NDA Atomic structure, periodic table and bonding previous year questions with solutions

Answer: (b) The mass of an electron is about 1/⁠2000 times that of a proton

Method. properties of subatomic particles

Electron mass is about 1/⁠1836 of the proton's (m_p/⁠m_e = 1836), i.e. about 1/⁠2000; the atom's mass sits in the nucleus (NS8-A41). (a) A neutron is a separate nucleon, not made by combining an electron and proton here; (c) iodine-131, not cobalt, treats goitre; (d) Thomson proposed the plum-pudding model, and the neutron is Chadwick's (NS8-A40).

Answer: (d) Neutron

Method. discovery of the neutron

J. Chadwick discovered the neutron in 1932; the electron is Thomson's and the proton is Rutherford's (NS8-A40, NS8-A27).

Answer: (a) There is a neutral centre in an atom called nucleus

Method. Rutherford's nuclear model

Rutherford's model has a positively charged nucleus, not a neutral one, so (a) was not observed. (b), (d) and (c) (electrons revolving in circular paths) are part of the model (NS8-A22, NS8-A35, KC2-A25).

Answer: (c) Niels Bohr

Method. Bohr's postulate

Bohr (1913) proposed that electrons revolve only in certain stable orbits (shells) without radiating energy (NS8-A30, NS8-A32).

Answer: (c) Atomic Nucleus

Method. Rutherford's alpha-scattering experiment

Large-angle deflection of a few alpha particles showed a small, dense, positively charged nucleus (NS8-A21, KC2-A23).

Answer: (c) Electrons move in a circular path of fixed energy called orbits.

Method. limits of Rutherford's model

Rutherford's experiment showed (a) empty space, (d) mass in the nucleus, and (b) a nuclear radius about 10^-5 times the atom's (NS8-A22, NS8-A23). Fixed-energy circular orbits were Bohr's postulate (NS8-A30), not derived from the scattering.

Answer: (a) James Chadwick

Method. discovery of the neutron

James Chadwick discovered the neutron in 1932 (NS8-A40).

Answer: (c) Nucleus

Method. Rutherford's alpha-scattering experiment

Most alpha particles passed through gold foil, a few were deflected through large angles: the positive charge and most of the mass sit in a tiny centre, the nucleus (NCERT Class 9 Science ch 8, NS8-A21, NS8-A22). Electron was found by Thomson, proton named by Rutherford later, helium is an element.

Answer: (c) x = 18 and y = 32

Method. protons, electrons and nucleons from the nuclide symbol

32S2-: Z = 16, so x = 16 + 2 = 18 electrons; nucleons y = mass number = 32 (NS8-A57).

Answer: (a) 2

Method. 2n^2 rule

First shell n = 1: 2 x 1^2 = 2 electrons (NS8-A62, NS8-A63).

Answer: (d) The valency of Al is 2

Method. particles in 27Al and valency

Z = 13: protons 13, electrons 13, neutrons 27 − 13 = 14. Al is 2, 8, 3: it loses 3 electrons, valency 3, not 2. So (d) is NOT correct (NS8-A57, NS8-A72).

Answer: (c) The number of electrons in argon is 18.

Method. protons, neutrons, electrons from Z and A

Argon: Z = 18, A = 40. Protons 18, electrons 18 (neutral atom), neutrons 40 − 18 = 22. Only (c) is true; protons plus electrons = 36, not 40 (NS8-A56, NS8-A57).

Answer: (d) 7

Method. Bohr shell distribution

Z = 35 (bromine): 2, 8, 18, 7. Valence shell (N) has 7 electrons (NS8-A62, NS8-A70).

Answer: (c) 18

Method. 2n^2 rule

M shell is n = 3: 2 x 3^2 = 18 electrons (NS8-A62).

Answer: (d) N^-

Method. isoelectronic with F-

F- has 9 + 1 = 10 electrons. O2-: 10; Na+: 10; Ne: 10; N-: 7 + 1 = 8. So N- is not isoelectronic (KC3-A81).

Answer: (d) 32

Method. 2n^2 rule for shell capacity

N shell is n = 4: maximum electrons = 2n^2 = 2 x 16 = 32 (NS8-A62).

Answer: (b) protons and neutrons only

Method. mass number = protons + neutrons

Atomic mass is carried by the nucleons: protons plus neutrons only; electrons contribute negligibly (NS8-A57, NS8-A41).

Answer: (b) Na⁺, O^2-

Method. isoelectronic = same number of electrons

(a) Mg2+ 10, Ar 18. (b) Na+ 11 − 1 = 10, O2- 8 + 2 = 10. (c) Al3+ 10, Cl- 18. (d) K+ 18, Ne 10.
Only the pair in (b) has equal electrons (KC3-A81).

Answer: (c) 4018Ar⁺

Method. isoelectronic species: count electrons

Cl (Z = 17) has 17 electrons.
(a) 32S: 16. (b) 34S+: 16 − 1 = 15. (c) 40Ar+: 18 − 1 = 17. (d) 35S2-: 16 + 2 = 18.
Only (c) has 17 electrons as printed (isoelectronic species, KC3-A81).

Answer: (a) 19% and 81%

Method. weighted average to find abundance

Let x% be 10B: (10x + 11(100 − x))/⁠100 = 10.81, so 1100 − x = 1081, x = 19. 10B 19%, 11B 81% (NS8-A82).

Answer: (b) 35.5 u

Method. weighted average atomic mass

(3 x 35 + 1 x 37)/⁠4 = 142/⁠4 = 35.5 u, as in NCERT (NS8-A81, NS8-A82).

Answer: (c) Cobalt

Method. uses of radioisotopes

Cobalt-60 (gamma source) is used in cancer treatment; iodine-131 is for thyroid and goitre (NS8-A80).

Answer: (b) Cobalt

Method. uses of radioisotopes

Cobalt-60 is used for cancer radiotherapy; iodine-131 is used for thyroid disorders such as goitre; uranium-235 is a reactor fuel (NS8-A80).

Answer: (d) 16.5 u

Method. weighted average atomic mass

Average = (3 x 16 + 1 x 18)/⁠4 = 66/⁠4 = 16.5 u (NS8-A82, same method as the chlorine example).

Answer: (c) Mg

Method. formula and bonding show the group

XCl2 solid with a high melting point is an ionic compound X2+ (Cl-)2, so X loses two electrons: Group 2, like Mg. Na and K give XCl, and Al gives AlCl3 (KC3-A110).

Answer: (d) I, Br, Cl, F

Method. oxidising power of halogens

Oxidising power falls down Group 17: F2 > Cl2 > Br2 > I2 (higher electronegativity, KC3-A104). Increasing order: I, Br, Cl, F.

Answer: (b) Mercury and Bromine

Method. metals and non-metals liquid at room temperature

Mercury and bromine are the two elements liquid at room temperature. Gallium (m.p. 29.8 C) and caesium (28.4 C) are solids at ordinary room temperature. So the pair in (b).

Answer: (b) Atomic number

Method. atomic number defines the element

Atomic number (number of protons) is the fixed identity of an element and decides its place in the periodic table; mass, colour and melting point do not (KC3-A27, NS8-A56).

Answer: (d) Caesium

Method. trend of melting point down Group 1

Melting points of the alkali metals fall down the group as metallic bonding weakens with increasing size (KC3-A90): Na 98 C, K 63 C, Rb 39 C, Cs 28.4 C. Caesium is lowest.

Answer: (a) Lithium

Method. reactivity of alkali metals with water increases down the group

Reactivity of Group 1 metals with water rises from Li to Cs because the outer electron is lost more easily as size grows (NCERT Class 11 Chemistry ch 3, KC3-A90). Lithium is therefore the least reactive of the four.

Answer: (b) II only

Method. hydrogen bonding

Statement I follows the textbook (H bonded to N, O or F; KC4-A134) so it is taken as correct. II is wrong: hydrogen bonds are both intermolecular and intramolecular (KC4-A138). III is correct: lower electronegativity gives a weaker bond (KC4-A136). Only II is not correct.

Statement I is taken as the textbook states it; strictly, Cl and S can form weak hydrogen bonds, which does not change which statement is asked as incorrect.

Answer: (a) the presence of strong electrostatic forces between ions in the crystal lattice.

Method. bonding and properties of ionic solids

Ionic solids are held by strong electrostatic attraction between ions in the lattice (high lattice enthalpy), so melting and boiling points are high (KC4-A37, KC4-A39). A sea of electrons is metallic bonding.

Answer: (a) Nitride

Method. charge of common anions

Nitride N3- has charge -3; nitrate NO3- is -1; sulphide S2- is -2; sulphate SO4 2- is -2.

Answer: (c) Nitrogen

Method. valency from configuration

Nitrogen 2, 5: needs 3 electrons for an octet, valency 3. Carbon is 4, oxygen 2, sodium 1 (NS8-A72, NS8-A73).

Answer: (c) 6, 6

Method. NaCl crystal lattice

Rock-salt structure: each Na+ is surrounded by 6 Cl- and each Cl- by 6 Na+, so the coordination numbers are 6 and 6 (KC4-A37).

Answer: (b) MgO

Method. lattice energy and melting point

MgO is Mg2+ O2-, with doubly charged ions and a very high lattice enthalpy (m.p. about 2850 C). Na2O (about 1130 C), CuO (about 1330 C) and Fe2O3 (about 1565 C) are lower (KC4-A39).

Answer: (a) (NH₄)₂CO₃

Method. balance charges to write the formula

Ammonium NH4+ (+1), carbonate CO3 2- (-2): two NH4+ per CO3 2- gives (NH4)2CO3 (criss-cross of valencies).

Answer: (c) Silicon carbide

Method. ionic versus covalent compounds

CaO, Na3N and ZnS are built from metal and non-metal (ionic). Silicon carbide is a giant covalent network of Si and C, both non-metal/⁠metalloid (KC4-A12).

Answer: (a) Ne < Mg < N < Si

Method. valency from electronic configuration

Ne (2, 8): 0. Mg (2, 8, 2): 2. N (2, 5): 3. Si (2, 8, 4): 4. Increasing order: Ne < Mg < N < Si (NS8-A72, KC3-A110).

Answer: (c) 5

Method. count bonds in CH3OH

H3C-O-H: three C-H, one C-O, one O-H = 5 covalent bonds.

Answer: (d) Mg, Ca, Ba

Method. valency from group

Mg, Ca, Ba are all Group 2 (ns2), valency 2 (NS8-A72, KC3-A110). In the other sets Na and K have valency 1 and Al valency 3.

Answer: (d) Water is a non-polar molecule.

Method. polarity of water and hydrogen bonding

Water is bent and the O-H bonds are polar, so the molecule is polar, not non-polar. Hydrogen bonding in liquid water gives its high boiling point and high heat of fusion (KC4-A134, KC4-A135).

Answer: (b) Two

Method. electronic configuration and ionic valency

Z = 8: 2, 6. Six valence electrons, needs two more to complete the octet, so it gains 2 electrons (forming O2- in Na2O) (NS8-A72, NS8-A73).

Answer: (d) total number of electrons in the outer most shell of an atom

Method. valency is fixed by the valence electrons

Valency is the number of electrons gained, lost or shared to complete the octet of the outermost shell (NS8-A72). It depends on the electrons in the outermost shell, not on protons, mass number or neutrons.

Answer: (d) All four

Method. classify four reactions

I: Fe displaces Cu from CuSO4, displacement (C1-A21). II: ion exchange giving BaSO4 precipitate, double displacement (C1-A23). III: CO is oxidised and Fe2O3 reduced, redox (C1-A28). IV: KClO3 heated to KCl and O2, thermal decomposition (C1-A56). All four are correctly matched.

Answer: (c) 3 4 1 2

Method. types of chemical reactions

A: CaO + H2O -> Ca(OH)2 is combination (3). B: CaCO3 -> CaO + CO2 is decomposition (4). C: Pb displaces Cu from CuCl2, single displacement (1). D: ions exchange partners, BaSO4 precipitates, double displacement (2). Code 3 4 1 2 (C1-A13, C1-A21, C1-A23).

Answer: (d) 2Pb(NO₃)₂ (s) → 2PbO (s) + 4NO₂ (g) + O₂ (g)

Method. thermal decomposition absorbs heat

(a) as printed is a combustion of methane (exothermic whatever its misprinted product); (b) respiration (exothermic, C1-A11); (c) Haber synthesis of NH3 (exothermic); (d) heating lead nitrate breaks it down, endothermic (C1-A15, C1-A62).

Option (a) misprints the product as 2O2(g) for 2H2O; judged as printed, it is not endothermic either, so the answer is unchanged.

Answer: (b) in sunlight

Method. photolytic decomposition

Silver chloride decomposes in sunlight to silver and chlorine; it is stored in dark bottles because it is light-sensitive (C1-A19).

Answer: (c) 3 2 4 1

Method. classify reaction types

A: 2H2O -> 2H2 + O2 is decomposition (3). B: C + O2 -> CO2 is combination (2). C: Fe + CuSO4 is displacement (4). D: 3BaCl2 + Al2(SO4)3 -> 3BaSO4 + 2AlCl3 is double displacement (1). Code 3 2 4 1 (C1-A13, C1-A21, C1-A23, C1-A68).

Answer: (d) Solid, liquid, gas

Method. physical states at room conditions

HgO is a red solid, mercury is a liquid metal, oxygen is a gas: solid, liquid, gas (C1-A13).

The heat sign is printed as a filled triangle, evidently the delta for heat; the answer does not depend on it.

Answer: (d) Displacement reaction

Method. displacement reaction

Fe + CuSO4 -> FeSO4 + Cu: more reactive iron displaces copper, which deposits as a brown coating (C1-A21, C1-A49).

Answer: (b) 2AgCl(s) → (sunlight) 2Ag(s) + Cl₂(g)

Method. a single compound breaks into simpler substances

(a) combustion; (b) one compound AgCl splits into Ag and Cl2 in sunlight, a photolytic decomposition; (c) CuO + H2 is displacement/⁠redox; (d) Fe + CuSO4 is displacement. Answer (b) (C1-A19).

Answer: (d) Exothermic reaction.

Method. type of reaction CaO + H2O

CaO + H2O -> Ca(OH)2 evolves a large amount of heat, so it is exothermic (and a combination reaction, which is not offered) (C1-A09). It is not a displacement, endothermic or decomposition reaction.