Kössel-Lewis Approach to Chemical Bonding — Class 11 Chemistry Notes (JEE & NEET)

01

The Kössel-Lewis Idea — Complete Theory

By 1916 chemists had electronic configurations and a puzzle: why do atoms bond at all? Kössel (Germany) and Lewis (USA) — working independently, on opposite sides of a world war — gave the same answer. Atoms bond by gaining, losing or sharing valence electrons so as to acquire the stable outer-shell configuration of the nearest noble gas — usually eight electrons in the outermost shell: the octet rule. Kössel’s transfer picture grew into ionic bonding (§ 4.2); Lewis’s sharing picture grew into covalent bonding and the whole of this chapter.

Lewis symbols. Write the element’s symbol, surround it with one dot per valence electron: Na carries a single dot, Mg two, C four, O six. Only valence electrons appear — the core stays invisible, which is precisely why the symbols predict bonding so cleanly. Two facts hide in the dots: the number of dots = the valence electrons available; and each dot lost or shared is one potential bond — the electrovalency of Na is 1, of Mg 2, the covalency of C is 4, of N 3 (plus one lone pair left over).

Lewis symbolsymbol + dots = valence e⁻ Octet rulegain · lose · share → 8 Significancepredicts formulae & bonding

The octet rule’s significance. It explains why NaCl is NaCl and not NaCl₂ — sodium loses one dot to reach Ne’s octet, chlorine needs exactly one to reach Ar’s. It predicts the formulas of MgCl₂, CaO, CH₄, CCl₄ from dot arithmetic alone. And it divides bonding cleanly into two families: transfer ( Kössel → ionic, § 4.2) and sharing (Lewis → covalent, § 4.5 onwards). For a rule from 1916, that is extraordinary mileage.

Writing Lewis structures. The workflow: count total valence electrons, place the least electronegative atom central, connect with single bonds, distribute remaining electrons as lone pairs to complete octets, and convert lone pairs to double/triple bonds if octets remain unsatisfied. Then check with formal charge:

Formal chargeFC = V − L − B/2 Selection rulesmallest |FC| wins ExampleO₃ central O: FC = 6−2−3 = +1

Here V = valence electrons of the free atom, L = lone-pair electrons it holds in the structure, B = bonding electrons it shares. Ozone’s central oxygen: V = 6, L = 2 (one lone pair), B = 6 (one double + one single bond) → FC = 6 − 2 − 3 = +1. Formal charges are bookkeeping, not real charges — but they rank competing Lewis structures: the arrangement with the smallest formal charges (and negative charge on the more electronegative atom) is the most plausible.

Keep the rule’s status precise: it is an empirical guideline, excellent for predicting formulae and drawing first-guess structures, powerless for hypervalent species and odd-electron radicals. The later sections — VSEPR, hybridisation, MOT — are the machinery that succeeds where the octet picture runs out.

02

Visualising the Dot Row & Running the Ledger

Ek dot-row, ek ledger — period 2’s Lewis symbols first, then any element’s octet journey computed live.

FIG. 1 — The dot row is the chapter’s alphabet: dots count valence electrons, and each dot is one bond waiting to happen. Metals lose their few dots (electrovalency); non-metals share or gain to complete the octet (covalency); neon already has eight and does nothing.

Try it live

Octet Ledger

Element

Valence electrons1

Path to octetlose 1

Resulting ion / bondsNa⁺

Bonding typeelectrovalent (ionic)

The ledger computes each element’s distance from an octet: metals lose their few dots (Kössel’s road → § 4.2), non-metals gain or share (Lewis’s road → § 4.5+). Neon’s ledger is empty — the octet is already banked.

03

Solved Examples (Step-by-Step)

Dots → octet arithmetic → formula. Jo chain yahan chalti hai, wahi ledger me live chalti hai.

EXAMPLE 01Foundation · Formula prediction

Why MgCl₂ and not MgCl — the dot arithmetic

Using Lewis symbols and the octet rule, show why magnesium forms MgCl₂ rather than MgCl.

Mg’s ledgerMg carries 2 dots → must lose 2 electrons to reach Ne’s octet → Mg²⁺.
Cl’s ledgerCl carries 7 dots → needs 1 electron to reach Ar’s octet → Cl⁻ per chlorine.
BalanceOne Mg²⁺ needs two Cl⁻ to balance charge: Mg²⁺ + 2Cl⁻ → MgCl₂
VerdictMgCl would leave magnesium with an incomplete octet — the octet rule forbids it ✓

2 dots lost ÷ 1 dot needed per Cl → MgCl₂

EXAMPLE 02JEE Main · Formal charge

Ozone’s central oxygen, formally charged

Calculate the formal charge on the central oxygen atom of ozone (O₃), whose Lewis structure carries one double bond, one single bond and one lone pair on the central atom.

FormulaFC = V − L − B/2
ValuesV = 6 · L = 2 (one lone pair) · B = 6 (4 in the double bond + 2 in the single)
ComputeFC = 6 − 2 − 6/2 = 6 − 2 − 3 = +1
UseFormal charges rank the resonance forms of § 4.3 — smallest |FC| = most plausible structure.

FC = +1 on the central oxygen

EXAMPLE 03JEE Main · Exception family

Three molecules the octet rule cannot explain

Classify BCl₃, NO and SF₆ by their relationship to the octet rule, with electron counts.

BCl₃B: 3 bonds, 6 electrons around it → incomplete octet (boron’s habit).
NO5 + 6 = 11 valence electrons — odd, so no distribution gives every atom an octet → odd-electron molecule.
SF₆S: 6 bonds → 12 electrons around sulphur → expanded octet (d-orbitals available from shell 3).

Incomplete · odd-electron · expanded — the three exception families

04

Practice Questions (With Solutions)

Attempt first — options lock after one shot, exactly like the real exam. Then read the working, chahe galti ho ya na ho.

Attempted 0/4 · Correct 0

Q1NEET · Lewis dots

The number of dots in the Lewis symbol of phosphorus (group 15) is:

Solution

Lewis dots = valence electrons = group number (15 → 5 valence e⁻ for main-group elements).
Option C confuses the octet target with the count; D counts total electrons — the core stays invisible.

(B) 5

Q2JEE Main · Formal charge

The formal charge on the single-bonded oxygen atoms of ozone (each carrying three lone pairs) is:

Solution

FC = V − L − B/2 = 6 − 6 − 2/2 = −1.
With the central oxygen’s +1, the molecule’s charges sum to zero ✓ — and the −1 sits on the more electronegative end atoms, as the selection rule prefers.

(C) −1 on each end oxygen

Q3NEET · Exceptions

NO and NO₂ are exceptions to the octet rule because they:

Solution

NO: 5 + 6 = 11 e⁻ · NO₂: 5 + 12 = 17 e⁻ — odd counts can never give every atom a full octet.
They are odd-electron molecules — radicals, permanently octet-incomplete. A and B describe the other two exception families.

(C) Odd electron count

Q4JEE Main · Octet significance

The octet rule successfully predicts the formula of:

Solution

C: 4 dots shared with 4 chlorines — every atom reaches an octet → CCl₄ is a clean octet-rule prediction.
PCl₅ (10 e⁻ on P), SF₆ (12 on S) and XeF₂ are all exception-family species — the rule cannot justify them.

(C) CCl₄

05

Key Rules & Takeaways

Rule card

Eight lines that solve this topic

Octet rule: gain · lose · share → 8The noble-gas destination — Kössel’s transfer, Lewis’s share.

Lewis symbol: dots = valence e⁻Group count on the outside; core invisible; each dot a bond slot.

Electrovalency = dots LOSTNa 1 · Mg 2 · Al 3 — metals empty their dots (§ 4.2).

Covalency = dots SHAREDC 4 · N 3 (+1 lp) · O 2 (+2 lp) — non-metals complete by sharing.

FC = V − L − B/2Formal charge ranks Lewis structures; smallest |FC| wins.

Exception 1: incomplete octetsLiCl · BeH₂ · BCl₃ — central atom under eight.

Exception 2: odd-electron moleculesNO (11 e⁻) · NO₂ (17 e⁻) — octets impossible in principle.

Exception 3: expanded octetsPF₅ (10 e⁻) · SF₆ (12 e⁻) — d-seats from shell 3 onward.

Formal charge worked: O₃ central O = 6 − 2 − 3 = +1 · O₃ end O = 6 − 6 − 1 = −1 · Exceptions: BCl₃ (6 e⁻) · NO (11 e⁻) · SF₆ (12 e⁻) · XeF₂

One idea, two theories — the octet drive splits into Kössel’s transfer (ionic, § 4.2) and Lewis’s sharing (covalent, § 4.5+); the dot row is the shared alphabet.
Dots are bonding capacity — count a symbol’s dots and you have its valence before any reaction is written.
Formal charge is the structure referee — V − L − B/2, smallest absolute values win; it sets up § 4.3’s resonance selection.
Know the three exception families by name — incomplete, odd-electron, expanded — each with its signature molecules.

06

FAQs

What is the octet rule?

Kössel and Lewis proposed that atoms combine by gaining, losing or sharing electrons so as to acquire eight electrons in their outermost shell — the stable electron arrangement of the nearest noble gas. The octet rule successfully explains the formation of many compounds such as NaCl, MgCl2 and CH4, though it has exceptions.

What are Lewis symbols and what do the dots represent?

Lewis symbols write the chemical symbol surrounded by dots, where each dot represents one valence electron. The number of dots equals the group's valence electron count — Na carries one dot, Mg two, C four, O six. For simplicity, bond formation involves only these valence electrons; inner-core electrons stay out of the picture.

What is formal charge and how is it calculated?

Formal charge is assigned to an atom in a Lewis structure by the formula: formal charge = valence electrons − non-bonding (lone) electrons − ½(bonding electrons). It helps select the most plausible structure among several — the one with the smallest formal charges is generally the most stable.

What are the limitations of the octet rule?

Three main limitations: incomplete octets, as in LiCl, BeH2 and BCl3 where the central atom has fewer than eight electrons; odd-electron molecules, such as NO and NO2 which cannot give every atom an octet no matter how electrons are arranged; and expanded octets, as in PF5 and SF6 where more than eight electrons surround the central atom. Noble gases forming compounds like XeF2 also fall outside the rule.