What are Lewis acids and bases? Give two examples for each.

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Q. What are Lewis acids and bases? Give two examples for each.

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Concept understanding — Lewis Acid-Base Theory

Gilbert N. Lewis's 1923 concept is the most general of the three, defined by the electron pair rather than the proton: a Lewis acid is a species that accepts an electron pair, and a Lewis base is a species that donates an electron pair. A Lewis acid is typically a positive ion or an electron-deficient molecule; a Lewis base is typically an anion or a neutral molecule with at least one lone pair.

In $BF_3+NH_3 \rightarrow F_3B{\leftarrow}NH_3$ , boron's vacant $2p$ orbital accepts the lone pair donated by nitrogen, forming a new coordinate covalent bond: $BF_3$ is the Lewis acid and $NH_3$ is the Lewis base. In $Cr^{3+}+6H_2O \rightarrow [Cr(H_2O)_6]^{3+}$ , each water molecule donates a lone pair to $Cr^{3+}$ , so $Cr^{3+}$ is the Lewis acid and $H_2O$ is the Lewis base -- the same logic underlies every coordination compound, where the ligands are Lewis bases and the central metal ion is the Lewis acid.

Families of Lewis acids: electron-deficient molecules ( $BF_3, AlCl_3, BeF_2$ ), all metal ions ( $Fe^{2+}, Cr^{3+}, Cu^{2+}$ ), molecules with a polar double bond ( $SO_2, CO_2, SO_3$ ), molecules whose central atom can expand its octet via empty d orbitals ( $SiF_4, SF_4, FeCl_3$ ), and carbonium ions ( $CH_3^+$ ).

Families of Lewis bases: molecules with one or more lone pairs ( $NH_3, H_2O, R{-}O{-}H$ ), all anions ( $F^-, CN^-, SO_4^{2-}$ ), molecules with a carbon-carbon multiple bond ( $CH_2{=}CH_2, CH{\equiv}CH$ ), all metal oxides ( $CaO, MgO, Na_2O$ ), and carbanions ( $CH_3^-$ ).

Where two possible Lewis bases compete for the same Lewis acid -- as when $NH_3$ displaces $H_2O$ from $[Cu(H_2O)_4]^{2+}$ to form the deep-blue $[Cu(NH_3)_4]^{2+}$ -- the base forming the more stable complex is the stronger Lewis base; here $NH_3$ is the stronger Lewis base than $H_2O$ .

"Lewis acid and base theory definition with examples" and "Lewis acid base class 11 important questions" are common search phrases tied to the Equilibrium chapter of the NCERT-aligned CBSE Class 11 Chemistry curriculum, since Lewis theory is the most general of the three acid-base theories tested in JEE Main and NEET. Classifying species into the Lewis-acid and Lewis-base families listed here is a frequent one-mark identification question in board exams.

Step 1. A Lewis acid is a species that ACCEPTS an electron pair -- typically a positive ion or an electron-deficient molecule.

Step 2. Two examples of Lewis acids: $BF_3$ (boron has only six electrons around it, an incomplete octet, so it readily accepts a lone pair) and $AlCl_3$ (aluminium is similarly electron-deficient).

Step 3. A Lewis base is a species that DONATES an electron pair -- typically an anion or a neutral molecule with at least one lone pair.

Step 4. Two examples of Lewis bases: $NH_3$ (nitrogen's lone pair is donated, e.g. to $BF_3$ forming the adduct $F_3B{\leftarrow}NH_3$ ) and $H_2O$ (oxygen's lone pairs are donated, e.g. to $Cr^{3+}$ forming $[Cr(H_2O)_6]^{3+}$ ).

✓Final answer

BF_3

Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

Showing the 12 most recent of 14 on this concept.

CBSE 2026Set ANNUAL1 markMCQ
$Q. Which of the following can act as Lewis acid?$
(a) OH-
(b) F-
(c) BCl3
(d) H-
›Reveal solutionSolution
A Lewis acid is an electron-pair ACCEPTOR. Among the options, BCl3 is the only species that is electron-deficient and can accept a lone pair; OH-, F-, and H- are all electron-pair DONORS (Lewis bases).

Lewis acid-base theory: a Lewis acid accepts an electron pair (has an empty/low-energy orbital available), while a Lewis base donates an electron pair (has a lone pair to give).

Check each option:

OH-: has lone pairs on oxygen to donate -- Lewis base.

F-: has lone pairs to donate -- Lewis base.

BCl3: boron has only 3 bonds (6 electrons) around it in this molecule, an incomplete octet, leaving an empty p-orbital that can accept an incoming electron pair -- classic Lewis acid (this is why BCl3 forms adducts like BCl3.NH3).

H-: the hydride ion has a lone pair (2 electrons) it can donate -- Lewis base.

Only BCl3 fits the definition of an electron-pair acceptor.

✓Final answer
$(c) BCl3.$
CBSE 2026Set ANNUAL1 mark
$Q. BF3 is a ................. acid.$
›Reveal solutionSolution

$BF_3$ is electron-deficient at boron, so it readily accepts a lone pair from a donor species — the hallmark of a Lewis acid.

Boron in $BF_3$ has only 6 electrons around it (3 bond pairs, no lone pair), giving it an empty $2p$ orbital. According to the Lewis concept, a species that can accept an electron pair to complete its octet is a Lewis acid. $BF_3$ readily accepts a lone pair from species like $NH_3$ or $F^-$ (forming $BF_3\cdot NH_3$ or $BF_4^-$ ), confirming it acts as an electron-pair acceptor.

✓Final answer
$BF_3$
CBSE 2026Set ANNUAL1 mark
$Q. Write true or false: G.N. Lewis defined base as electron pair donor.$
›Reveal solutionSolution
True. In the Lewis theory, a base is defined as an electron pair donor, and an acid as an electron pair acceptor.

G.N. Lewis proposed a broader definition of acids and bases based on electron pairs rather than protons:

Lewis acid: a species that can accept a lone pair of electrons (electron-deficient species), e.g., BF3, AlCl3, H+.

Lewis base: a species that can donate a lone pair of electrons, e.g., NH3, H2O, OH-, halide ions.

When a Lewis base donates its electron pair to a Lewis acid, a coordinate (dative) covalent bond is formed, e.g., NH3 + BF3 -> H3N->BF3. This definition is broader than the Bronsted-Lowry (proton-based) theory because it also explains acid-base behaviour in reactions that don't involve H+ transfer at all.

✓Final answer
$True — G.N. Lewis defined a base as a substance that donates an electron pair.$
CBSE 2025Set ANNUAL1 markMCQ
$Q. Metal ion like Ag+, Cu+ etc. acts as$
(a) Brönsted acid
(b) Brönsted base
(c) Lewis acid
(d) Lewis base
›Reveal solutionSolution

Metal ions such as Ag+ and Cu+ act as Lewis acids.

A Lewis acid is defined as a species that can accept a lone pair of electrons to form a coordinate (dative) bond. Metal cations like Ag+ and Cu+ have empty (vacant) orbitals and a positive charge, allowing them to accept electron pairs from Lewis bases (e.g. NH3, CN-, H2O) to form complex ions such as [Ag(NH3)2]+ or [Cu(NH3)4]2+. This makes them Lewis acids, not Brönsted acids/bases (which are defined in terms of proton donation/acceptance, not applicable here since these ions don't have a proton to donate).

✓Final answer
$(C) Lewis acid.$
CBSE 2024Set ANNUAL1 markMCQ
$Q. Which of the following is not a Lewis acid?$
(a) AlCl3
(b) BF3
(c) NH3
(d) FeCl3
›Reveal solutionSolution

NH3 is a Lewis base (electron-pair donor), not a Lewis acid.

A Lewis acid is a species that can accept a pair of electrons (electron-deficient, often with an incomplete octet). AlCl3, BF3, and FeCl3 all have a central atom with fewer than 8 valence electrons, so they readily accept an electron pair, making them Lewis acids. NH3, by contrast, has a lone pair of electrons on nitrogen that it can donate — this makes it a Lewis base, not a Lewis acid.

✓Final answer
$(C) NH3.$
CBSE 2024Set ANNUAL1 markMCQ
$Q. Species NH3 is$
(a) Lewis acid
(b) Lewis base
(c) Both (A) and (B)
(d) None of these
›Reveal solutionSolution
A Lewis base is an electron-pair donor; ammonia's nitrogen carries a lone pair that it readily donates, so NH3 is a Lewis base.

By the Lewis definition:

Lewis acid = electron-pair acceptor (typically has an empty orbital, e.g. BF3, H+).

Lewis base = electron-pair donor (has a lone pair available to share, e.g. NH3, H2O, OH-).

Ammonia, NH3, has a lone pair of electrons on the nitrogen atom. It uses this lone pair to form a coordinate (dative) bond with electron-deficient species (e.g. NH3 + BF3 -> H3N-BF3, or NH3 + H+ -> NH4+), classifying it as a Lewis base.

✓Final answer
$(b) Lewis base.$
CBSE 2023Set ANNUAL1 markMCQ
$Q. Which of the following is Lewis acid?$
(a) OH^-
(b) NH3
(c) BF3
(d) NH4^+
›Reveal solutionSolution
A Lewis acid is an electron-pair ACCEPTOR. BF3 has an incomplete octet on boron (only 6 electrons), so it readily accepts a lone pair from a Lewis base, e.g. forming BF3·NH3.

Lewis's definition: a Lewis acid accepts an electron pair; a Lewis base donates an electron pair.

OH− has a lone pair to donate → Lewis base

NH3 has a lone pair on N to donate → Lewis base

NH4+ already has a complete octet on N with no lone pair and no readily available vacant orbital → not a typical Lewis acid

BF3: boron has only 3 bonds (6 electrons total around it), leaving an empty 2p orbital — electron-deficient, eagerly accepts an electron pair (e.g. from NH3, forming the adduct F3B←NH3). This is the classic Lewis acid.

✓Final answer
$(c) BF3.$
CBSE 2023Set ANNUAL1 markMCQ
$Q. Which of the following is not a Lewis base?$
(a) HO-
(b) BCl3
(c) F-
(d) H2O
›Reveal solutionSolution
A Lewis base donates an electron pair; BCl3 instead accepts one, so it is a Lewis acid, not a Lewis base.

A Lewis base is a species that can donate a lone pair of electrons.

$OH^-$ : has lone pairs on oxygen to donate — Lewis base.

$BCl_3$ : boron has only 6 electrons around it (electron-deficient, incomplete octet) and readily accepts a lone pair to complete its octet — it is a Lewis acid, not a base.

$F^-$ : has lone pairs to donate — Lewis base.

$H_2O$ : has lone pairs on oxygen to donate — Lewis base.

✓Final answer
$BCl3 is not a Lewis base (option b) — it is a Lewis acid.$
CBSE 2023Set ANNUAL1 markMCQ
$Q. Which of the following is not a Lewis acid?$
(a) AlCl3.6H2O
(b) AlCl3
(c) SnCl4
(d) FeCl3
›Reveal solutionSolution

AlCl3.6H2O is not a Lewis acid because its aluminium centre is already coordinatively saturated by water.

A Lewis acid is an electron-pair acceptor. AlCl3, SnCl4 and FeCl3 are all classic Lewis acids: the central metal atom has an incomplete octet / empty low-lying orbitals available to accept an electron pair from a Lewis base.

In AlCl3.6H2O, however, the aluminium is already surrounded by six water molecules in an octahedral hydrated complex, $[Al(H_2O)_6]^{3+}\cdot 3Cl^-$ — all its coordination sites are filled, so it has no vacant orbital left to accept a new electron pair, and it does not behave as a Lewis acid.

✓Final answer
$Option (a): AlCl3.6H2O is not a Lewis acid.$
CBSE 2019Set ANNUAL1 markMCQ
$Q. Match the columns — Column A: (A) Arrhenius. Choose the correct matching term from Column B.$
(i) Estimation of nitrogen
(ii) Estimation of halogen
(iii) V ∝ T
(iv) Concept of Ionisation
(v) Gain of electrons
›Reveal solutionSolution

Arrhenius is associated with the concept of ionisation — his theory defined acids as substances that release H+ ions and bases as substances that release OH− ions in aqueous solution.

Svante Arrhenius proposed that electrolytes, when dissolved in water, dissociate (ionise) into positively and negatively charged ions, and that this ionisation is responsible for their electrical conductivity and their acidic/basic behaviour (H+ for acids, OH− for bases). This is why, of the five listed terms, Arrhenius is correctly matched with 'Concept of Ionisation'.

✓Final answer
$(iv) Concept of Ionisation.$
CBSE 2018Set ANNUAL1 mark
$Q. (b) Why is AlCl3 considered as Lewis acid?$
›Reveal solutionSolution

AlCl3's central Al has an incomplete octet, making it an electron-pair acceptor (Lewis acid).

Step 1 -- electron count on Al in AlCl3: Aluminium has 3 valence electrons, all used to form 3 single covalent bonds with the 3 chlorine atoms. This leaves aluminium with only 6 electrons in its valence shell (3 bond pairs), 2 electrons short of a complete octet.

Step 2 -- Lewis acid definition: A Lewis acid is any species that can accept a pair of electrons (an electron-pair acceptor) to form a new covalent (coordinate/dative) bond, typically because it has an empty orbital or an incomplete octet.

Step 3 -- application to AlCl3: Because aluminium in AlCl3 has an empty (vacant) orbital and an incomplete octet, it can readily accept a lone pair of electrons donated by an electron-rich species (a Lewis base, e.g. Cl- or NH3), forming a coordinate bond and completing its octet. (This is why AlCl3 is commonly used as a catalyst in Friedel-Crafts reactions -- it accepts a lone pair/chloride to generate the electrophile.)

✓Final answer
$AlCl3 acts as a Lewis acid because aluminium has an incomplete octet (only 6 electrons, an empty orbital available), allowing it to accept an electron pair from a Lewis base.$
CBSE 2018Set ANNUAL1 markMCQ
$Q. An example of Lewis acid is:$
(a) MgCl2
(b) BCl3
(c) NaCl
(d) SnCl4
›Reveal solutionSolution
BCl3 is an electron-deficient molecule (boron has only 6 electrons around it), so it readily accepts a lone pair of electrons — the defining property of a Lewis acid.

A Lewis acid is any species that can accept a pair of electrons to form a new covalent (coordinate) bond. Checking the options:

MgCl2 and NaCl are simple ionic salts; in basic classification they are not treated as Lewis acids.

BCl3: boron has only 3 valence electrons and forms 3 bonds to Cl, leaving it with just 6 electrons (an incomplete octet). This vacant orbital lets BCl3 accept an electron pair from a donor (e.g., NH3 → Cl3B←NH3), the textbook example of a Lewis acid.

SnCl4 can also behave as a Lewis acid in some contexts, but BCl3 is the standard example used in this syllabus (alongside BF3).

✓Final answer
$Option (b) BCl3 is an example of a Lewis acid — it has an incomplete octet on boron and accepts an electron pair.$

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Q.What are Lewis acids and bases? Give two examples for each.

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