Chemical Bonding and Molecular Structure

Student Expectations

The student is expected to construct electron dot formulas to illustrate ionic and covalent bonds and describe the nature of metallic bonding and apply the theory to explain metallic properties such as thermal and electrical conductivity, malleability, and ductility AND expected to predict molecular structure for molecules with linear, trigonal planar, or tetrahedral electron pair geometries using Valance Shell Electron Pair Repulsion (VSEPR) theory.

Key Concepts

• • Electron dot formulas are used to illustrate how bonds are formed between the valence electrons of atoms. In ionic bonds, the anions and cations are shown in brackets with their respective charges. Ionic bonds form formula units. In covalent bonds, the valence electrons of each atom are shared and are localized between two atoms. Starting with the central atom, the electron pairs are placed around each atom in order to fulfill the octet rule for each atom. Covalent bonds form molecules.

  • In metallic bonding, the electrons are delocalized in that they do not remain close to any one atom. In the solid state, the valence electrons of the metal atoms move freely among the monatomic cations within the structure, forming what is known as the “electron sea”. Metallic bonding is formed due to the attraction of the electrons for the metal cations.

  • The delocalized electrons in metallic bonding makes these types of bonds more flexible than ionic or covalent bonds, allowing metals to be more ductile and malleable than ionic or covalent substances. As a result of the electrons’ ability to move freely among the cations, metals are good thermal and electrical conductors.

  • The Valence Shell Electron Pair Repulsion (VSEPR) model is used to predict the geometric shape of a molecule. A bond angle is formed by a central atom and the atoms that surround it. Any unbonded pairs of electrons that may also be present around the central atom will affect the bond angle.

CHEMICAL BONDING AND MOLECULAR STRUCTURE

Electron Dot Formulas Illustrate Bonding

Electron dot formulas, also known as Lewis valence electron dot formulas or Lewis structures, are used to illustrate how bonds are formed between the valence electrons of atoms. These formulas are represented as dots around the chemical symbol of the central atom. Valence electrons are the negatively charged particles in the outermost energy shell of an atom. More specifically, the electron dot formulas represent the total number of electrons in the highest occupied s and p energy sublevels of an atom’s electron orbitals. Electron dot formulas show the structure of a molecule, ion, or formula unit.

Ionic Bond Electron Dot Formulas

In ionic bonds, one or more valence electrons move from one atom (usually a metal) to another atom (usually a nonmetal). The atom that loses electrons becomes a positively charged ion, or cation. The atom that gains electrons becomes a negatively charged ion, or anion. The attraction between the newly formed cation and anion results in the formation of an ionic bond. In ionic bonds, the anions and cations are shown in brackets with their respective charges. For example, sodium chloride (NaCl) is an ionic compound of sodium (Na) and chlorine (Cl). The electron dot structure of sodium contains one electron, and the electron dot structure of chlorine contains seven electrons. To form an ionic bond, one electron transfers from sodium to chlorine. This gives the anion, chloride, a stable nucleus surrounded by eight valence electrons. It also gives the cation, sodium, a stable nucleus surrounded by eight valence electrons. The basic unit of ionic bonds is known as a formula unit. The formula unit of sodium chloride is a sodium cation and a chloride anion.

Covalent Bond Electron Dot Formulas

In covalent bonds, the valence electrons of each atom are shared and are localized between two atoms. Starting with the central atom, the electron pairs are placed around each atom in order to fulfill the octet rule. Two important exceptions to the octet rule are hydrogen and helium as each of these atoms can only have a maximum of two valence electrons.

Molecules can also form between atoms that share more than one electron pair. For example, in a molecule of oxygen gas (O₂), each oxygen atom (O) has six valence electrons, but it needs two electrons to complete its valence orbital. Thus, two electron pairs are shared between the two oxygen atoms. The sharing of two electron pairs between atoms is known as a double bond.

Similarly, the nitrogen atoms (N) in a molecule of nitrogen gas (N₂), share three electron pairs. This type of covalent bond is known as a triple bond. Double and triple bonds may also occur between different elements.

Metallic Bonding and the Electron Sea

In metallic bonding, the valence electrons are delocalized. This means that they do not remain close to any one particular atom. In the solid state, the valence electrons of metal atoms move freely among the monatomic cations within the structure, forming what is known as the electron sea.“ These bonds, which occur between closely grouped metal atoms, are the strongest type of chemical bond. Examples of substances that contain metallic bonds are gold bars, sheets of aluminum foil, cooking pans, and copper wires.

Metallic Properties

Metallic bonding is formed from the attraction of the valence electrons to the metal cations. Metallic bonding is used in comparison with covalent bonds. As a result of the electrons' ability to move freely among the cations, metals are good thermal and electrical conductors. The structure of the electron sea in a metallic bond determines many of the other properties observed in metals. The fact that the electrons are able to move freely throughout the electron sea and that the cations are able to slide past each other make metallic bonds more flexible than both ionic and covalent bonds. This flexibility makes metals both malleable (able to be stretched and bent into shapes) and ductile (able to be shaped into long, thin wires).

Molecular Geometry

The valence shell electron pair repulsion (VSEPR) model is used to predict the geometric shape of a molecule. According to the VSEPR theory, electrons have negative charges and repel each other, so the electrons in each atom of a molecule exist in a configuration that minimizes the repulsion between electrons. In other words, the electrons will space themselves as far away from each other as possible. A bond angle is formed by a central atom and the atoms that surround it. This occurs as the central atom in a molecule forms bonds with the other atoms at specific angles, and these angles are determined by electron repulsion. Any un-bonded pairs of electrons found around the central atom will affect the bond angle.

Some examples of molecular geometry are:

• • A linear molecule consists of a central atom with two atoms on both sides and no lone pairs of electrons on the central atom. Linear molecules have a bond angle of 180^o. For example, a carbon dioxide molecule is a linear molecule.

  • A bent molecule occurs if one or two lone pairs of electrons are added to the central atom. This will change the bond angle from linear to bent. For example, the water that you drink every day has a bent shape. The angle in a water molecule, as in other bent molecules, is 104.45^o.

  • A trigonal planar molecule consists of a central atom with three atoms surrounding it and no lone pairs of electrons on the central atom. This configuration gives this molecule a bond angle of 120^o. The trigonal planar shape is symmetrical due to the symmetrical force from the three identical atoms. For example, boron trifluoride is trigonal planar.

  • A pyramidal molecule occurs with the addition of one lone pair of electrons on the central atom, decreasing the bond angle and changing the molecular shape from trigonal planar to pyramidal. Ammonia and xenon trioxide molecules are pyramidal molecules.

  • A tetrahedral molecule consists of a central atom with four atoms surrounding it and no lone pairs of electrons on the central atom. This gives this molecule a bond angle of 109.5^o. This is a symmetrical shape in a three-dimentional space. The molecule methane is tetrahedral. The angle can be calculated mathematically with the following formula: cos (109.5^o) = -1/3