Why shielding effect of electrons makes cation formation easy?
Answers
Learning Objective
Calculate effective nuclear charges experienced by valence electrons.
Key Points
The shielding effect describes the balance between the pull of the protons on valence electrons and the repulsion forces from inner electrons.
The shielding effect explains why valence-shell electrons are more easily removed from the atom. The effect also explains atomic size. The more shielding, the further the valence shell can spread out and the bigger atoms will be.
The effective nuclear charge is the net positive charge experienced by valence electrons. It can be approximated by the equation: Zeff = Z – S, where Z is the atomic number and S is the number of shielding electrons.
Terms
effective nuclear chargeThat experienced by an electron in a multi-electron atom, typically less for electrons that are shielded by core electrons.
nucleusThe positively charged central part of an atom, made up of protons and neutrons.
core electronsThose that are not part of the valence shell and as such, are not involved in bonding.
valence shell electron pair repulsion theoryA set of rules used to predict the shape of individual molecules.
cationA positively charged ion, as opposed to an anion.
valence shellThe outermost shell of electrons in an atom; these electrons take part in bonding with other atoms.
anionA negatively charged ion, as opposed to a cation.
The Shielding Effect
Electrons in an atom can shield each other from the pull of the nucleus. This effect, called the shielding effect, describes the decrease in attraction between an electron and the nucleus in any atom with more than one electron shell. The more electron shells there are, the greater the shielding effect experienced by the outermost electrons.
In hydrogen-like atoms, which have just one electron, the net force on the electron is as large as the electric attraction from the nucleus. However, when more electrons are involved, each electron (in the n-shell) feels not only the electromagnetic attraction from the positive nucleus but also repulsion forces from other electrons in shells from 1 to n-1. This causes the net electrostatic force on electrons in outer shells to be significantly smaller in magnitude. Therefore, these electrons are not as strongly bound as electrons closer to the nucleus.
The shielding effect explains why valence shell electrons are more easily removed from the atom. The nucleus can pull the valence shell in tighter when the attraction is strong and less tight when the attraction is weakened. The more shielding that occurs, the further the valence shell can spread out. As a result, atoms will be larger.
Example:
Why is cesium bigger than elemental sodium?
Solution:
The element sodium has the electron configuration 1s22s22p63s1. The outer energy level is n = 3 and there is one valence electron. The attraction between this lone valence electron and the nucleus with 11 protons is shielded by the other 10 core electrons.
The electron configuration for cesium is 1s22s22p63s23p64s23d104p65s24d105p66s1. While there are more protons in a cesium atom, there are also many more electrons shielding the outer electron from the nucleus. The outermost electron, 6s1, therefore, is held very loosely. Because of shielding, the nucleus has less control over this 6s1 electron than it does over a 3s1 electron.
Effective Nuclear Charge
The magnitude of the shielding effect is difficult to calculate precisely. As an approximation, we can estimate the effective nuclear charge on each electron.
Effective nuclear charge diagramDiagram of the concept of effective nuclear charge based on electron shielding.
The effective nuclear charge (often symbolized as Zeff or Z*) is the net positive cha
Answer:
When the shielding effect comes into action, due to less attractive force between the valence electron and nucleus of the atom, the electrons can be removed easily. Due to the easy removal of the electron, the cation formation is facilitated with less effort.