equation 10.18. this equation cannot be applied to many-electron atoms. one way to modify it for the more complex atoms is to replace z with ^z 2 σh, where z is the atomic number and σ is a positive dimensionless quantity called the shielding constant. consider the helium atom as an example. the physical significance of σ is that it represents the extent of shielding that the two 1s electrons exert on each other. thus, the quantity ^z 2 σh is appropriately called the effective nuclear charge. calculate the value of σ if the first ionization energy of helium is 3.94 × 10−18 j per atom. ^in your calculations, ignore the minus sign in the given equation. h