Barium oxide (\(\text{BaO}\)) is a compound formed by the elements Barium (\(\text{Ba}\)) and Oxygen (\(\text{O}\)). Like all chemical substances, its fundamental characteristics are determined by the forces that hold its constituent atoms together. The primary question when examining Barium oxide is determining the nature of the chemical bonds that link the Barium and Oxygen atoms. The type of bond dictates nearly every physical and chemical property the substance exhibits.
The Difference Between Ionic and Covalent Bonds
Chemical bonds are generally categorized into two idealized types: ionic and covalent. The distinction lies in how the outermost electrons, known as valence electrons, are distributed between the atoms.
An ionic bond involves the complete transfer of one or more valence electrons from one atom to another. This transfer results in the formation of charged particles called ions: a positively charged cation and a negatively charged anion. The strong electrostatic attraction between these oppositely charged ions forms the bond, and this interaction typically occurs between a metal and a nonmetal.
In contrast, a covalent bond forms when two atoms share one or more pairs of valence electrons. This sharing allows each atom to effectively complete its outer electron shell. Covalent bonds usually form between two nonmetal atoms, creating a discrete unit known as a molecule. While the sharing in some bonds is equal, many covalent bonds are polar, meaning the electrons are shared unequally.
Measuring Bond Character with Electronegativity
Chemists use the concept of electronegativity to quantify where a specific bond falls on the continuous spectrum between purely covalent and purely ionic. Electronegativity is defined as an atom’s ability to attract a shared pair of electrons toward itself within a chemical bond. The values for this property are measured on the Pauling scale, ranging from approximately \(0.7\) to \(3.98\).
To assess the nature of a bond between two atoms, the difference in their electronegativity values (\(\Delta \text{EN}\)) is calculated. A small difference indicates that the atoms have a similar pull on the electrons, resulting in a covalent bond where sharing is dominant. As the difference increases, the bond becomes more polar, shifting the electron density significantly toward the more electronegative atom.
General guidelines categorize the bond type based on this calculated difference. A \(\Delta \text{EN}\) less than \(0.5\) is considered nonpolar covalent, while a difference between \(0.5\) and \(1.7\) suggests a polar covalent bond. When the electronegativity difference is greater than \(1.7\), the bond is predominantly classified as ionic, indicating a near-complete transfer of electrons.
Classifying the Bonds in Barium Oxide
To classify the bond in Barium oxide, we first look at the elements involved, Barium (\(\text{Ba}\)) and Oxygen (\(\text{O}\)). Barium is an alkaline earth metal in Group 2, meaning it has a strong tendency to lose its two valence electrons. Oxygen is a nonmetal in Group 16, which has a strong tendency to gain two electrons. This metal-nonmetal pairing is the first strong indicator of an ionic bond.
Applying the electronegativity method confirms this assessment. The Pauling electronegativity value for Barium is approximately \(0.89\). Conversely, the electronegativity value for Oxygen is approximately \(3.44\). The difference in electronegativity (\(\Delta \text{EN}\)) for the \(\text{Ba}-\text{O}\) bond is calculated as \(3.44 – 0.89 = 2.55\).
Since the calculated \(\Delta \text{EN}\) of \(2.55\) is significantly greater than the \(1.7\) threshold, Barium oxide is classified as a predominantly ionic compound. The large difference means the Barium atom effectively transfers its two valence electrons to the Oxygen atom. This transfer forms a Barium cation (\(\text{Ba}^{2+}\)) and an Oxide anion (\(\text{O}^{2-}\)), which are held together by powerful electrostatic forces in a neutral, repeating lattice.
The Properties that Result from BaO’s Bond Type
The ionic nature of Barium oxide dictates its observable physical characteristics. The strong electrostatic forces holding the \(\text{Ba}^{2+}\) and \(\text{O}^{2-}\) ions together require a substantial amount of energy to overcome. This results in Barium oxide having a high melting point of approximately \(1923^\circ \text{C}\) and a high boiling point of around \(2000^\circ \text{C}\).
At room temperature, the compound exists as a hard, white crystalline solid with a cubic crystal lattice structure. In this solid state, the ions are locked in place and cannot move freely, making Barium oxide a poor conductor of electricity. When the compound is melted or dissolved in a suitable solvent, the ions become mobile, allowing the substance to conduct an electrical current.