In NO3^- ion, the number of bond pairs and lone pairs of electrons on nitrogen atom are (i) 2, 2 (ii) 3, 1 (ii

NCERT Exemplar · Q6

Q. In NO3^- ion, the number of bond pairs and lone pairs of electrons on nitrogen atom are

(i) 2, 2

(ii) 3, 1

(iii) 1, 3

(iv) 4, 0

Uttar Pradesh UpmspMCQ· 1mImportance★★★★★est

45% · 50/112 Questions

🔒 Locked · start free trial →

You're viewing a preview — the full solution, concept, methods & PYQ mapping are locked.

Start your 14-day free trial to unlock the full solution →

Concept understanding — Lewis Dot Structures

Why do we need Lewis dot structures?

Atoms are held together in molecules by chemical bonds — but what exactly is a bond? In the early 20th century, Gilbert N. Lewis realised that the key lies in the valence electrons (the outermost electrons). He noticed that atoms of noble gases (like Ne, Ar) are extremely stable and unreactive, and they all have 8 electrons in their outermost shell (except helium, which has 2). This led to the octet rule: atoms tend to gain, lose, or share electrons to achieve a full outer shell of 8 electrons (or 2 for hydrogen).

Lewis dot structures are simply a shorthand picture of this idea. They show:

Which atoms are connected to which
How many valence electrons each atom contributes
How those electrons are arranged as bonding pairs (shared) or lone pairs (unshared)

The precise statement

A Lewis dot structure (or electron dot structure) represents the valence electrons of an atom or molecule using dots placed around the element's symbol. Each dot stands for one valence electron. Shared pairs (bonds) are shown as lines, and unshared pairs as pairs of dots.

For a single atom, you write the element symbol and place dots on its four sides (top, bottom, left, right) — up to 8 dots. The order of filling doesn't matter for the final picture, but conventionally you place one dot on each side first, then pair them up.

For example:

Carbon (group 14, 4 valence electrons): $\cdot \text{C} \cdot$ (four single dots)
Oxygen (group 16, 6 valence electrons): $\cdot \ddot{\text{O}} \cdot$ (two single dots and two pairs)

How to draw a Lewis structure for a molecule

Here's the step-by-step method you'll use in exams:

Count total valence electrons — add up valence electrons from all atoms. For ions, add 1 electron for each negative charge, subtract 1 for each positive charge.
Identify the central atom — usually the least electronegative element (not hydrogen or fluorine). Place it in the centre.
Connect atoms with single bonds — each bond uses 2 electrons. Subtract these from your total.
Complete octets of outer atoms — place remaining electrons as lone pairs on terminal atoms (except hydrogen, which only needs 2).
Place leftover electrons on the central atom — if any remain.
If the central atom has fewer than 8 electrons, form multiple bonds — move lone pairs from outer atoms to create double or triple bonds until the central atom has an octet.

Watch out

A common mistake: forgetting that hydrogen only needs 2 electrons (a duet), not 8. Never put more than 2 electrons around H.

A concrete example: water (H₂O)

Total valence electrons: O has 6, each H has 1 → $6 + 1 + 1 = 8$ electrons.
Central atom: oxygen (least electronegative after H).
Connect: O—H bonds (2 bonds × 2 electrons = 4 electrons used).
Remaining: $8 - 4 = 4$ electrons → place as two lone pairs on oxygen.
Check: O has 2 bonds (4 electrons) + 2 lone pairs (4 electrons) = 8. Each H has 1 bond (2 electrons) = 2. Done.

The structure: $\text{H} - \ddot{\text{O}} - \text{H}$

What the structure tells you

Once drawn, a Lewis structure reveals:

Bond order (single, double, triple)
Lone pairs (which affect molecular shape and reactivity)
Formal charge (a bookkeeping tool to check which structure is most stable) …

Start your 14-day free trial to unlock the full solution →

Nitrogen in $\mathrm{NO_3^-}$ forms three bonds (one double, two coordinate) with no lone pairs remaining; the answer is (D) 4, 0.

The Lewis structure reveals what nitrogen actually does with its five valence electrons in the nitrate ion. Nitrogen sits at the center bonded to three oxygen atoms, and the ion carries a single negative charge. To count bond pairs and lone pairs on nitrogen, we need to draw the structure carefully and see where all the electrons end up.

Why Lewis structures matter here

Nitrogen has five valence electrons. Each bond it forms uses one of those electrons (paired with an electron from the other atom). Any electrons left over sit as lone pairs on nitrogen itself. The trick with polyatomic ions is accounting for the extra charge: $\mathrm{NO_3^-}$ has one more electron than the neutral atoms would provide.

Building the nitrate ion step by step

Count total valence electrons. Nitrogen contributes 5, each oxygen contributes 6, and the negative charge adds 1 more:

5 + 3(6) + 1 = 24 \text{ electrons total.}

Sketch the skeleton. Nitrogen is less electronegative than oxygen, so it goes in the center with three oxygens around it:

\mathrm{O} - \mathrm{N} - \mathrm{O} \quad \text{with a third O attached.}

Place single bonds first.

Three $\mathrm{N–O}$ single bonds use $3 \times 2 = 6$ electrons, leaving $24 - 6 = 18$ electrons to distribute.
Complete octets on the outer atoms (oxygen).

Each oxygen needs 8 electrons total. Each already has 2 from its bond to nitrogen, so needs 6 more (three lone pairs). Three oxygens × 6 electrons = 18 electrons, which exactly uses up what we have left.
Check nitrogen's octet.

Right now nitrogen has only three single bonds (6 electrons). It needs 8. We fix this by converting one or more lone pairs from oxygen into double bonds. Move one lone pair from one oxygen to form a $\mathrm{N=O}$ double bond. Now nitrogen has:
- One double bond (4 electrons)
- Two single bonds (4 electrons)
- Total: 8 electrons around nitrogen ✓ …

Start your 14-day free trial to unlock the full solution →

←Previous questionQ5 — In PO4³- ion the formal charge on the oxygen atom of P–O bond is (i) +…→Next questionQ7 — Which of the following species has tetrahedral geometry? (i) BH4^- (ii…

Q.In NO3^- ion, the number of bond pairs and lone pairs of electrons on nitrogen atom are

Why Lewis structures matter here

Building the nitrate ion step by step

🎓Unlock everything free for 14 days

Unlock everything free for 14 days