valence level

Examples of valence level in the following topics:

• Writing Lewis Symbols for Atoms

  • Recall that any valence level can have up to eight electrons, except for the first principal energy level, which can only have two.
  • Neon (Ne), argon (Ar), krypton (Kr), etc., each contain eight electrons in their valence level.
  • Therefore, these elements have a full valence level that has the maximum number of electrons possible.
  • Helium (He), at the very top of this column is an exception because it has two valence electrons; its valence level is the first principal energy level which can only have two electrons, so it has the maximum number of electrons in its valence level as well.
  • We explain this phenomenon by attributing their stability to having a 'full' valence level.

• Introduction to Lewis Structures for Covalent Molecules

  • In covalent molecules, atoms share pairs of electrons in order to achieve a full valence level.
  • Eight electrons fill the valence level for all noble gases, except helium, which has two electrons in its full valence level.
  • These are exceptions to the octet rule because they only require 2 electrons to have a full valence level.
  • After a bond has formed, each F atom has 6 electrons in its valence level which are not used to form a bond.
  • Notice that only the outer (valence level) electrons are involved, and that in each F atom, 6 valence electrons do not participate in bonding.

• Representing Valence Electrons in Lewis Symbols

  • 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.
  • Atoms can achieve this more stable state by having a valence level which contains as many electrons as it can hold.
  • Only the electrons in the valence level are shown using this notation.
  • Electrons that are not in the valence level are not shown in the Lewis symbol.

• Physical Properties of Covalent Molecules

  • The octet rule is a chemical rule that generalizes that atoms of low atomic number (< 20) will combine in a way that results in their having 8 electrons in their valence shells.
  • Having 8 valence electrons is favorable for stability and is similar to the electron configuration of the inert noble gases.
  • A H atom needs one additional electron to fill its valence level, and the halogens need one more electron to fill the octet in their valence levels.
  • Lewis bonding theory states that these atoms will share their valence electrons, effectively allowing each atom to create its own octet.

• Atomic Size

  • As the atomic number increases along each row of the periodic table, the additional electrons go into the same outermost principal energy level (also known as valence level).
  • Experiments have shown that the first case is what happens: the increase in nuclear charge overcomes the repulsion between the additional electrons in the valence level.
  • The principal energy levels hold electrons at increasing radii from the nucleus.
  • In a noble gas, the outermost level is completely filled; therefore, the additional electron that the following alkali metal (Group I) possesses will go into the next principal energy level, accounting for the increase in the atomic radius.

• The Incomplete Octet

  • Valence electrons can be counted using a Lewis electron dot diagram.
  • For example, with the duet rule of the first principal energy level, the noble gas helium, He, has two electrons in its outer level.
  • Since there is no 1p subshell, 1s is followed immediately by 2s, and thus level 1 can only have at most two valence electrons.
  • The bonding is relatively simple to model with a Lewis structure if we allow each valence level electron in the boron atom to be shared in a covalent bond with each fluorine atom.
  • This covalent compound (NH3BF3) shows that boron can have an octet of electrons in its valence level.

• Expectancy Theory

  • the desirability of the result (known as valence) to the individual
  • Vroom introduces three variables within his expectancy theory: valence (V), expectancy (E), and instrumentality (I).
  • These three components of expectancy theory (expectancy, instrumentality, and valence) fit together in this fashion:
  • Instrumentality is low when the outcome is the same for all possible levels of performance.
  • V(O): Valence is the value individuals place on outcomes (O) based on their needs, goals, values, and sources of motivation.

• The Shielding Effect and Effective Nuclear Charge

  • The shielding effect explains why valence shell electrons are more easily removed from the atom.
  • The more shielding that occurs, the further the valence shell can spread out.
  • The outer energy level is n = 3 and there is one valence electron.
  • The valence shell is shell 2 and contains 8 valence electrons.
  • Thus the number of nonvalence electrons is 2 (10 total electrons - 8 valence).

• Electron Configurations of Atoms

  • The study of organic chemistry must at some point extend to the molecular level, for the physical and chemical properties of a substance are ultimately explained in terms of the structure and bonding of molecules.
  • According to the Aufbau principle, the electrons of an atom occupy quantum levels or orbitals starting from the lowest energy level, and proceeding to the highest, with each orbital holding a maximum of two paired electrons (opposite spins).
  • 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.
  • These atoms have only one electron in the valence shell, and on losing this electron arrive at the lower shell valence octet.

• sp Hybridization

  • When atomic orbitals hybridize, the valence electrons occupy the newly created orbitals.
  • The hybridization process involves mixing of the valence s orbital with one of the valence p orbitals to yield two equivalent sp hybrid orbitals that are oriented in a linear geometry.
  • The electronic differences in an isolated Be atom and in the bonded Be atom can be illustrated using an orbital energy-level diagram.
  • The left orbital energy-level diagram shows both electrons of Be in the 2s orbital.
  • The right energy-level diagrams shows sp hybridized orbitals on Be in the linear BeCl2 molecule.