CA02 · Quiz — Structure of Atom & Molecules

Question 1 of 20

Atoms of the same element with different mass numbers (different number of neutrons) are called:

AIsobars BIsotopes CIsotones DAllotropes

Isotopes: same atomic number (same protons, same element), different mass number (different neutrons). Examples: H-1, H-2 (deuterium), H-3 (tritium); C-12, C-14; U-235, U-238. Isobars = same mass number, different atomic numbers.

Question 2 of 20

The number of protons in the nucleus of an atom is called the:

AMass number BNeutron number CAtomic number DValence number

The atomic number (Z) = number of protons in the nucleus. It uniquely identifies the element. For a neutral atom, Z also equals the number of electrons. Mass number (A) = protons + neutrons.

Question 3 of 20

Rutherford's gold foil experiment proved that:

AElectrons are embedded in a diffuse sphere of uniform positive charge BThe atom is mostly empty space with a tiny, dense, positive nucleus CAtoms are indivisible particles with no internal structure whatsoever DElectrons orbit in fixed energy shells without radiating energy

Rutherford's alpha particle scattering experiment (1911): most alpha particles passed straight through (mostly empty space); a few deflected slightly; very few bounced back → tiny, dense, positively charged nucleus. This replaced Thomson's plum pudding model.

Question 4 of 20

Which quantum number determines the shape of an orbital?

APrincipal quantum number (n) BAzimuthal quantum number (l) CMagnetic quantum number (ml) DSpin quantum number (ms)

Azimuthal quantum number (l): l=0 → s orbital (spherical); l=1 → p orbital (dumbbell); l=2 → d orbital (cloverleaf); l=3 → f orbital. Principal quantum number (n) determines the energy level/shell. Magnetic quantum number (ml) determines orbital orientation.

Question 5 of 20

The electronic configuration of sodium (Na, Z=11) is:

A2, 8, 2 B2, 8, 1 C2, 9 D3, 8

Na (Z=11): 2 electrons in 1st shell, 8 in 2nd shell, 1 in 3rd shell = 2,8,1. Or in orbital notation: 1s² 2s² 2p⁶ 3s¹. Na has 1 valence electron → readily loses it to form Na⁺ → highly reactive metal.

Question 6 of 20

Isotopes of an element have the same:

AMass number but different atomic number BAtomic number and same chemical properties, but different mass number CNumber of neutrons but different protons DPhysical properties but different chemical properties

Isotopes: same atomic number (same element, same protons) → same chemical properties (same electron configuration). Different mass number (different neutrons) → different physical properties (density, melting point). Example: ¹²C and ¹⁴C have identical chemistry but different atomic masses.

Question 7 of 20

Bohr's model of the atom proposed that electrons:

AMove randomly in the space around the nucleus BOccupy fixed circular orbits at specific energy levels without radiating energy CAre embedded in the nucleus alongside protons DExist in clouds of probability described by wave functions

Bohr's model (1913): electrons orbit the nucleus in fixed circular paths (shells) at specific energy levels. An electron only emits/absorbs energy when it jumps between levels. Explained the hydrogen spectrum. Limitations: does not explain multi-electron atoms or fine spectral structure.

Question 8 of 20

The maximum number of electrons that can be accommodated in a shell is given by:

An² B2n² Cn/2 D3n²

Maximum electrons in shell n = 2n². Shell 1 (n=1): 2×1² = 2. Shell 2 (n=2): 2×2² = 8. Shell 3 (n=3): 2×3² = 18. Shell 4 (n=4): 2×4² = 32. This governs the periodic table structure.

Question 9 of 20

The valency of chlorine (Cl, Z=17) in HCl is:

A1 B2 C3 D7

Cl has electronic configuration 2,8,7 — 7 valence electrons. It needs 1 more to complete its octet → valency = 1. In HCl, one bond forms between H and Cl (Cl gains 1 electron). Valency = number of electrons gained, lost, or shared.

Question 10 of 20

Covalent bonds are formed by:

AComplete transfer of electrons from one atom to another BSharing of electron pairs between atoms CAttraction between oppositely charged ions DDelocalisation of electrons across a metallic lattice

Covalent bonds form when atoms share electron pairs to achieve stable electronic configurations (octet/duplet rule). Example: H₂ (H shares 1 electron with H). Non-metals typically form covalent bonds. Ionic bonds = electron transfer; metallic bonds = delocalised electrons.

Question 11 of 20

The ionic bond in NaCl is formed because:

ANa and Cl atoms share electrons equally BNa loses 1 electron to Cl, forming Na⁺ and Cl⁻ ions CCl transfers an electron to Na DNa and Cl form a coordinate bond

Na (2,8,1) loses its 1 valence electron to Cl (2,8,7) → Na⁺ (2,8) + Cl⁻ (2,8,8). The oppositely charged ions attract → ionic bond. Na achieves Ne configuration; Cl achieves Ar configuration. Both achieve stable octets.

Question 12 of 20

The number of bonds in a nitrogen molecule (N₂) is:

ASingle bond (1 pair of electrons) BDouble bond (2 pairs) CTriple bond (3 pairs) DCoordinate bond

N₂ contains a triple bond (N≡N). Each N has 5 valence electrons; needs 3 more for octet → shares 3 pairs of electrons → triple bond. This makes N₂ very stable (bond dissociation energy = 945 kJ/mol — one of the strongest bonds). N₂ is inert at room temperature.

Question 13 of 20

Which of the following represents the correct electron dot (Lewis) structure representation?

AOnly valence electrons are shown as dots around the element symbol BAll electrons including core inner-shell electrons are shown as dots COnly bonding electrons are shown; lone pairs on atoms are omitted DAll electrons are shown exclusively as lines between bonded atoms

Lewis dot structures show only valence electrons (outer shell electrons) as dots around the element symbol. These are the electrons involved in bonding. Lone pairs are included. Example: Na has 1 dot; Cl has 7 dots; O has 6 dots.

Question 14 of 20

Cathode rays (electrons) were discovered by:

ARutherford BChadwick CJ.J. Thomson DBohr

J.J. Thomson (1897) discovered electrons using cathode ray tubes. He measured the charge-to-mass ratio (e/m) of cathode rays and showed they were identical regardless of cathode material → fundamental particle. Proposed the plum pudding model. Chadwick discovered the neutron.

Question 15 of 20

The outermost shell of an atom is called the:

ACore shell BValence shell CNucleus DTransition shell

The valence shell is the outermost electron shell. Electrons in the valence shell are called valence electrons — they determine chemical properties, reactivity, and bonding behaviour. The number of valence electrons equals the group number (for main group elements).

Question 16 of 20

The charge of an electron was determined by:

AThomson's cathode ray experiment (charge-to-mass ratio) BMillikan's oil drop experiment (absolute charge) CRutherford's gold foil experiment DBohr's spectroscopic analysis of hydrogen

Robert Millikan's oil drop experiment (1909) determined the absolute charge of an electron: e = 1.6 × 10⁻¹⁹ Coulombs. Thomson had measured e/m ratio (1.76 × 10¹¹ C/kg); combining Thomson's e/m with Millikan's e gave the electron's mass.

Question 17 of 20

Isobars are atoms that have:

ASame atomic number, different mass number BSame mass number, different atomic number CSame number of neutrons DSame number of valence electrons

Isobars: same mass number (A = protons + neutrons) but different atomic numbers (different elements). Example: ⁴⁰Ar (Z=18) and ⁴⁰Ca (Z=20) are isobars — both have A=40. Isotopes = same Z, different A. Isotones = same number of neutrons.

Question 18 of 20

The Aufbau principle states that electrons fill orbitals:

AIn any random order BStarting from the highest energy orbital CIn order of increasing energy (lowest energy first) DTwo electrons per orbital with parallel spins

Aufbau (German: 'building up') principle: electrons fill orbitals in order of increasing energy — 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → ... (Madelung rule). The orbital with the lowest (n+l) value fills first; if (n+l) is equal, the lower n orbital fills first.

Question 19 of 20

According to Hund's rule of maximum multiplicity:

AElectrons pair up in one orbital before any empty orbital in the subshell is filled BElectrons occupy subshell orbitals singly with parallel spins before pairing begins CNo two electrons in an atom may have the same four quantum numbers DElectrons always fill the highest energy orbital in the subshell first

Hund's rule: within the same subshell (e.g., 2p has 3 orbitals), electrons occupy orbitals singly with parallel spins before pairing. Example: carbon (2p²) has 2 electrons in 2 different 2p orbitals, both spin-up — not paired in one orbital. This minimises electron repulsion.

Question 20 of 20

The neutron was discovered by:

AJ.J. Thomson BErnest Rutherford CJames Chadwick DNiels Bohr

James Chadwick discovered the neutron in 1932 by bombarding beryllium with alpha particles → neutral particles of mass ~1 amu were emitted. This explained why atomic masses are roughly twice the atomic numbers (protons + neutrons in nucleus). He received the Nobel Prize in 1935.

CA02 — Structure of Atom & Molecules