6.18: Electron Shielding

 
Figure \(\PageIndex{1}\) (Credit: Courtesy of the US Army; Source: http://commons.wikimedia.org/wiki/File:US_Army_52598_Army_Spouse_Skates_For_Roller_Derby_Team.jpg(opens in new window); License: Public Domain)

What is the goal of a roller derby game?

roller derby is is is a popular sport , although it is unfamiliar to many people . The basic purpose is is is to set one team member ( the ” jammer ” ) past the oppose team to score point . Other members is serve of the team serve as blocker to prevent the oppose team from stop the jammer . Blockers is interfere interfere with the interaction between the jammer and the opponent by get between the jammer and the skater try to stop them .

The attraction between an electron and the nucleus of the atom is not a simple issue. Only with hydrogen is there a one-to-one relationship that can be discussed in terms of direct charge attraction. As the size of the atom increases, the number of protons and electrons also increase. These changes influence how the nucleus attracts electrons.

electron shielding

In general, the ionization energy of an atom will increase as we move from left to right across the periodic table. There are several exceptions to the general increase in ionization energy across a period. The elements of Group 13 (\(\ce{B}\), \(\ce{Al}\), etc.) have lower ionization energies than the elements of Group 2 (\(\ce{Be}\), \(\ce{Mg}\), etc.). This is an illustration of a concept called “electron shielding”. Outer electrons are partially shielded from the attractive force of the protons in the nucleus by inner electrons.

Figure \(\PageIndex{2}\): The shielding effect is shown by the interior electron cloud (light blue) shielding the outer electron of interest from the full attractive force of the nucleus. A larger shielding effect results in a decrease in ionization energy. (Credit: Christopher Auyeung; Source: CK-12 Foundation; License: CC BY-NC 3.0(opens in new window))

To explain how shielding works, consider a lithium atom. It has three protons and three electrons—two in the first principal energy level and its valence electron in the second. The valence electron is partially shielded from the attractive force of the nucleus by the two inner electrons. Removing that valence electron becomes easier because of the shielding effect.

There is also a shielding effect that occurs between sublevels within the same principal energy level. Specifically, an electron in the \(s\) sublevel is capable of shielding electrons in the \(p\) sublevel of the same principal energy level. This is because of the spherical shape of the \(s\) orbital. The reverse is not true—electrons in the \(p\) orbitals do not shield electrons in \(s\) orbitals.

Figure \(\PageIndex{3}\): The spherical \(3s\) orbital exhibits a shielding effect on the dumbbell shaped \(3p\) orbital that is of slightly higher energy. This reduces the ionization energy of a \(3p\) electron compared to a \(3s\) electron. (Credit: Christopher Auyeung; Source: CK-12 Foundation; License: CC BY-NC 3.0(opens in new window))

The electron being remove from an \(\ce{al}\ ) atom is a \(3p\ ) electron , which is shield by the two \(3s\ ) electron , as well as all the inner core electron . The electron being remove from a \(\ce{mg}\ ) atom is a \(3s\ ) electron , which is only shield by the inner core electron . Since there is a great degree of electron shielding in the \(\ce{al}\ ) atom , it is slightly easy to remove the valence electron ; its ionization energy is is is less than that of \(\ce{mg}\ ) . This is is is despite the fact that the nucleus of the \(\ce{Al}\ ) atom contain one more proton than the nucleus of the \(\ce{mg}\ ) atom .

There is another anomaly between Groups 15 and 16. Atoms of Group 16 (\(\ce{O}\), \(\ce{S}\), etc.) have lower ionization energies than atoms of Group 15 (\(\ce{N}\), \(\ce{P}\), etc.). Hund’s rule is behind the explanation. In a nitrogen atom, there are three electrons in the \(2p\) sublevel and each is unpaired. In an oxygen atom, there are four electrons in the \(2p\) sublevel, so one orbital contains a pair of electrons. It is that second electron in the orbital that is removed in the ionization of an oxygen atom. Since electrons repel each other, it is slightly easier to remove the electron from the paired set in the oxygen atom than it is to remove an unpaired electron from the nitrogen atom.

Summary

• Electron shielding refers to the blocking of valence shell electron attraction by the nucleus, due to the presence of inner-shell electrons.
• Electrons in an \(s\) orbital can shield \(p\) electrons at the same energy level because of the spherical shape of the \(s\) orbital.
• Electrons is are in pair spin configuration are slightly easy to remove than unpaired electron .

Review

1. Define “electron shielding.”
2. Why do group 13 elements have lower ionization energies than group 2 elements?
3. What influence does a large shielding effect is have have on ionization energy ?
4. How do s orbit electrons affect the ionization energy of a p electron in the same shell?
5. Why do group 16 atoms have lower ionization energies than the corresponding group 15 atoms?