What does 'sigma' signify in Slater's rules?

In Slater's rules, 'sigma' represents the total shielding constant that accounts for the effective nuclear charge experienced by an electron in a multi-electron atom. It is indeed calculated as the sum of the number of electrons present in various orbitals, each multiplied by specific weighting coefficients that reflect the shielding effect of those electrons.

These coefficients are based on the principal quantum number and the type of orbital, which helps in understanding how different electrons influence the effective nuclear charge felt by a particular electron in the atom. By using these coefficients, you can evaluate how well inner electrons shield the outer electrons from the nuclear charge, ultimately affecting their energy levels and reactivity.

The other options do not accurately describe the role of 'sigma' in Slater's rules. For instance, while shielding does have to do with the outermost shell, it is just a broader aspect of what 'sigma' quantifies specifically. Neutrons and total magnetic moment are not part of Slater's rules and therefore are not relevant in this context.