Shielding Effect

Shielding effect is a concept in chemistry, which describes the effect of core electrons on the valence electrons. The former shields the latter from the nuclear charge of the nucleus. Read the following article to gain more information about this subject.
The most fundamental or basic building element of any matter is called an atom. It is mainly constituted of protons, electrons, and neutrons. Of these subatomic particles, the protons and the neutrons form the structure of the atomic nucleus, and electrons move around the center in defined orbits. The number of orbits around an electron is often known as an electron cloud. Now, the electrons contain the negative charge, and protons carry the positive charge. The neutrons are neutral in charge, with a slightly higher mass than protons. In a neutral atom, the number of protons and electrons are equal, and this number defines the atomic number of the atom.

If you observe the atomic structure, then you will find that the distribution of electrons in different shell within the atom are different. Precisely, some electrons are in orbits near the atomic nucleus, and some are positioned away from the nucleus. As per the Aufbau principle of electronic configuration, the electrons arrange themselves in an order of increasing energy, which implies that the ones nearer to the nucleus will have greater attraction (or nuclear charge), than the ones in the outer orbits. Thus, a shielding effect arises when the inner electrons, due to a high attraction, do not allow the nuclear charge to pass through.

Electron Shielding Effect

Definition:

Shielding effect can be defined as a reduction in the effective nuclear charge on the electron cloud, due to a difference in the attraction forces of the electrons on the nucleus.

This chemistry concept is also popularly known as atomic shielding or screening effect. If an atom contains only one electron, it represents the absolute charge of the nucleus. However, if there are more electrons, then as per the electronic configuration, the ones in orbits nearer to the nucleus are attracted more towards the positive charge in the nucleus. But, the electrons in the outer orbits, although attracted towards the nuclear charge, are repulsed by the immediate negatively charged electrons in the intermediate orbits, due to which there is a palpable reduction in the effective nuclear charge of an atom. Let's say, the electrons in the innermost orbit contain the maximum charge, and then in the next orbit, the charge will be less than in the innermost one, and so on. The electrons in the outermost orbit contain the least nuclear charge, and are more loosely bound by the nucleus, than the ones in the innermost shell. This also is a reason as to why chemical reactions are balanced so that the atoms attain stability in nuclear charge. The total amount of nuclear charge in an atom can be calculated as:

ZEff = Z - S,

where,
Z: number of protons
S: average number of electrons between the nucleus and the inner orbit of electrons.

This effect is solely responsible for controlling the size of an atom. Also, the magnitude of the atomic shielding depends on the number of electrons in the inner orbits. Thus, higher the number of inner electrons, stronger will the effect. Hence, there is a screening effect constant to define this value, denoted by symbol σ. To calculate this constant, one must be aware of the following two concepts:

Electron orbitals
An atom basically contains sub-levels, each consisting of some defined number of orbitals (a space occupied with 2 electrons). Thus, it is evident that each sub-level as per the electronic configuration contains a different number of orbitals, and hence, different number of electrons. There are basically 4 sub-levels in an atom, and their electron containing capacity is listed below.

SublevelNumber of OrbitsNumber of Electrons
s12
p36
d510
f714

The number of orbitals can be used to calculate the energy level of an atom; this number is represented as n2, where 'n' is the sub-level. Say, if 'p' has 3 sub-levels, then it contains 9 electrons. This way, total number of electrons in an atom can be calculated as 2n2. Thus, if the sub-level is 4, then the number of electrons is 2x42 = 32 electrons.

Slater rules
In 1930, John C Slater devised a method to calculate the shielding/screening constant for evaluating the shielding effect. On the basis of the above-defined electronic configuration, the electronic structure of the atom is written as : (1s) (2s, 2p) (3s, 3p) (3d) (4s, 4p) (4d) (4f) (5s, 5p), etc. Here, the electrons in the higher group (i.e., towards the right side in the expression), also known as valence electrons, do not shield the electrons in the inner orbitals.

For the valence electrons pertaining to 's' and 'p' energy levels, the screening constant increases with respect to the inner orbitals, which means that the outer electrons of the same shell (n) contribute a value of 0.35, the electrons in the next smaller shell (n-1) contribute a value of 0.85, and the electrons in the subsequent smaller shells contribute a value of 1.00. Similarly, electrons in the same shells of 'd' and 'f' energy levels, contribute to a value of 0.35, and electrons to the left (inner orbitals), contribute to a value of 1.00.

Calculating the Shielding Effect Constant
This constant can be calculated as a sum of all the contribution values in all the energy levels.

S (σ) = 1.00 N2 + 0.85 N1 + 0.35 N0,

where,
N0 = Number of electrons in the same shell n
N1 = Number of electrons in the inner shell n-1
N2 = Number of electrons in the inner shell n-2
(When the value of n > 1), otherwise, S = 0.35.

So, this way, the effective nuclear charge can be calculated as: Z - S. This concept will further help you in understanding the balance of many complex chemical equations. Also, solving exercises related to this subject will provide you with a much clearer idea about the subject of atomic shielding.