Which of the following formulas represents an ionic compound?
Understanding how to spot an ionic formula is a fundamental skill in chemistry because it tells you about the bonding, physical properties, and reactivity of a substance. In this guide we will walk through the criteria that define an ionic compound, apply those rules to a set of example formulas, and clarify why some substances that look similar are actually covalent. By the end you’ll be able to look at any chemical formula and decide confidently whether it describes an ionic lattice or a molecular entity Easy to understand, harder to ignore..
Introduction: What Makes a Compound Ionic?
An ionic compound forms when atoms transfer electrons to achieve stable electron configurations, resulting in positively charged cations and negatively charged anions. These oppositely charged ions attract each other through strong electrostatic forces, creating a repeating three‑dimensional lattice. The key signatures of an ionic formula are:
- Presence of a metal (or polyatomic cation) combined with a nonmetal (or polyatomic anion).
- A large electronegativity difference (generally > 1.7 on the Pauling scale) between the constituent atoms, which favors electron transfer rather than sharing.
- The formula reflects the simplest whole‑number ratio of ions needed to balance total charge (the formula unit).
- High melting and boiling points, brittleness, and conductivity when molten or dissolved—properties that stem from the lattice energy.
If a formula meets most of these points, it is very likely ionic. Conversely, formulas that consist only of nonmetals, show small electronegativity differences, or represent discrete molecules are typically covalent.
Step‑by‑Step Guide to Identifying an Ionic Formula
Follow these practical steps whenever you encounter a new formula:
| Step | Action | What to Look For |
|---|---|---|
| 1 | Identify the elements (or polyatomic ions) present. Worth adding: | Metals (left side of periodic table) vs. That's why nonmetals (right side). Which means |
| 2 | Determine the typical charges of each species. | Use group numbers for main‑group elements (e.g., Group 1 = +1, Group 2 = +2, Group 15 = −3, Group 16 = −2, Group 17 = −1). Consider this: polyatomic ions have memorized charges (e. Now, g. , NO₃⁻ = −1, SO₄²⁻ = −2). Worth adding: |
| 3 | Check if the total positive charge equals the total negative charge. | If not, adjust subscripts until charge balance is achieved. |
| 4 | Estimate the electronegativity difference (ΔEN). | ΔEN > 1.Now, 7 → strong ionic character; 0. 5‑1.7 → polar covalent; < 0.Day to day, 5 → nonpolar covalent. |
| 5 | Consider the physical state and properties (if known). That's why | High melting point, soluble in water, conductive when molten → ionic. |
| 6 | Conclude. | If steps 1‑4 point to electron transfer and charge balance, label the formula ionic. |
Quick note before moving on That's the part that actually makes a difference..
Applying the Rules: Example Formulas
Below is a list of ten common formulas. We will evaluate each using the steps above and state whether it represents an ionic compound.
| Formula | Elements / Ions | Typical Charges | Charge Balance? 20, O 3.Because of that, ) | Verdict | |---------|----------------|----------------|----------------|--------------|---------| | NaCl | Na (metal), Cl (nonmetal) | Na⁺, Cl⁻ | 1 × (+1) + 1 × (−1) = 0 | 2. Plus, 24 (H 2. 00, F 3.Because of that, 93, Cl 3. Plus, 90, O 3. 98) | Ionic | | CH₄ | C (nonmetal), H (nonmetal) | C⁴⁻ (covalent), H⁺ (partial) | Neutral molecule | 0.Also, 61, O 3. 31, Cl 3.That's why 08 (Ca 1. Think about it: 83 (K 0. 44) | Ionic | | CaF₂ | Ca (metal), F (nonmetal) | Ca²⁺, F⁻ | 1 × (+2) + 2 × (−1) = 0 | 3.Here's the thing — 96 (Mg 1. 96) | Ionic | | Al₂O₃ | Al (metal), O (nonmetal) | Al³⁺, O²⁻ | 2 × (+3) + 3 × (−2) = 0 | 2.Now, 23 (Na 0. 44) | Covalent (network) | | KBr | K (metal), Br (nonmetal) | K⁺, Br⁻ | 1 × (+1) + 1 × (−1) = 0 | 1.Day to day, 04, H 2. 04 (Al 1.82, Br 2.16) | Ionic | | SiO₂ | Si (metalloid), O (nonmetal) | Si⁴⁺ (covalent), O²⁻ (covalent) | Network covalent solid | 1.44) | Polar covalent | | NH₃ | N (nonmetal), H (nonmetal) | N³⁻ (covalent), H⁺ (partial) | Neutral molecule | 0.89 (C 2.84 (N 3.54 (Si 1.44) | Covalent | | H₂O | H (nonmetal), O (nonmetal) | H⁺ (partial), O²⁻ (partial) | Neutral molecule | 1.Plus, | ΔEN (approx. 16) | Ionic | | CO₂ | C (nonmetal), O (nonmetal) | C⁴⁺ (covalent), O²⁻ (covalent) | No formal charge sharing; molecule neutral | 0.Practically speaking, 20) | Covalent | | MgCl₂ | Mg (metal), Cl (nonmetal) | Mg²⁺, Cl⁻ | 1 × (+2) + 2 × (−1) = 0 | 1. But 55, O 3. 35 (C 2.55, H 2 And that's really what it comes down to..
Takeaway: The formulas that satisfy the metal‑nonmetal combination, charge balance,
