Question

how can you tell if electron screening is small or large between elements?

Answer 1

The magnitude of shielding is calculated by Slater's Rule. It allows you to estimate the effective nuclear charge from protons in the nucleus and the effective shielding of electrons in each orbital. Slater gave a set of rules which can determine the shielding experienced by any electron. The steps are as follows:

1. Step 1: Write the electron configuration of the atom in the following form: (1s) (2s, 2p) (3s, 3p) (3d) (4s, 4p) (4d) (4f) (5s, 5p) . . .
2. Step 2: Identify the electron of interest, and ignore all electrons in higher groups (to the right in the list from Step 1). These do not shield electrons in lower groups.
3. Step 3: Slater's Rules is now broken into two cases:

The shielding experienced by an s- or p- electron,

• electrons within same group shield 0.35, except the 1s which shield 0.30
• electrons within the n-1 group shield 0.85
• electrons within the n-2 or lower groups shield 1.00

​ The shielding experienced by nd or nf valence electrons

• electrons within same group shield 0.35
• electrons within the lower groups shield 1.00

​Lets solve an example to make it more clear:

Calculating the shielding constant S of 2p electron of Oxygen:

Shielding constant is the sum of shielding experienced by the electron of interest by all the electrons in lower shells.

Step 1: write the electronic configuration according to Slater's rule

O = (1s²)(2s²,2p⁴)   

Calculating S

1. Electrons within the same group shield to an extent of 0.35 and we have 5 electrons in the same shell excluding our electron of interest, therefore 0.35x5 2. Electrons in (n-1) shield to an extent of 0.85 and we 2 electrons in (n-1)shell, therefore 0.85x2

S = (0.35x5) + (0.85x2) = 3.45

Hope I cleared your doubt!