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Electron dot structures look like art, so students draw them by eye and lose marks for a missing pair of dots. There is a simple count that catches almost every error before the examiner does. This post gives a three-step method, works NCERT's own questions with the count, and lists the slips that come up every year.
Where is this in the syllabus?
The CBSE Class X science syllabus for 2026-27 starts the carbon chapter with "Covalent bonds – formation and properties of covalent compounds". The textbook is section 4.1 of NCERT Class 10 Science, Chapter 4, Carbon and its Compounds (2026-27 reprint). The same skill continues in Class 11 as the "Kossel-Lewis approach" listed in the JEE (Main) 2026 syllabus, so it is worth getting right now.
What does an electron dot structure show?
NCERT draws only the outer-shell (valence) electrons, as dots or crosses. A shared pair between two atoms is a single covalent bond and can also be drawn as a line. Its examples:
H₂: each hydrogen shares its one electron, so each reaches two electrons, like helium.
O₂: each oxygen has six valence electrons and shares two, giving two shared pairs: a double bond.
N₂: each nitrogen contributes three electrons, giving three shared pairs: a triple bond.
CH₄: carbon's four valence electrons are shared with four hydrogens.
NCERT explains why carbon shares rather than forming ions: gaining four electrons (C⁴⁻) or losing four (C⁴⁺) would both be very difficult.
A three-step method with a built-in check
Count valence electrons. H 1, C 4, N 5, O 6, S 6, Cl 7. Add them for the whole molecule.
Connect and complete. Join the atoms with single bonds (carbon atoms first, as NCERT does for ethane). Then give every atom eight electrons around it, or two for hydrogen, using lone pairs, or by turning single bonds into double or triple bonds if you run short.
Check the count. Twice the number of shared pairs, plus all the lone-pair electrons, must equal the total from step 1. Then check that every carbon has four bonds and every hydrogen one.
Worked example 1: carbon dioxide (NCERT question)
Count: 4 + 6 + 6 = 16 electrons.
Single bonds O–C–O leave carbon with only four electrons around it. Make both bonds double: O=C=O. Carbon now has eight, and each oxygen needs two lone pairs to reach eight.
Check: 4 shared pairs × 2 = 8, plus 2 lone pairs on each oxygen = 8. Total 16.
Worked example 2: hydrogen sulphide (NCERT exercise)
Count: 1 + 1 + 6 = 8 electrons.
H–S–H, with two lone pairs on sulphur.
Check: 2 shared pairs × 2 = 4, plus 4 lone-pair electrons = 8.
Water has the same pattern, since oxygen and sulphur both have six valence electrons.
Worked example 3: ethanoic acid, CH₃COOH (NCERT exercise)
Count: 2 carbons × 4 + 4 hydrogens × 1 + 2 oxygens × 6 = 24 electrons.
Skeleton: a CH₃ group bonded to a carbon that carries a double-bonded oxygen and an –OH group.
Shared pairs: three C–H, one C–C, two in C=O, one C–O and one O–H, so 8 pairs, or 16 electrons.
Each oxygen has two lone pairs: 8 electrons.
Check: 16 + 8 = 24.
Worked example 4: propanone, S₈ and cyclopentane
Propanone, CH₃COCH₃. 3 × 4 + 6 × 1 + 6 = 24. Shared pairs: six C–H, two C–C and two in C=O, so 10 pairs or 20 electrons, plus two lone pairs on oxygen (4). Total 24.
Sulphur, S₈ (NCERT question, hint: a ring). 8 × 6 = 48. A ring of eight atoms has 8 S–S bonds, 16 electrons. Each sulphur then needs two lone pairs: 8 × 4 = 32. Total 48.
Cyclopentane, C₅H₁₀ (NCERT exercise). 5 × 4 + 10 = 30. Five C–C bonds and ten C–H bonds make 15 shared pairs, 30 electrons, with no lone pairs.
Which mistakes cost marks?
Drawing all the electrons instead of only the valence shell.
Leaving lone pairs off oxygen, sulphur, nitrogen or chlorine. The count in step 3 catches this.
Giving carbon three or five bonds. Count the lines at every carbon.
Drawing CO₂ with single bonds. Carbon would have only four electrons.
Drawing S₈ as an open chain. NCERT's hint says ring; a chain would leave the end atoms short.
A practice routine
Rules alone are not enough. Cooper and colleagues (2010) studied how students at all levels draw Lewis structures and found that, despite effort on "foolproof" rules, students still have problems with the skill and many do not connect it with the reasons for learning it. So after each structure, write one property it explains, such as NCERT's point that sharing electrons forms no ions, which is why most covalent compounds conduct electricity poorly.
Draw Cl₂, NH₃ and ethyne, NCERT's in-text questions, plus ethene, and run the count on each.
Draw NCERT's exercise molecules from memory two days later.
For each, write one sentence linking the structure to a property in Table 4.1 or section 4.1.
When your count fails and you cannot see why, the Fermi Problem Library has a step-by-step solver that explains the reasoning rather than printing an answer. For naming and reactions in the same chapter, read our guide to carbon and its compounds. When you reach Class 11, formal charge and Lewis structures builds on this.
Start with ethyne tonight: count its valence electrons first, then draw.
