Q. how to know if a compound is ionic or covalent

Goal. Determine whether a given chemical compound is ionic or covalent, with a clear step by step procedure and illustrative examples.

Step 1. Identify the atoms in the compound and note whether they are metals or nonmetals. Ionic compounds most often form between metals and nonmetals. Covalent compounds most often form between two nonmetals. If both atoms are metals, consider metallic bonding or alloys, not ionic or covalent in the usual sense.

Step 2. Use electronegativity to quantify the bond character. Compute the electronegativity difference using the formula

\[ \Delta \chi \;=\; \left| \chi_{A} \;-\; \chi_{B} \right|. \]

Step 3. Compare the computed electronegativity difference to commonly used ranges. These ranges are approximate, but they give a useful decision rule.

– If \( \Delta \chi \gtrsim 1.7 \), the bond is typically classified as ionic, meaning electron transfer and formation of ions is dominant.

– If \( 0.4 \lesssim \Delta \chi \lesssim 1.7 \), the bond is typically polar covalent, meaning electrons are shared unequally.

– If \( \Delta \chi \lesssim 0.4 \), the bond is typically nonpolar covalent, meaning electrons are shared approximately equally.

Step 4. Consider special cases and additional evidence. If the compound contains a recognizable polyatomic ion, such as \( \mathrm{NH_{4}^{+}} \) or \( \mathrm{SO_{4}^{2-}} \), and another oppositely charged ion, the substance is ionic overall even though bonds inside the polyatomic ion are covalent. Also consider physical properties: ionic solids tend to have high \Delta ing points and conduct electricity when molten or dissolved, whereas simple molecular covalent substances often have lower \Delta ing points and do not conduct electricity when molten or dissolved. Covalent network solids, such as \( \mathrm{SiO_{2}} \), are covalent but have very high \Delta ing points.

Worked example 1. Sodium chloride, \( \mathrm{NaCl} \).

Values: \( \chi_{\mathrm{Na}} = 0.93 \), \( \chi_{\mathrm{Cl}} = 3.16 \).

Compute the difference:

\[ \Delta \chi \;=\; \left| 0.93 \;-\; 3.16 \right| \;=\; 2.23. \]

Decision: Since \( \Delta \chi = 2.23 \gtrsim 1.7 \), the bond is ionic. This agrees with the fact that \( \mathrm{NaCl} \) is a lattice of \( \mathrm{Na^{+}} \) and \( \mathrm{Cl^{-}} \) ions.

Worked example 2. Carbon dioxide, \( \mathrm{CO_{2}} \).

Values: \( \chi_{\mathrm{C}} = 2.55 \), \( \chi_{\mathrm{O}} = 3.44 \).

Compute the difference for each C–O bond:

\[ \Delta \chi \;=\; \left| 2.55 \;-\; 3.44 \right| \;=\; 0.89. \]

Decision: Since \( 0.4 \lesssim 0.89 \lesssim 1.7 \), each C–O bond is polar covalent. The molecule as a whole is linear and symmetrical, so the bond dipoles cancel and the molecule is nonpolar overall. Thus the bonding is covalent, not ionic.

Worked example 3. Hydrogen gas, \( \mathrm{H_{2}} \).

Values: \( \chi_{\mathrm{H}} = 2.20 \) for both atoms.

Compute the difference:

\[ \Delta \chi \;=\; \left| 2.20 \;-\; 2.20 \right| \;=\; 0.00. \]

Decision: Since \( \Delta \chi \) is near zero, the H–H bond is nonpolar covalent.

Worked example 4. Ammonium chloride, \( \mathrm{NH_{4}Cl} \).

Structure: \( \mathrm{NH_{4}^{+}} \) is a covalently bonded polyatomic cation, and \( \mathrm{Cl^{-}} \) is an anion. The substance is an ionic salt composed of oppositely charged ions. Decision: ionic overall, even though bonds inside \( \mathrm{NH_{4}^{+}} \) are covalent.

Step 5. Summary decision procedure you can apply to any compound, step by step.

1. Identify the elements. If metal plus nonmetal, suspect ionic. If nonmetal plus nonmetal, suspect covalent.

2. Compute \( \Delta \chi = \left| \chi_{A} – \chi_{B} \right| \) using tabulated electronegativities.

3. Classify using approximate thresholds: \( \Delta \chi \gtrsim 1.7 \) ionic, \( 0.4 \lesssim \Delta \chi \lesssim 1.7 \) polar covalent, \( \Delta \chi \lesssim 0.4 \) nonpolar covalent.

4. Check for polyatomic ions, lattice behavior, electrical conductivity when molten or dissolved, and other chemical evidence to confirm the classification.

Notes and cautions. The electronegativity thresholds are approximate. Many bonds have mixed character, and some solids are better described as having partial ionic character while still being covalent. Use multiple lines of evidence for a confident classification.