formal charge

(noun)

The charge assigned to an atom in a molecule, assuming that electrons in a chemical bond are shared equally between atoms. This helps determine which of a few Lewis structures is most correct.

Related Terms

  • resonant structures
  • octet rule
  • resonance structure

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.
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