Question

Argon atoms have a mass number of 40, but a relative atomic mass of 39.948 why is this so?

Answer 1

Hint :To answer this question, we first need to understand what atoms are. The smallest unit of ordinary matter that makes up a chemical element is an atom. Atoms that are neutral or ionized make up every solid, liquid, gas, and plasma. Atoms are incredibly small, measuring about 100 picometers in diameter.

Complete Step By Step Answer:
Mass number – The total number of protons and neutrons in an atomic nucleus is known as the mass number, often known as the atomic mass number or nucleon number. It's about the same as the atom's atomic mass expressed in atomic mass units. Because protons and neutrons are both baryons, their mass number A is the same as the nucleus' baryon number B. (and also of the whole atom or ion). For each isotope of a chemical element, the mass number is different. As a result, the number of neutrons is equal to the difference between the mass number and the atomic number Z.
Atomic mass - The mass of an atom is its atomic mass. Although the kilograms (symbol: kg) is the SI unit of mass, the atomic mass is sometimes stated in the non-SI unit Dalton, which is defined as 1/12 of the mass of a single carbon-12 atom at rest. The nucleus' protons and neutrons account for virtually all of an atom's total mass, with electrons and nuclear binding energy playing a modest role. As a result, when stated in Daltons, the numeric value of the atomic mass is roughly equal to the mass number. The atomic mass constant can be used to convert mass in kilograms to mass in Daltons.
Reason - The atomic masses of a chemical element's naturally occurring isotopes determine its average atomic mass. The atomic number, or the number of protons in an atom's nucleus, determines the chemical element's identity. Any atom with a nucleus containing 19 protons is an argon atom.
$$\begin{gathered} massnumber=A=Z+no.ofneutronsno.ofneutrons=A-Z=40-18=22 \\ Theaverageatomicmassoftheelementargonwillbe \\ avg.atomicmass=\sum i\left(isotopei\times abundancei\right) \\ Averageatomicmass=35.96755u\times 0.003336+37.96273u\times 0.000629+39.96238u\times 0.996035 \\ Averageatomicmass=39.948u \end{gathered}$$
So, we conclude that Argon atoms have a mass number of 40, but a relative atomic mass of 39.948 is proved in the above solution.

Note :
The mass of a certain chemical element or chemical compound (g) divided by the amount of substance is the molar mass (mol). The molar mass of a compound can be estimated by summing the constituent atoms' standard atomic masses (in g/mol).