1.4 : Ligações Químicas

Atoms participate in a chemical bond formationto acquire a completed valence-shell electron configuration similar to that of the noble gas nearest to it in atomic number. Ionic, covalent, and metallic bonds are some of the important types of chemical bonds. Bond energy and bond length determine the strength of a chemical bond.

Types of Chemical Bonds

An ionic bond is formed due to electrostatic attraction between cations and anions. Often, the ions are formed by the transfer of electrons from one participating atom to the other. However, these bonds do not have a defined directionality because the electrostatic force of attraction is distributed uniformly throughout the three-dimensional space.

A covalent bond is a chemical bond formed by the sharing of electron pairs between adjacent atoms. The shared pair of electrons is called the bonding pair. Covalent bonds are directional in nature.

A metallic bond is formed between two metal atoms. Metallic bonding is described by the “Electron Sea model”. Based on the low ionization energies of metals, the model states that metal atoms lose their valence electrons easily and become cations. These valence electrons create a pool of the delocalized electrons surrounding the cations over the entire metal.

Bond Energies and Bond Length

The strength of a covalent bond is measured by the energy required to break it—that is, the energy necessary to separate the bonded atoms. Separating any pair of bonded atoms requires energy. The stronger a bond, the greater the energy required to break it.

The energy required to break a specific covalent bond in one mole of gaseous molecules is called the bond energy or the bond dissociation energy. The bond energy for a diatomic molecule is defined as the standard enthalpy change for the endothermic reaction. Molecules with three or more atoms have two or more bonds. The sum of all bond energies in such a molecule is equal to the standard enthalpy change for the endothermic reaction that breaks all the bonds in the molecule.

The strength of a bond between two atoms increases as the number of electron pairs in the bond increases. Generally, the greater the number of bonds between two atoms, the shorter the bond length and the greater the bond strength. Thus, triple bonds are stronger and shorter than double bonds between the same two atoms; likewise, double bonds are stronger and shorter than single bonds between the same two atoms. When one atom bonds to various atoms in a group, the bond strength typically decreases as we move down the group.

This text is adapted fromOpenstax, Chemistry 2e, Section 7.1: Ionic Bonding,Openstax, Section 7.2: Covalent Bonding,Section 10.5: The Solid State of Matter, andSection7.5. Bond Strength: Covalent Bonds.