Question

The ${{Z}_{effective}}$ for He is?

Answer 1

Hint: When in poly electronic species the net positive charge that an electron experiences is called an effective nuclear charge. It is termed as Z effective. There is the shielding of the electrons present in the lower orbitals. The effective nuclear charge for electrons placed in various orbitals is different due to different shielding.
Formula used:
${{Z}_{effective}}$= Z – S, where Z is atomic number and S is the shielding constant.

Complete answer:
In an atom there are various electrons with a positive center that is the nucleus. The electrons experience some positive nuclear charge from the center. This value varies due to the various levels of orbitals present and the electrons present in them. So, the net positive charges that any electron in a particular orbital experiences is called effective nuclear charge.
The presence of orbitals having electrons from lower to a higher energy level affects the shielding that affects the effective nuclear charge. The orbitals have a different shielding effect that alters the value of the effective nuclear charge on a particular electron.
The formula to find the effective nuclear charge is ${{Z}_{effective}}$= Z – S. We have been given He, whose atomic number Z is 2 with configuration $1{{s}^{2}}$. According to Slater’s rule the shielding constant for 1s electron is 0.30, so
${{Z}_{effective}}$= 2 – 0.30
${{Z}_{effective}}$= 1.70
So, the ${{Z}_{effective}}$ for He is 1.70.

Note:
According to Slater’s rule the electrons in 1s orbital have a shielding constant 0.30, while others have 0.35. For ns and np it will be 0.85 for each electron with principal quantum number (n-1) and for (n-2) it will be 1.00 for each electron. For d and f orbitals it will be 1.00 for each electron. So, for a particular electron the shielding constant for the orbitals preceding that electron are counted and added as S.