A neutral atom of the isotope chlorine-37 contains 17 protons, 20 neutrons
PHYSICS · Multi-part · A Level · Physics · Particles and radiation · Particles and radiation
Problem
A neutral atom of the isotope chlorine-37 contains 17 protons, 20 neutrons, and 17 electrons. The standard atomic mass listed for chlorine-37 is very close to 37 u, where 1 u is defined as one-twelfth the mass of a carbon-12 atom.
Using only qualitative reasoning based on the information given above and any generally accepted physical constants that need not be numerically specified, address the issues raised in the parts that follow.
(1) Describe two distinct physical reasons why the tabulated atomic mass of chlorine-37 is extremely close to, yet not exactly equal to, 37 u.
(2) Explain why the removal of one electron from a chlorine-37 atom (forming the ion Cl⁻¹) changes the total mass of the system by a value that is typically ignored in most chemical mass calculations.
(3) Discuss whether the relative contribution of electron mass to the total mass of an atom becomes more or less significant as the atomic number increases, and justify your answer qualitatively.