In terms of chemical bonding, the bonding in barium oxide (BaO) is best described as which dominant type of bond?

Introduction / Context:
Classifying the type of bonding in compounds is an essential skill in basic chemistry. Barium oxide, BaO, is a simple binary compound of a metal and a nonmetal. Students are often asked whether such compounds are ionic or covalent and how to justify the choice. This question focuses on the dominant bonding in BaO and tests your understanding of periodic trends and the behaviour of metal and nonmetal combinations.

Given Data / Assumptions:

• Compound: barium oxide, formula BaO.
• Barium is an alkaline earth metal in group 2 of the periodic table.
• Oxygen is a nonmetal in group 16.
• Barium commonly forms Ba²⁺ ions and oxygen commonly forms O²⁻ oxide ions.
• We are asked for the best description of the bonding type.

Concept / Approach:
The main concept is that compounds formed between a highly electropositive metal and a highly electronegative nonmetal tend to be ionic, especially when the metal is from group 1 or group 2. Barium has a low ionisation energy and readily loses two electrons to form a Ba²⁺ cation. Oxygen has a high electronegativity compared with metals and can gain two electrons to form an O²⁻ anion. The strong electrostatic attraction between these oppositely charged ions forms an ionic lattice. While no bond is one hundred percent ionic or covalent, the dominant character in BaO is clearly ionic.

Step-by-Step Solution:
Step 1: Identify barium as a group 2 metal that commonly forms Ba²⁺ ions by losing two valence electrons. Step 2: Identify oxygen as a nonmetal that commonly gains two electrons to form O²⁻ oxide ions. Step 3: Recognise that electrostatic attraction between Ba²⁺ and O²⁻ ions forms a crystal lattice characteristic of ionic solids. Step 4: Compare this picture to typical covalent bonding, where electrons are shared rather than completely transferred, which is more common between nonmetals. Step 5: Conclude that the bonding in BaO is best described as predominantly ionic bonding between Ba²⁺ and O²⁻ ions.

Verification / Alternative check:
Additional evidence for ionic bonding in BaO includes its physical properties. Barium oxide has a high melting point and boiling point, typical of ionic solids with strong ionic attractions. It also conducts electricity when molten, because ions are free to move in the liquid state. In water, BaO reacts to form barium hydroxide, again behaving like an ionic oxide of an alkaline earth metal. These observable properties align well with an ionic bonding model and not with purely covalent or metallic bonding.

Why Other Options Are Wrong:
Option B is wrong because nonpolar covalent bonds form mainly between identical nonmetal atoms, not between a metal like barium and a nonmetal like oxygen. Option C suggests polar covalent bonds, but the electronegativity difference and metal nonmetal combination favour ionic character rather than shared electron pairs. Option D describes metallic bonding, which is found in pure metals or alloys, not in metal oxides of this type. Option E mentions hydrogen bonding, which requires hydrogen atoms bonded to highly electronegative atoms such as O, N or F; BaO has no hydrogen and therefore cannot form hydrogen bonds. Only option A correctly describes the bonding in BaO as predominantly ionic between Ba²⁺ and O²⁻ ions.

Common Pitfalls:
A common mistake is to assume that every bond has to be classified as purely ionic or purely covalent, when in reality bonding is often a spectrum. However, for exam questions, simple rules based on metal and nonmetal combinations usually apply. Another pitfall is confusing metal oxides like BaO with covalent oxides such as CO2 or SO2. Remember that oxides of highly electropositive metals in groups 1 and 2 are almost always ionic in character.

Final Answer:
The correct description is Predominantly ionic bonding between Ba²⁺ and O²⁻ ions, which best explains the structure and properties of barium oxide.