Sodium oxide (\(\text{Na}_2\text{O}\)) is definitively an ionic compound. Understanding why requires examining the fundamental principles of chemical bonding and how valence electrons are distributed between sodium and oxygen atoms.
Defining Ionic and Covalent Bonds
Chemical bonds are categorized based on how electrons are handled between atoms. A covalent bond forms when two atoms, typically non-metals, share a pair of electrons. This sharing can be equal or unequal, but the electrons remain associated with both nuclei.
An ionic bond, conversely, involves the complete transfer of one or more valence electrons from one atom to another. This transfer creates electrically charged ions: cations (positive) and anions (negative). The resulting compound is held together by the strong electrostatic force of attraction between these oppositely charged ions.
Predicting Bond Type Using Electronegativity
Chemists use electronegativity to predict the likelihood of electron sharing or transfer. Electronegativity measures an atom’s ability to attract a shared pair of electrons within a chemical bond. This property is quantified using scales like the Pauling scale.
The difference in electronegativity (\(\Delta\text{EN}\)) between bonded atoms determines the bond type. A \(\Delta\text{EN}\) less than \(0.4\) indicates a nonpolar covalent bond, where electrons are shared equally. Differences between \(0.4\) and \(1.7\) result in a polar covalent bond, showing unequal sharing.
When the electronegativity difference exceeds \(1.7\), the attractive force of one atom is strong enough to remove the electron from the other. This large disparity indicates that the bond has a predominantly ionic character.
Determining the Bond Type in Sodium Oxide
Sodium oxide (\(\text{Na}_2\text{O}\)) consists of sodium (\(\text{Na}\)) and oxygen (\(\text{O}\)). Sodium is an alkali metal with a low electronegativity of \(0.93\). Oxygen is a non-metal with a high electronegativity of \(3.44\).
The difference in electronegativity (\(\Delta\text{EN}\)) is \(3.44 – 0.93 = 2.51\). This value is significantly higher than the \(1.7\) threshold used to classify a bond as ionic. This calculation confirms that the bond between sodium and oxygen atoms is overwhelmingly ionic.
The substantial difference means oxygen exerts a much greater pull on the valence electrons than sodium. Consequently, each of the two sodium atoms transfers its single valence electron to the oxygen atom. This transfer forms a sodium cation (\(\text{Na}^{+}\)) and an oxide anion (\(\text{O}^{2-}\)), held together by powerful electrostatic forces.
Physical Properties Resulting from the Ionic Bond
The ionic structure results in physical characteristics typical of ionic compounds. Strong electrostatic forces lock the \(\text{Na}^{+}\) and \(\text{O}^{2-}\) ions into a highly ordered crystal lattice. This rigid arrangement requires substantial energy to disrupt, resulting in a high melting point of approximately \(1,132^\circ\text{C}\).
Sodium oxide exists as a white, crystalline solid at room temperature and is brittle. Ionic compounds shatter easily because shifting the crystal structure brings like-charged ions into repulsion. While solid \(\text{Na}_2\text{O}\) does not conduct electricity, its ions are mobile when melted or dissolved, allowing it to act as an electrical conductor.