Chemistry Basic Concepts Atomic Structure Competitive Exams
100 questions · General Science
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Question 1 of 100
How many molecules are present in 0.5 moles of a substance?
ExplanationNumber of molecules = moles × Avogadro's number = 0.5 × 6.022×10²³ = 3.011×10²³.
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Question 2 of 100
An ion is formed when an atom:
ExplanationIons form when atoms gain electrons (anions, negative charge) or lose electrons (cations, positive charge).
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Question 3 of 100
Canal rays, used in the discovery of the proton, carry what type of charge?
ExplanationCanal rays (anode rays) are positively charged particles, leading to the identification of the proton by Eugen Goldstein and later studies.
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Question 4 of 100
Matter exists in how many fundamental states (classical)?
ExplanationClassically, matter exists in three states: solid, liquid, and gas (plasma and Bose-Einstein condensate are additional modern states).
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Question 5 of 100
An atom that gains one or more electrons becomes a(n):
ExplanationGaining electrons gives more electrons than protons, resulting in a net negative charge (anion).
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Question 6 of 100
De Broglie's hypothesis relates a particle's wavelength to its:
ExplanationDe Broglie proposed λ = h/p, relating a particle's wavelength (λ) inversely to its momentum (p).
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Question 7 of 100
Distillation is used to separate:
ExplanationDistillation separates liquids with different boiling points by heating and condensing vapors selectively.
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Question 8 of 100
A catalyst in a chemical reaction:
ExplanationA catalyst increases reaction rate by providing an alternate lower-energy pathway, without itself being used up.
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Question 9 of 100
The electronic configuration of Helium (atomic number 2) is:
ExplanationHelium has two electrons, both filling the 1s orbital, giving configuration 1s².
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Question 10 of 100
Which particle was discovered later than both the electron and proton, due to the difficulty in detecting a neutral particle?
ExplanationThe neutron, being electrically neutral, was harder to detect and was discovered by Chadwick in 1932, after the electron (1897) and proton (early 1900s).
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Question 11 of 100
Isobars are atoms of different elements having the same:
ExplanationIsobars have the same mass number but different atomic numbers (different elements), e.g., Argon-40 and Calcium-40.
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Question 12 of 100
J.J. Thomson's atomic model is commonly known as the:
ExplanationThomson proposed electrons embedded in a positively charged sphere, resembling plums in a pudding.
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Question 13 of 100
The atomic number (Z) of an atom equals the number of:
ExplanationAtomic number equals the number of protons, which uniquely identifies the element.
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Question 14 of 100
Which subshell can hold a maximum of 2 electrons?
ExplanationThe s subshell has one orbital, holding a maximum of 2 electrons.
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Question 15 of 100
The smallest particle of an element that retains its chemical properties is called a(n):
ExplanationAn atom is the smallest unit of an element that retains the element's chemical identity.
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Question 16 of 100
Molality is defined as the number of moles of solute per:
ExplanationMolality (m) = moles of solute / mass of solvent in kilograms, making it independent of temperature unlike molarity.
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Question 17 of 100
Which particle was discovered using the cathode ray tube experiment?
ExplanationJ.J. Thomson's cathode ray tube experiments led to the discovery of the electron.
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Question 18 of 100
In Bohr's model of the atom, electrons move in:
ExplanationBohr proposed electrons occupy quantized, fixed circular orbits, each corresponding to a specific energy level.
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Question 19 of 100
The molar mass of a substance is numerically equal to its:
ExplanationMolar mass (g/mol) is numerically equal to the atomic or molecular mass (in amu) of the substance.
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Question 20 of 100
A major limitation of Bohr's model is that it could not explain the spectra of:
ExplanationBohr's model successfully explained hydrogen's spectrum but failed for atoms with more than one electron due to electron-electron interactions.
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Question 21 of 100
Which of the following represents a homogeneous mixture?
ExplanationA salt solution is uniform throughout (homogeneous); sand-water, oil-water, and salt-sand mixtures are heterogeneous.
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Question 22 of 100
The principal quantum number (n) determines the:
ExplanationThe principal quantum number (n = 1, 2, 3...) indicates the main energy level (shell) and roughly the size of the orbital.
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Question 23 of 100
In the reaction 2H₂ + O₂ → 2H₂O, the coefficient '2' before H₂ indicates:
ExplanationA coefficient in front of a formula denotes the number of molecules (or moles) of that substance taking part in the reaction.
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Question 24 of 100
Rutherford's gold foil experiment led to the discovery of the:
ExplanationRutherford's alpha particle scattering experiment revealed a small, dense, positively charged nucleus at the atom's center.
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Question 25 of 100
Hund's Rule states that electrons occupying orbitals of equal energy will:
ExplanationHund's Rule states that electrons fill degenerate (equal energy) orbitals singly first, with parallel spins, before pairing.
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Question 26 of 100
The mass number (A) of an atom equals:
ExplanationMass number is the total count of protons and neutrons (nucleons) in the nucleus.
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Question 27 of 100
Sublimation is the direct change of state from:
ExplanationSublimation is the direct transition of a substance from solid to gas phase, bypassing the liquid phase (e.g., dry ice, camphor).
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Question 28 of 100
Which of the following best describes a limiting reagent in a chemical reaction?
ExplanationThe limiting reagent runs out first, determining the maximum amount of product that can form in a reaction.
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Question 29 of 100
The Law of Conservation of Mass states that mass:
ExplanationAntoine Lavoisier's Law of Conservation of Mass states total mass of reactants equals total mass of products in a chemical reaction.
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Question 30 of 100
The empirical formula of a compound represents:
ExplanationAn empirical formula shows the simplest ratio of elements, while the molecular formula shows the actual number of atoms.
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Question 31 of 100
The number of orbitals in the p subshell is:
ExplanationThe p subshell consists of three orbitals (px, py, pz), each capable of holding 2 electrons.
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Question 32 of 100
Heisenberg's Uncertainty Principle has significant implications primarily for:
ExplanationThe uncertainty principle's effects are significant for very small particles like electrons, while negligible for macroscopic objects.
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Question 33 of 100
A compound is formed when two or more elements combine in a:
ExplanationCompounds have a fixed, definite ratio of elements by mass, as per the Law of Definite Proportions.
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Question 34 of 100
The SI unit of temperature is:
ExplanationKelvin is the SI base unit of thermodynamic temperature.
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Question 35 of 100
Which subshell can hold a maximum of 6 electrons?
ExplanationThe p subshell has three orbitals, each holding 2 electrons, for a maximum of 6 electrons.
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Question 36 of 100
The relationship PV = nRT is known as the:
ExplanationPV = nRT is the Ideal Gas Equation, combining Boyle's Law, Charles's Law, and Avogadro's Law into one unified relationship.
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Question 37 of 100
The maximum number of electrons that can occupy a shell is given by the formula:
ExplanationThe maximum number of electrons in the nth shell is 2n² (e.g., n=2 shell holds up to 8 electrons).
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Question 38 of 100
The quantum mechanical model of the atom is based on which equation?
ExplanationThe modern quantum mechanical model uses Schrödinger's wave equation to describe electron behavior as probability distributions (orbitals).
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Question 39 of 100
The chemical formula for water is:
ExplanationWater consists of two hydrogen atoms and one oxygen atom, giving the formula H₂O.
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Question 40 of 100
The spin quantum number (mₛ) can have values of:
ExplanationThe spin quantum number describes the intrinsic spin of an electron, which can be either +1/2 or −1/2.
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Question 41 of 100
The percentage composition of an element in a compound is calculated using:
ExplanationPercentage composition = (total mass of a given element in one mole of the compound ÷ molar mass of the compound) × 100.
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Question 42 of 100
Which subatomic particle has the least mass?
ExplanationThe electron's mass (~9.1×10⁻³¹ kg) is about 1/1836 the mass of a proton or neutron.
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Question 43 of 100
Which of the following is a pure substance?
ExplanationDistilled water is a pure substance (single compound); air, salt water, and milk are mixtures.
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Question 44 of 100
Who proposed the first modern atomic theory stating atoms are indivisible?
ExplanationJohn Dalton proposed in 1803 that atoms are indivisible, indestructible particles that make up all matter.
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Question 45 of 100
Which of the following best represents a chemical equation obeying mass conservation?
ExplanationBalancing: 1C=1C, 4H=4H (2H₂O), and O: left 2×2=4 O, right 2(CO₂)+2(H₂O)=2+2=4 O — this equation balances correctly.
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Question 46 of 100
Which technique separates a dissolved solid from its solution by evaporating the solvent?
ExplanationEvaporation removes the liquid solvent, leaving behind the dissolved solid.
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Question 47 of 100
An atomic orbital represents a region of space where the probability of finding an electron is:
ExplanationAn orbital is a three-dimensional region around the nucleus where there's a high probability (typically ≥90%) of finding an electron.
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Question 48 of 100
The energy of an electron in the nth orbit of a hydrogen atom, according to Bohr, is proportional to:
ExplanationBohr derived that the energy of an electron in the nth orbit of hydrogen is given by E ∝ −1/n², becoming less negative (higher energy) as n increases.
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Question 49 of 100
Which of the following is an example of a chemical formula showing a polyatomic ion?
ExplanationSulfate (SO₄²⁻) is a polyatomic ion, made of multiple atoms bonded together carrying an overall charge; Na⁺, Cl⁻, and Mg²⁺ are simple monatomic ions.
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Question 50 of 100
The Law of Definite Proportions states that a compound always contains:
ExplanationProust's Law of Definite Proportions states a given compound always contains the same elements combined in the same fixed mass ratio.
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Question 51 of 100
According to Bohr's model, electrons absorb energy and emit a photon when they:
ExplanationElectrons absorb energy to jump to higher orbits and then emit energy as photons when they fall back to lower orbits.
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Question 52 of 100
The Aufbau principle states that electrons fill orbitals:
ExplanationThe Aufbau principle states electrons occupy the lowest energy orbitals available before filling higher energy ones.
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Question 53 of 100
The electronic configuration of Hydrogen (atomic number 1) is:
ExplanationHydrogen has one electron, occupying the 1s orbital, giving configuration 1s¹.
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Question 54 of 100
Electrons in the outermost shell of an atom are called:
ExplanationValence electrons occupy the outermost shell and primarily determine an atom's chemical bonding behavior.
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Question 55 of 100
The SI unit of amount of substance is the:
ExplanationThe mole is the SI base unit for measuring the amount of substance.
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Question 56 of 100
The Pauli Exclusion Principle states that no two electrons in an atom can have:
ExplanationPauli's Exclusion Principle states that each electron in an atom must have a unique combination of all four quantum numbers.
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Question 57 of 100
Which subshell can hold a maximum of 10 electrons?
ExplanationThe d subshell has five orbitals, each holding 2 electrons, for a maximum of 10 electrons.
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Question 58 of 100
Which of the following elements has an exception to the normal Aufbau filling order due to extra stability of half-filled/filled subshells?
ExplanationChromium's configuration is [Ar] 3d⁵4s¹ instead of the expected [Ar] 3d⁴4s², due to the extra stability of a half-filled d subshell.
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Question 59 of 100
The Lyman series of hydrogen spectral lines corresponds to electron transitions ending at:
ExplanationThe Lyman series involves transitions ending at the ground state (n=1), producing lines in the ultraviolet region.
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Question 60 of 100
The chemical formula for carbon dioxide is:
ExplanationCarbon dioxide consists of one carbon atom and two oxygen atoms, giving the formula CO₂.
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Question 61 of 100
Isotones are atoms that have the same number of:
ExplanationIsotones have the same number of neutrons but different numbers of protons, hence different elements.
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Question 62 of 100
Which of the following correctly represents the electron configuration shorthand for Oxygen (atomic number 8) using noble gas notation?
ExplanationOxygen has 8 electrons; using Helium (2 electrons) as the core gives [He] 2s² 2p⁴, totaling 2+2+4=8 electrons.
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Question 63 of 100
The Balmer series of hydrogen spectral lines corresponds to electron transitions ending at:
ExplanationThe Balmer series consists of spectral lines from transitions where electrons fall to the n=2 energy level, falling mostly in the visible range.
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Question 64 of 100
Which of the following is a mixture?
ExplanationAir is a mixture of gases (nitrogen, oxygen, etc.); water, salt, and CO2 are pure compounds.
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Question 65 of 100
The relative atomic mass of an element is measured relative to:
ExplanationThe modern standard for atomic mass is Carbon-12, assigned a mass of exactly 12 atomic mass units (amu).
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Question 66 of 100
The azimuthal (angular momentum) quantum number (l) determines the:
ExplanationThe azimuthal quantum number (l) defines the subshell type (s, p, d, f) and thus the shape of the orbital.
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Question 67 of 100
The charge on an electron is:
ExplanationElectrons carry a negative elementary charge.
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Question 68 of 100
Which of these is a chemical property of a substance?
ExplanationReactivity with acid describes how a substance chemically transforms, unlike color, melting point, and density, which are physical properties.
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Question 69 of 100
The magnetic quantum number (mₗ) determines the:
ExplanationThe magnetic quantum number specifies the spatial orientation of an orbital within a given subshell.
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Question 70 of 100
Which of the following correctly calculates the number of moles in 22 grams of CO₂ (molar mass = 44 g/mol)?
ExplanationMoles = mass / molar mass = 22 / 44 = 0.5 moles.
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Question 71 of 100
The modern periodic table arranges elements primarily based on increasing:
ExplanationThe modern periodic law states that properties of elements are periodic functions of their atomic number, which is the basis for the modern periodic table's arrangement.
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Question 72 of 100
Neutrons carry what electric charge?
ExplanationNeutrons are electrically neutral particles found in the nucleus alongside protons.
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Question 73 of 100
A solution that contains the maximum amount of solute that can dissolve at a given temperature is called:
ExplanationA saturated solution holds the maximum solute it can dissolve at a specific temperature without excess solute settling out.
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Question 74 of 100
The charge on a proton is:
ExplanationProtons carry a positive elementary charge, equal in magnitude to the electron's negative charge.
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Question 75 of 100
The number of orbitals in the d subshell is:
ExplanationThe d subshell consists of five orbitals, together holding up to 10 electrons.
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Question 76 of 100
An element is a substance that:
ExplanationElements are the simplest form of pure substances and cannot be chemically decomposed further.
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Question 77 of 100
The electronic configuration of Sodium (atomic number 11) is:
ExplanationSodium has 11 electrons, filling in order: 1s²(2) + 2s²(2) + 2p⁶(6) + 3s¹(1) = 11 electrons total.
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Question 78 of 100
Molarity is defined as the number of moles of solute per:
ExplanationMolarity (M) = moles of solute / volume of solution in liters.
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Question 79 of 100
One mole of any substance contains how many particles?
ExplanationAvogadro's number, 6.022 × 10²³, is the number of particles (atoms, molecules, etc.) in one mole of a substance.
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Question 80 of 100
Melting of ice into water is an example of a:
ExplanationMelting is a physical change since no new substance is formed; only the state changes.
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Question 81 of 100
For a neutral atom, the number of electrons equals the number of:
ExplanationIn a neutral atom, the number of negatively charged electrons balances the number of positively charged protons.
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Question 82 of 100
In a balanced chemical equation, the number of atoms of each element on both sides must be:
ExplanationA balanced equation obeys the Law of Conservation of Mass, requiring equal numbers of atoms of each element on both sides.
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Question 83 of 100
Burning of paper is an example of a:
ExplanationBurning produces new substances (ash, gases), making it a chemical change.
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Question 84 of 100
The term 'stoichiometry' in chemistry refers to:
ExplanationStoichiometry deals with calculating quantities of reactants and products based on balanced chemical equations.
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Question 85 of 100
Which subshell can hold a maximum of 14 electrons?
ExplanationThe f subshell has seven orbitals, each holding 2 electrons, for a maximum of 14 electrons.
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Question 86 of 100
Valency of an element is defined as its combining capacity with:
ExplanationValency traditionally refers to an element's combining power, often measured relative to hydrogen.
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Question 87 of 100
An atom that loses one or more electrons becomes a:
ExplanationLosing electrons leaves more protons than electrons, giving a net positive charge (cation).
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Question 88 of 100
The Law of Multiple Proportions applies when two elements form:
ExplanationDalton's Law of Multiple Proportions states that when two elements form multiple compounds, the masses of one element combining with a fixed mass of the other are in simple whole number ratios.
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Question 89 of 100
Which of the following is NOT one of the three classical states of matter?
ExplanationClassical chemistry traditionally discusses solid, liquid, and gas; plasma is considered a separate, additional state of matter.
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Question 90 of 100
In Rutherford's model, most of the atom's volume is occupied by:
ExplanationRutherford's experiment showed that atoms are mostly empty space, with a tiny dense nucleus at the center.
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Question 91 of 100
Two or more atoms chemically bonded together form a:
ExplanationA molecule is formed when two or more atoms bond chemically.
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Question 92 of 100
Avogadro's number is also known as:
ExplanationAvogadro's number (6.022×10²³ per mole) is formally called the Avogadro constant.
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Question 93 of 100
Density is defined as mass per unit:
ExplanationDensity = mass / volume.
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Question 94 of 100
Filtration is used to separate:
ExplanationFiltration separates insoluble solid particles from a liquid using a filter medium.
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Question 95 of 100
If the empirical formula of a compound is CH₂O and its molecular mass is 180, what is its molecular formula?
ExplanationEmpirical formula mass of CH₂O = 30. Molecular mass/empirical mass = 180/30 = 6, so molecular formula = (CH₂O)₆ = C₆H₁₂O₆.
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Question 96 of 100
Which of the following is an example of an exothermic reaction?
ExplanationCombustion releases heat energy (exothermic); photosynthesis, melting, and evaporation absorb energy (endothermic).
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Question 97 of 100
The number of significant figures in the measurement 0.0405 is:
ExplanationLeading zeros are not significant; 4, 0, and 5 are significant, giving three significant figures.
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Question 98 of 100
Which of the following is an example of an endothermic process?
ExplanationMelting absorbs heat from surroundings (endothermic); combustion, typical neutralization, and rusting release heat (exothermic).
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Question 99 of 100
A supersaturated solution contains:
ExplanationA supersaturated solution is unstable, holding more dissolved solute than equilibrium (saturation) would normally allow, often via careful cooling.
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Question 100 of 100
Isotopes of an element have the same atomic number but different:
ExplanationIsotopes share the same number of protons (and electrons when neutral) but differ in neutron number, giving different mass numbers.
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