Lewis Structure of Molecules with Odd Electrons

General Chemistry • Chemical Bonds

Written by STEM Calculators Team Published December 6, 2025 Updated April 21, 2026

Odd-electron species (radicals): NO, NO₂, ClO₂, O₂^-

Choose one of the classic odd-electron radicals to inspect its Lewis structure, total valence-electron count, radical center, and electron bookkeeping. The structure panel below preserves the same electron-dot distribution used in your earlier SVGs, while the surrounding layout adds cleaner teaching support and comparison tools.

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Odd-electron species (radicals)

An odd-electron species is a molecule or ion with an odd total number of valence electrons. This means that after forming bonds and lone pairs, there is always one unpaired electron, which makes the species a radical. Radicals are usually paramagnetic (attracted by a magnetic field) and quite reactive.

In many of these species the atom in the center (like \( \ce{N} \) or \( \ce{Cl} \)) ends up with 7, 9, or 11 electrons in its valence shell instead of an octet, so we cannot draw a normal Lewis structure that gives all atoms 8 electrons. The extra, unpaired electron is shown as a single dot in the Lewis structure.

How to recognize an odd-electron molecule

Count valence electrons: add up the valence electrons of each atom and adjust for charge.
If the total \( N_{\text{val}} \) is odd, then there must be one unpaired electron, and the species is a radical.
When drawing the Lewis structure, give octets where possible; the “leftover” electron is placed as a dot on the atom that best accommodates it (often the central atom or delocalized over several atoms via resonance).

Species in this tool

\(\ce{NO}\) (nitric oxide):
\( N_{\text{val}} = 5_{\ce{(N)}} + 6_{\ce{(O)}} = 11 \) electrons (odd). The Lewis structure is approximately \( \ce{:N{\equiv}O:} \) with one unpaired electron mostly on \( \ce{N} \). Linear, paramagnetic radical.
\(\ce{NO2}\) (nitrogen dioxide):
\( N_{\text{val}} = 5_{\ce{(N)}} + 2\times 6_{\ce{(O)}} = 17 \) electrons (odd). \( \ce{NO2} \) is bent; the unpaired electron and part of the negative charge are delocalized over \( \ce{N} \) and the two \( \ce{O} \) atoms, so we draw resonance structures with different \( \ce{N=O} \) double bonds.
\(\ce{ClO2}\) (chlorine dioxide):
\( N_{\text{val}} = 7_{\ce{(Cl)}} + 2\times 6_{\ce{(O)}} = 19 \) electrons (odd). \( \ce{ClO2} \) is also bent. The odd electron is mainly on \( \ce{Cl} \) and delocalized into the \( \ce{Cl–O} \) bonds. It is a powerful oxidizing radical used in water treatment and bleaching.
\(\ce{O2^-}\) (superoxide ion):
\( N_{\text{val}} = 2\times 6_{\ce{(O)}} + 1_{\text{extra}} = 13 \) electrons (odd). The two oxygens share an unpaired electron in a molecular orbital, so both atoms effectively have 7 electrons in their valence shell. Superoxide is a key reactive oxygen species in biology and atmospheric chemistry and is strongly paramagnetic.

The Lewis structures shown in the calculator highlight where this unpaired electron sits and how octets are approximated or sacrificed to accommodate an odd total of valence electrons.

Frequently Asked Questions

What is an odd-electron Lewis structure?

It is a Lewis structure for a species with an odd total number of valence electrons, so one electron cannot be paired. The unpaired electron is shown as a single dot, and the species is classified as a radical.

How many valence electrons do NO, NO2, ClO2, and O2- have?

NO has 11 (5 + 6), NO2 has 17 (5 + 2x6), and ClO2 has 19 (7 + 2x6). Superoxide O2- has 13 total valence electrons (2x6 + 1), which is also odd.

Why do radicals often violate the octet rule in a Lewis diagram?

With an odd number of valence electrons, at least one electron must remain unpaired, so at least one atom cannot achieve a complete octet in the simplest Lewis picture. The Lewis diagram is an electron-bookkeeping model that makes the unpaired electron explicit.

What is the superoxide ion (O2-) and why is it a radical?

Superoxide is the diatomic oxygen anion with a -1 charge. Adding one electron to O2 gives an odd total valence electron count, leaving one unpaired electron and making it a radical.

Where is the unpaired electron shown in these odd-electron structures?

The unpaired electron is shown as a single dot in the Lewis diagram. The tool also provides a short note indicating which atom the odd electron is mainly associated with for the selected species.

Odd-electron species (radicals): NO, NO2, ClO2, O2-

Calculation steps

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Frequently Asked Questions

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Odd-electron species (radicals): NO, NO₂, ClO₂, O₂^-