valence shell

Examples of valence shell in the following topics:

• Electron Configurations of Atoms

  • The highest occupied electron shell is called the valence shell, and the electrons occupying this shell are called valence electrons.
  • Helium is unique since its valence shell consists of a single s-orbital.
  • The other members of group 8 have a characteristic valence shell electron octet (ns2 + npx2 + npy2 + npz2).
  • In their chemical reactions halogen atoms achieve a valence shell octet by capturing or borrowing the eighth electron from another atom or molecule.
  • These atoms have only one electron in the valence shell, and on losing this electron arrive at the lower shell valence octet.

• Electron Configuration of Cations and Anions

  • Monatomic ions are formed by the addition or removal of electrons from an atom's valence shell.
  • For example, Group 1 element sodium (Na) has a single electron in its valence shell, with full shells of 2 and 8 electrons beneath.
  • Sodium could gain electrons, but it would require seven more to achieve a full valence shell.
  • On the other hand, a chlorine atom (Cl) has seven electrons in its valence shell, which is one short of a stable, full shell with 8 electrons.
  • When combined, the uncharged atoms can exchange electrons and in doing so, achieve complete valence shells.

• Electron Shells and the Bohr Model

  • This is known as the octet rule which states that, with the exception of the innermost shell, atoms are more stable energetically when they have eight electrons in their valence shell, the outermost electron shell.
  • An atom may gain or lose electrons to achieve a full valence shell, the most stable electron configuration.
  • Group 18 elements (helium, neon, and argon are shown) have a full outer, or valence, shell.
  • A full valence shell is the most stable electron configuration.
  • Elements in other groups have partially-filled valence shells and gain or lose electrons to achieve a stable electron configuration.

• The Shielding Effect and Effective Nuclear Charge

  • 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.
  • The valence shell is shell 2 and contains 8 valence electrons.
  • Once again, the electron configuration is the same as in the previous examples and the number of nonvalence electrons is 2 (by losing one electron, the valence shell becomes the n=2 shell).

• Formal Charge and Lewis Structure

  • Generally, most Lewis structures follow the octet rule; they will share electrons until they achieve 8 electrons in their outermost valence shell.
  • However, there are exceptions to the octet rule, such as boron, which is stable with only 6 electrons in its valence shell.
  • The elements hydrogen (H) and helium (He) follow the duet rule, which says their outermost valence shell is full with 2 electrons in it.
  • Because the bonding pair is shared, the atom that had the lone pair still has an octet, and the other atom gains two or more electrons in its valence shell.
  • FC = 6 valence electrons - (4 non-bonding valence electrons + 4/2 electrons in covalent bonds)

• Chemical Bonding & Valence

  • Transfer of the lone 3s electron of a sodium atom to the half-filled 3p orbital of a chlorine atom generates a sodium cation (neon valence shell) and a chloride anion (argon valence shell).
  • Non-bonding valence electrons are shown as dots.
  • Boron compounds such as BH3 and BF3 are exceptional in that conventional covalent bonding does not expand the valence shell occupancy of boron to an octet.
  • The number of valence shell electrons an atom must gain or lose to achieve a valence octet is called valence.
  • From the formulas written above, we arrive at the following general valence assignments:

• Explanation of Valence Bond Theory

  • Each hydrogen atom needs one more electron to complete its valence energy shell.
  • The nitrogen atom needs three more electrons to complete its valence energy shell.
  • The nitrogen atom will share three of its electrons so that each of the hydrogen atoms now has a complete valence shell.
  • Each of the hydrogen atoms will share its electron with the nitrogen atom to complete its valence shell.
  • To complete their valence shells, they bond and share one electron with each other.

• The Incomplete Octet

  • The octet rule states that atoms below atomic number 20 tend to combine so that they each have eight electrons in their valence shells, which gives them the same electronic configuration as a noble gas.
  • Valence electrons can be counted using a Lewis electron dot diagram.
  • This leads to hydrogen and lithium both having two electrons in their valence shell—the same electronic configuration as helium—when they form molecules by bonding to other elements.
  • In this compound, the boron atom only has six valence shell electrons, but the octet rule is satisfied by the fluorine atoms.
  • However, the stability of aluminum hydride ions (AlH4-) indicates that Al can also support an octet of valence shell electrons.

• Writing Lewis Symbols for Atoms

  • Each of these elements has one valence electron.
  • Helium is one of the noble gases and contains a full valence shell.
  • Electrons can inhabit a number of energy shells.
  • Different shells are different distances from the nucleus.
  • The electrons in the outermost electron shell are called valence electrons, and are responsible for many of the chemical properties of an atom.

• Representing Valence Electrons in Lewis Symbols

  • Lewis symbols use dots to visually represent the valence electrons of an atom.
  • Chemical reactivity of all of the different elements in the periodic table depends on the number of electrons in that last, outermost level, called the valence level or valence shell.
  • In the case of gold, there is only one valence electron in its valence level.
  • Only the electrons in the valence level are shown using this notation.
  • Each of the four valence electrons is represented as a dot.