMS
- What is the mass number of an element which can be depicted as
?Ans: As we already know, the general representation of an element is as 
, where A is the mass number, Z is the atomic number and X is the element itself. Hence when we compare this notation to , we can see that mass number = A = 191. Hence, the mass number of the given isotope of Osmium is 191. Osmium is known to be the densest substance on Earth with a density of 22 g/cc. - How can we get the number of protons from the mass number?Ans: Mass number i.e. A is a measure of the number of nucleons. Hence we can say that A = number of protons + number of neutrons. We know that number of protons is simply the atomic number of the element i.e. Z. Hence, A = number of neutrons + Z. Thus, we can say that the number of neutrons = A - Z.
- Can the mass number be zero?Ans: Since mass number talks about the number of nucleons present, it is not possible for it to take a zero or negative value. Logically it is not possible for the number of nucleons to be zero. We also need to remember that mass numbers can not take fractional values.
- What parameters remain constant in a set of isotopes and what parameters change?Ans: Consider a pair of isotopes oxygen-16 and oxygen-18. Here, we know that since the chemical element is oxygen, we can say that Z is the same and Z = 8. But we can see that the mass number is different. Hence, we see that A = 16 and A = 18. So we can see that in a pair of isotopes, atomic number remains constant whereas mass number changes as a consequence of the number of neutrons changing.
FREQUENTLY ASKED QUESTIONS
Q1. What is relative atomic mass?
Ans: Relative atomic mass of an element can be understood at the weighted average of masses of individual atoms along with their isotopic concentrations in a given sample. Since it is also a ratio of the atomic mass with respect to a reference element atomic mass, it is dimensionless quantity.
Q2. What is the most abundant element, by mass?
Ans: In the Earth’s crust, oxygen is the most abundant element by mass. It makes up about 46% of all of the planet’s mass, forming an important component of both organic and inorganic matter. However in the universe as we know it, Hydrogen is the most abundant element.
Q3. Who discovered the atom?
Ans: A few centuries ago, brilliant scientist John Dalton proposed the existence of the atom. Even before Dalton, there was centuries worth of discussion. Countless theories have been noted since and numerous developments have been observed. However, Dalton is credited with the discovery of the atom.
Q4. Can we break a proton?
Ans: As per latest scientific developments, protons are made up of smaller particles called quarks. They are held together by gluons. It is noteworthy that all subatomic particles are made of quarks but the number, orientation and kind of quarks are different. While the existence of a proton was first theorised in 1815 by William Prout, it wasn’t until Rutherford’s famous alpha particle scattering experiment that the world heard of “protons” in atoms in the years 1917 - 1920.
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