Question

What causes shielding effect?

Answer 1

A greater number of electron between the nucleus and the electron in question.

Explanation:

To understand the shielding effect, consider `Z`eff or the effective nuclear charge.

The value of `Z`effective will stay constant as you go across a period and increase as you travel down. Why do you think this is?
To understand this, try think of the atomic orbital theory.

Remember that `Z`eff = Z - S

where Z is the atomic number and S is the amount of electron between the nucleus and the valence electron.

Example: Why is Cesium a larger element that Potassium?

Cs = `1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^6 5s^2 4d^10 5p^6 6s^1`

K = `1s^2 2s^2 2p^6 3s^1 3p^6 4s^1`

As you can see there are more orbitals separating the valence electrons from the nucleus of the atom, so the nucleus will have less control of the `6s^1` electron than it will of the `4s^1` electron. This is the shielding affect.