What is the purpose of the gas constant r

HI! Here is the DEFINITION
gas constant (universal molar gas constant)
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The gas constant, also known as the universal molar gas constant, is a physical constant that appears in an equation defining the behavior of a gas under theoretically ideal conditions. The gas constant is, by convention, symbolized R.The pressure, volume, amount of substance, and temperature of a gas are related according to the following equation, known as the universal gas equation:PV = NRTwhere P represents the pressure in pascals, V represents the volume in meters cubed, N represents the amount of substance in moles, and T represents the absolute temperature in kelvins.In terms of the pressure, volume, amount of substance, and temperature, the gas constant is equal to:R = (PV) / (NT)The gas constant has been found, by experiment, to have a value of approximately 8.3145 joules per kelvin per mole (J · K-1 · mol-1).Also see Table of Physical Constants.

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.