Empirical formula gives the simplest integer ratio of atoms of each element in a compound, not the actual molecular composition.
If empirical formula is CH₂ and molar mass is 42 g/mol, molecular formula is
A C₂H₄
B C₃H₆
C C₄H₈
D C₅H₁₀
Empirical mass of CH₂ = 14. Factor = 42/14 = 3. Multiply subscripts by 3 → C₃H₆.
In the reaction 2H₂ + O₂ → 2H₂O, 4 moles of H₂ require O₂ moles equal to
A 1
B 2
C 3
D 4
Stoichiometric ratio H₂:O₂ = 2:1. For 4 moles H₂, required O₂ = 4 × (1/2) = 2 moles.
Limiting reagent is the reactant
A Present in excess amount
B That finishes first and limits product
C With higher molar mass
D That forms catalyst
The limiting reagent is consumed completely first, determining maximum product formed.
1 mole of Mg contains electrons (atomic number 12) equal to
A 6.022×10236.022 \times 10^{23}6.022×1023
B 1.204×10241.204 \times 10^{24}1.204×1024
C 7.226×10247.226 \times 10^{24}7.226×1024
D 3.011×10233.011 \times 10^{23}3.011×1023
One Mg atom has 12 electrons. Total electrons in 1 mol = 12×NA=12×6.022×1023≈7.226×102412 \times N_A = 12 \times 6.022\times10^{23} \approx 7.226\times10^{24}12×NA=12×6.022×1023≈7.226×1024.
The maximum number of electrons in a shell with n = 3 is
A 8
B 18
C 32
D 50
Max electrons in shell = 2n². For n=3, 2×9=18.
The number of orbitals in a subshell with l = 2 is
A 1
B 3
C 5
D 7
For a subshell, orbitals = (2l + 1). For l=2 (d-subshell), orbitals = 5.
Which quantum number decides shape of orbital
A n
B l
C m
D s
Azimuthal quantum number (l) determines subshell and orbital shape (s, p, d, f).
For a p-subshell, allowed values of m are
A 0 only
B +1 only
C 0, +1
D −1, 0, +1
For p, l=1, so m ranges from −l to +l → −1, 0, +1.
The number of molecules present in 18 g of water is closest to
A 6.022×10236.022 \times 10^{23}6.022×1023 molecules
B 3.011×10233.011 \times 10^{23}3.011×1023 molecules
C 1.204×10241.204 \times 10^{24}1.204×1024 molecules
D 9.033×10239.033 \times 10^{23}9.033×1023 molecules
Molar mass of water = 18 g/mol, so 18 g = 1 mol. One mole contains Avogadro number of molecules, i.e., 6.022×10236.022 \times 10^{23}6.022×1023.
Pauli exclusion principle states that
A Electrons fill lowest energy orbitals first
B No two electrons in an atom have same set of 4 quantum numbers
C Orbitals of equal energy fill singly first
D Electron has wave nature
Pauli principle limits electron pairing in an orbital to opposite spins and unique quantum-number sets.
Hund’s rule is related to
A Electron spin only
B Filling order of shells n=1,2,3
C Maximum multiplicity in degenerate orbitals
D Nuclear stability
Electrons occupy degenerate orbitals singly with parallel spins before pairing to reduce repulsion.
Aufbau principle is used to predict
A Valency only
B Electron configuration based on increasing energy
C Atomic size directly
D Boiling point trend
Aufbau means “building up”; electrons enter lower-energy orbitals first.
The electronic configuration of Cl (Z=17) is
A 1s²2s²2p⁶3s²3p⁵
B 1s²2s²2p⁶3s²3p⁶
C 1s²2s²2p⁶3s¹3p⁶
D 1s²2s²2p⁶3s²3p⁴
Chlorine has 17 electrons; filling gives [Ne]3s²3p⁵.
Which species is isoelectronic with Ne
A F⁻
B Na⁺
C Mg²⁺
D All of these
Ne has 10 electrons. F⁻ = 9+1=10, Na⁺ = 11−1=10, Mg²⁺ = 12−2=10.
Atomic radius generally decreases across a period because
A Shielding increases
B Nuclear charge decreases
C Effective nuclear charge increases
D Electrons move to higher shells
Across a period, nuclear charge rises while electrons enter same shell, increasing Z_eff and pulling electrons closer.
First ionization enthalpy generally increases across a period because
A Atomic size increases
B Atomic size decreases and Z_eff increases
C Electronegativity decreases
D Shielding dominates strongly
Higher Z_eff holds valence electrons more strongly, requiring more energy to remove.
Electron affinity is generally highest (most negative) for
A Alkali metals
B Noble gases
C Halogens
D Alkaline earth metals
Halogens need one electron to complete octet, so they release maximum energy when gaining an electron.
Which element has the highest electronegativity
A Oxygen
B Fluorine
C Chlorine
D Nitrogen
Fluorine is the most electronegative element due to small size and high nuclear attraction.
Metallic character decreases across a period because
A Atoms gain electrons easily
B Atoms lose electrons more easily
C Ionization enthalpy increases
D Atomic mass decreases
As ionization enthalpy rises, losing electrons becomes harder, so metallic character reduces.
Which has the largest atomic radius
A Na
B Mg
C Al
D Si
In period 3, atomic radius decreases from left to right due to increasing Z_eff; hence Na is largest.
Which has the highest first ionization enthalpy
A Li
B Be
C B
D C
Be has a filled 2s² subshell which is stable, making electron removal harder than B (2p¹).
An exception in ionization enthalpy trend is between
A Be and B
B C and N
C N and O
D All of these show known irregularities
Be>B (filled s vs entering p), N>O (half-filled p³ more stable than p⁴), etc. Such exceptions arise from subshell stability.
The number of protons in an atom is given by
A Mass number
B Atomic number
C Neutron number
D Isotopic number
Atomic number (Z) equals number of protons in nucleus.
Isotopes have same
A Mass number
B Number of neutrons
C Atomic number
D Number of nucleons always different
Isotopes are atoms of same element (same Z) but different mass number due to different neutrons.
The mass of 1 mole of O atoms is
A 8 g
B 16 g
C 32 g
D 64 g
Atomic mass of oxygen is 16 u, so 1 mol oxygen atoms has mass 16 g.
The number of moles in 9 g of Al (atomic mass 27) is
A 1/9
B 1/6
C 1/3
D 2/3
Moles = mass/molar mass = 9/27 = 1/3 mol.
Percentage of oxygen in H₂O is
A 11.11%
B 33.33%
C 66.67%
D 88.89%
Mass of O in water = 16, total = 18. So % = 16/18 ×100 = 88.89%.
If 2.24 L of gas at STP is taken, the moles are
A 0.01
B 0.05
C 0.1
D 1
Moles = 2.24/22.4 = 0.1 mol.
The total number of neutrons in 1123^{23}_{11}1123Na is
A 11
B 12
C 23
D 34
Neutrons = Mass number − Atomic number = 23 − 11 = 12.
Which set represents a valid set of quantum numbers (n, l, m, s)
A (2, 2, 0, +1/2)
B (3, 1, 2, −1/2)
C (3, 2, −2, +1/2)
D (1, 1, 0, +1/2)
For n=3, l can be 0,1,2. If l=2, m can be −2 to +2, so m=−2 allowed. Others violate l
The maximum number of electrons in a d-subshell is
A 2
B 6
C 10
D 14
d-subshell has 5 orbitals and each holds 2 electrons → 10.
Which orbital has a spherical shape
A s
B p
C d
D f
s-orbitals are spherically symmetric (only one orientation).
Which pair shows diagonal relationship
A Li and Mg
B Be and Al
C B and Si
D All of these
Diagonal relationship occurs between 2nd and 3rd period elements: Li–Mg, Be–Al, B–Si due to similar size/charge density.
The most reactive non-metal in the periodic table is
A Oxygen
B Fluorine
C Chlorine
D Bromine
Fluorine has highest electronegativity and strong oxidizing power, making it most reactive non-metal.
Atomic number of an element is 20. It belongs to
A Group 1
B Group 2
C Group 17
D Group 18
Z=20 is calcium: configuration [Ar]4s² → two valence electrons → group 2.
The element with configuration [Ne]3s²3p³ is
A Silicon
B Phosphorus
C Sulfur
D Chlorine
[Ne]=10, plus 3s²3p³ = 5 → total 15 → phosphorus.
The correct increasing order of atomic size is
A Cl < S < P < Si
B Si < P < S < Cl
C P < S < Cl < Si
D Cl < P < S < Si
In a period, atomic size decreases left to right, so Si is largest and Cl smallest.
Which has greater electron affinity
A Cl
B F
C Br
D I
Chlorine has slightly more negative electron affinity than fluorine due to less e–e repulsion in larger 3p orbital compared to compact 2p of F.
Which is most basic oxide
A Na₂O
B MgO
C Al₂O₃
D SiO₂
Metallic character is highest for Na; its oxide is strongly basic. MgO is basic but less; Al₂O₃ is amphoteric; SiO₂ is acidic.
The number of moles of atoms in 4.6 g of ethanol (C₂H₆O, molar mass 46) is
A 0.9 mol
B 0.8 mol
C 0.7 mol
D 0.6 mol
Moles of ethanol = 4.6/46 = 0.1 mol. Each molecule has 9 atoms. Moles of atoms = 0.1×9 = 0.9 mol.
Which has maximum effective nuclear charge on valence shell
A Na
B Mg
C Al
D Cl
Across period 3, Z increases while shielding changes little, so Z_eff increases and is highest for Cl.
Second ionization enthalpy of Na is very high because
A Na has large size
B Na⁺ has noble gas configuration
C Na has low electronegativity
D Na is a metal
After losing one electron, Na becomes Na⁺ with [Ne]. Removing another electron breaks stable configuration, needing very high energy.
Which is the correct order of first ionization enthalpy
A B > Be
B Be > B
C O > N
D Ne < F
Be has filled 2s² stable subshell, while B has 2p¹; removing 2p electron is easier, so Be has higher ionization enthalpy.
The element with highest electron affinity in period 3 is
A Si
B P
C S
D Cl
Chlorine gains one electron to achieve noble gas configuration and releases maximum energy (most negative EA) in period 3.