valence electrons

Examples of valence electrons 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.
  • 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.
  • Lewis symbols for the elements depict the number of valence electrons as dots.
  • 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.
  • 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.
  • Electrons that are not in the valence level are not shown in the Lewis symbol.
  • Each of the four valence electrons is represented as a dot.

• Covalent Bonds

  • A fluorine atom has seven valence electrons.
  • If it shares one electron with a carbon atom (which has four valence electrons), the fluorine will have a full octet (its seven electrons plus the one it is sharing with carbon).
  • Carbon will then have five valence electrons (its four and the one its sharing with fluorine).
  • A fluorine atom has seven valence electrons.
  • Carbon will then have five valence electrons (its four and the one its sharing with fluorine).

• 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)

• Ionic Bonds

  • This exchange of valence electrons allows ions to achieve electron configurations that mimic those of the noble gases, satisfying the octet rule.
  • The octet rule states that an atom is most stable when there are eight electrons in its valence shell.
  • Atoms with less than eight electrons tend to satisfy the duet rule, having two electrons in their valence shell.
  • Fluorine has seven valence electrons and usually forms the F - ion because it gains one electron to satisfy the octet rule.
  • Fluorine has seven valence electrons and as such, usually forms the F- ion because it gains one electron to satisfy the octet rule.

• 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.
  • It therefore has 7 valence electrons and only needs 1 more in order to have an octet.
  • 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.

• The Shielding Effect and Effective Nuclear Charge

  • The shielding effect, approximated by the effective nuclear charge, is due to inner electrons shielding valence electrons from the nucleus.
  • The shielding effect explains why valence shell electrons are more easily removed from the atom.
  • 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).

• Physical Properties of Covalent Molecules

  • These cases of electron sharing can be predicted by the octet rule.
  • 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.

• 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).
  • Covalent bonding occurs by a sharing of valence electrons, rather than an outright electron transfer.
  • 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.

• Bonding in Coordination Compounds: Valence Bond Theory

  • According to Pauling's theory, a covalent bond is formed between two atoms by the overlap of their half-filled valence orbitals, each of which contains one unpaired electron.
  • Sigma bonds occur when the like orbitals of shared electrons overlap.
  • For instance, when two s-orbital electrons overlap, we see a sigma bond.
  • When we apply valence bond theory to a coordination compound, the original electrons from the d orbital of the transition metal move into non-hybridized d orbitals.
  • The electrons donated by the ligand move into hybridized orbitals of higher energy, which are then filled by electron pairs donated by the ligand.