formal charge

Examples of formal charge in the following topics:

• Formal Charge and Lewis Structure

  • To assist with this problem, chemists often calculate the formal charge of each atom.
  • The formal charge is the electric charge an atom would have if all the electrons were shared equally.
  • The formal charge of an atom can be determined by the following formula:
  • The oxygen atom in carbon dioxide has a formal charge of 0.
  • Depending on the compound, the shifting of electrons may cause a change in formal charges.

• Charge Distribution in Molecules

  • In the formula for ozone the central oxygen atom has three bonds and a full positive charge while the right hand oxygen has a single bond and is negatively charged.
  • The overall charge of the ozone molecule is therefore zero.
  • Similarly, nitromethane has a positive-charged nitrogen and a negative-charged oxygen, the total molecular charge again being zero.
  • Finally, azide anion has two negative-charged nitrogens and one positive-charged nitrogen, the total charge being minus one.
  • The formal charge on an atom may also be calculated by the following formula:

• Nucleophilicity of Sulfur Compounds

  • Remarkably, sulfoxides (equation # 2), sulfinate salts (# 3) and sulfite anion (# 4) also alkylate on sulfur, despite the partial negative formal charge on oxygen and partial positive charge on sulfur.

• Resonance

  • If the double bond is broken heterolytically, formal charge pairs result, as shown in the other two structures.
  • The preferred charge distribution will have the positive charge on the less electronegative atom (carbon) and the negative charge on the more electronegative atom (oxygen).
  • Formal charge separation.
  • Electronegativity of charge bearing atoms and charge density.
  • (High charge density is destabilizing.

• Oxidation States

  • Oxidation state is the hypothetical charge of an atom if all of its bonds to other atoms were completely ionic.
  • For a simple (monoatomic) ion, the oxidation state is equal to the net charge on the ion.
  • Do not confuse the formal charge on an atom with its formal oxidation state, as these may be different (and often are different, in polyatomic ions).
  • For example, the charge on the nitrogen atom in ammonium ion NH4+ is 1+, but the formal oxidation state is -3—the same as it is for nitrogen in ammonia.
  • In the case between ammonium and ammonia, the formal charge on the N atom changes, but its oxidation state does not.

• Coloring Agents

  • The electronic structure can be described by a relatively ionic model that ascribes formal charges to the metals and ligands.
  • The chemical applications of group theory can aid in the understanding of crystal or ligand field theory, by allowing simple, symmetry-based solutions to the formal equations.
  • A charge transfer band entails promotion of an electron from a metal-based orbital into an empty ligand-based orbital (Metal-to-Ligand Charge Transfer or MLCT).
  • The overall charge of the system remains the same, but the localization of the electron changes.
  • Discuss the relationship between charge transfer and the color of a metal complex.

• Ions

  • The positive electric charge of a proton is equal in magnitude to the negative charge of an electron; therefore, the net electric charge of an ion is equal to its number of protons minus its number of electrons.
  • They are repelled by like electric charges and are attracted to opposite charges.
  • Conventionally the net charge is written with the magnitude before the sign; the magnitude of singly charged molecules/atoms is generally omitted.
  • Monoatomic ions are sometimes also represented by Roman numerals, which designate the formal oxidation state of the element, whereas the superscripted numerals denote the net charge.
  • Sodium has a +1 charge because sodium has eleven electrons.

• Electron Configuration of Cations and Anions

  • These charged atoms are known as ions.
  • Having gained a positive charge, the sodium ion is called a cation.
  • The net charge is written with the magnitude before the sign, that is, a doubly charged cation is indicated as 2+ instead of +2.
  • However, the magnitude of the charge is omitted for singly charged molecules or atoms; for example, the sodium cation is indicated as Na+ and not Na1+.
  • The Roman numeral designates the formal oxidation state of an element, whereas the superscripted numerals denotes the net charge.

• Naming Ionic Compounds

  • Usually, the positively charged portion consists of metal cations and the negatively charged portion is an anion or polyatomic ion.
  • But to be considered an ion, they must carry a positive or negative charge.
  • For cations that take on multiple charges (typically transition metals), the charge is written using Roman numerals in parentheses immediately following the element name.
  • Since the net charge of the ionic compound must be zero, the Cu ion has a 2+ charge.
  • Systematic names are formal names that are always used in print.

• Oxidation Numbers of Metals in Coordination Compounds

  • In simple ions, the oxidation number of the atom is the charge on the ion.
  • The total charge of the oxygens is -6.
  • Because there are 4 oxygen atoms, the total charge of the oxygens is -8.
  • Potassium has an oxidation number of +1, giving an overall charge of +2.
  • The only compounds in which gallium has a formal oxidation state of +2 are dimeric compounds, such as [Ga2Cl6]2−, which contain a Ga-Ga bond formed from the unpaired electron on each Ga atom.