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MCAT Content / Covalent Bonds / Partial Ionic Character

Partial Ionic Character

Topic: Covalent Bonds

If the electronegativities of two atoms in a polar covalent bond differ vastly, the bond is said to have partial ionic character.

Compounds are frequently classified by the bonds between constituent atoms. There are multiple kinds of attractive forces, including covalent, ionic, and metallic bonds. The bond formed between any two atoms is not purely ionic. All bonding interactions have some covalent character because the electron density remains shared between the atoms. The degree of ionic versus the covalent character of a bond is determined by the difference in electronegativity between the constituent atoms.

Electronegativity is a property that describes the tendency of an atom to attract electrons (or electron density) toward itself. An atom’s electronegativity is affected by both its atomic number and the size of the atom. The higher its electronegativity, the more an element attracts electrons.

The larger the difference in electronegativity between two elements in a bond, the more ionic the nature of the bond. In the conventional presentation, bonds are designated as ionic when the ionic aspect is greater than the covalent aspect of the bond. Bonds that fall in between the two extremes, having both ionic and covalent character, are classified as polar covalent bonds. Such bonds are thought of as consisting of partially charged positive and negative poles.

There are two ways to look at polarity:

  • Bond polarity: when atoms from different elements are covalently bonded, the shared pair of electrons will be attracted more strongly to the atom with the higher electronegativity. As a result, the electrons will not be shared equally. Such bonds are said to be ‘polar’ and possess partial ionic character.
  • Molecular polarity: when an entire molecule, which can be made out of several covalent bonds, has a net polarity, with one end having a higher concentration of negative charge and another end having a surplus of a positive charge. A polar molecule acts as an electric dipole which can interact with electric fields that are created artificially, or that arise from interactions with nearby ions or other polar molecules.

Dipoles are conventionally represented as arrows pointing in the direction of the negative end. The strength of a dipole’s interaction with an electric field is given by the electric dipole moment of the bond or molecule. The dipole moment is calculated by evaluating the product of the magnitude of separated charge, q, and the bond length, r. 

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Key Points

• Bonding between elements can be viewed on a spectrum of ionic to covalent character.

• Bonds which are not purely ionic or covalent due to differences in electronegativity are polar molecules.

• Polarity can be in a bond when the two elements have a different electronegativity or in a molecule when one atom draws the majority of the electron density creating an area of negative charge.

• Dipole moments can be used to measure the strength of the dipole by multiplying the separated charge and bond length.

Key Terms

Electronegativity: The tendency of an atom or molecule to attract electrons and form bonds.

Bond polarity: A covalent bond is polar if one atom is more electronegative than its bonding partner, resulting in a net dipole moment between the two atoms.

Dipole moment: A measure of the polarity of a covalent bond or an entire molecule. It is the product of the charge on either pole of the dipole and the distance separating them.

Molecular polarity: A molecule is polar if it has a net dipole moment, which depends on the existence of polar covalent bonds and the molecule’s three-dimensional structure or geometry.

Dipole: Usually refers to the separation of charges within a molecule between two covalently bonded atoms or atoms that share an ionic bond

Polar covalent bonds: A covalent bond between two atoms where the electrons forming the bond are unequally distributed.

Covalent bond: The interatomic linkage that results from the sharing of an electron pair between two atoms.

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