valence

Examples of valence in the following topics:

• Writing Lewis Symbols for Atoms

  • Since we have established that the number of valence electrons determines the chemical reactivity of an element, the table orders the elements by number of valence electrons.
  • Each of these elements has one valence electron.
  • Proceeding to the column labeled 'III', we find that those elements (B, Al, Ga, In, ...) have three valence electrons in their outermost or valence level.
  • 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.
  • Helium is one of the noble gases and contains a full valence shell.

• Bonding in Coordination Compounds: Valence Bond Theory

  • Valence bond theory is used to explain covalent bond formation in many molecules.
  • Valence bond theory is a synthesis of early understandings of how chemical bonds form.
  • Valence bond structures are similar to Lewis structures, except where a single Lewis structure is insufficient, several valence bond structures can be used.
  • It is in this aspect of valence bond theory that we see the concept of resonance.
  • Calculate the theoretical hybridization of a metal in a coordination complex based on valence bond theory

• 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.

• 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).
  • These illustrations use a simple Bohr notation, with valence electrons designated by colored dots.
  • Non-bonding valence electrons are shown as dots.
  • 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:

• Formal Charge and Lewis Structure

  • The total number of valence electrons in the entire compound is equal to the sum of the valence electrons of each atom in the compound.
  • Non-valence electrons are not represented when drawing the Lewis structures.
  • Valence electrons are placed as lone pairs (two electrons) around each atom.
  • For example, CO2 is a neutral molecule with 16 total valence electrons.
  • FC = 6 valence electrons - (4 non-bonding valence electrons + 4/2 electrons in covalent bonds)

• 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.
  • Other elements in the periodic table react to form bonds in which valence electrons are exchanged or shared in order to achieve a valence level which is filled, just like in the noble gases.
  • Notice that only the outer (valence level) electrons are involved, and that in each F atom, 6 valence electrons do not participate in bonding.
  • Four covalent bonds are formed so that C has an octet of valence electrons, and each H has two valence electrons—one from the carbon atom and one from one of the hydrogen atoms.

• Covalent Bonds

  • A fluorine atom has seven valence electrons.
  • Carbon will then have five valence electrons (its four and the one its sharing with fluorine).
  • Covalent bonds are a class of chemical bonds where valence electrons are shared between two atoms, typically two nonmetals.
  • A fluorine atom has seven valence electrons.
  • Carbon will then have five valence electrons (its four and the one its sharing with fluorine).

• Analysis of Molecular Formulas

  • The number of hydrogen atoms that can be bonded to a given number of carbon atoms is limited by the valence of carbon.
  • The presence of oxygen (valence = 2) does not change this relationship, so the previously described C4H10O isomers follow the rule, n=4 & 2n + 2 = 10.
  • Halogen atoms (valence = 1) should be counted equivalent to hydrogen, as illustrated by C3H5Cl3, n = 3 & 2n + 2 = 8 = (5 + 3).
  • If nitrogen is present, each nitrogen atom (valence = 3) increases the maximum number of hydrogens by one.
  • For stable organic compounds the total number of odd-valenced atoms is even.

• 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).

• Explanation of Valence Bond Theory

  • Valence bond theory states that overlap between two atomic orbitals forms a covalent bond between two atoms.
  • 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.
  • 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.