CC02 — Structure of Atom

📗 CDS General Knowledge20 Questions · No Negative Marking

Question 1 of 20

Rutherford's gold foil experiment proved that the atom has:

AElectrons embedded uniformly in a positive sphere BA tiny, dense, positively charged nucleus with mostly empty space around it CElectrons and protons uniformly distributed DNeutrons at the centre surrounded by protons

Most alpha particles passed straight through gold foil (mostly empty space); a few deflected at angles; a very few bounced back (dense nucleus). This disproved Thomson's plum pudding model and established the nuclear model of the atom.

Question 2 of 20

The atomic number (Z) of an element equals the number of:

ANeutrons in the nucleus BProtons in the nucleus CProtons + neutrons (mass number) DElectron shells

Atomic number Z = number of protons = number of electrons (neutral atom). Z uniquely identifies the element — all atoms of the same element have the same Z. Mass number A = protons + neutrons.

Question 3 of 20

Isotopes have the same atomic number but different:

ANumber of electrons BNumber of protons CMass number (different neutrons) DChemical properties

Isotopes: same Z (same protons, same electrons, same chemical properties), different A (different neutrons). Examples: C-12 and C-14; U-235 and U-238; H-1, H-2 (deuterium), H-3 (tritium).

Question 4 of 20

Bohr's model proposed that electrons move in:

ARandom positions around the nucleus BFixed circular orbits (energy levels) CElliptical orbits of varying size DA probability cloud around the nucleus

Bohr (1913): electrons exist in fixed circular shells at specific energy levels without radiating. Energy is emitted/absorbed only when electrons jump between levels. Explained the hydrogen spectrum (Balmer series, etc.) quantitatively.

Question 5 of 20

Maximum electrons in the M shell (n=3) is:

A8 B16 C18 D32

Max electrons in shell n = 2n². M shell (n=3): 2 x 9 = 18. K(2), L(8), M(18), N(32). The outermost shell however never holds more than 8 electrons in valence configuration.

Question 6 of 20

Electronic configuration of chlorine (Z=17) is:

A2, 8, 7 B2, 8, 6, 1 C2, 7, 8 D2, 9, 6

Cl (Z=17): 2 in K + 8 in L + 7 in M = 2,8,7. Chlorine has 7 valence electrons, needs 1 more for octet, valency = 1. Forms Cl- ion or single covalent bond.

Question 7 of 20

Who discovered the electron using cathode ray tube experiments?

ARutherford BChadwick CJ.J. Thomson (1897) DBohr

J.J. Thomson (1897) measured e/m ratio of cathode rays, proved they were fundamental particles. Proposed plum pudding model. Nobel Prize 1906. Chadwick discovered the neutron (1932); Rutherford discovered the nucleus (1911).

Question 8 of 20

Isobars have the same mass number but different atomic numbers. Which pair are isobars?

AH-1 and H-2 BC-14 and N-14 (both A=14 CCa-40 and Ca-40 DNa-23 and Na-24

C-14 (Z=6) and N-14 (Z=7) are isobars — same A=14, different atomic numbers. H-1 and H-2 = isotopes; Na-23 and Na-24 = isotopes of sodium.

Question 9 of 20

James Chadwick discovered the neutron by:

AMeasuring e/m ratio of particles BBombarding beryllium with alpha particles CX-ray diffraction of crystals DAnalysing hydrogen emission spectrum

Chadwick (1932): alpha + Be -> neutral particles (neutrons). This explained why atomic masses are ~double atomic numbers. Nobel Prize 1935.

Question 10 of 20

Valence electrons are located in:

AThe innermost shell BThe outermost electron shell CThe d-subshell only DThe core shells

Valence electrons are in the outermost shell. They determine chemical reactivity, bonding, and valency. Number of valence electrons = group number for main-group elements.

Question 11 of 20

A neutron has:

ANegative charge, mass ~1 amu BNo charge, mass approximately equal to CPositive charge, mass ~1/1836 amu DPositive charge, mass 1 amu

Neutron: mass ~ 1.675 x 10^-27 kg ~ 1 amu, charge = 0. Located in nucleus. Proton: mass ~1 amu, charge = +1. Electron: mass ~ 1/1836 amu, charge = -1.

Question 12 of 20

Aufbau principle states electrons fill orbitals:

ARandomly to minimise repulsion BFrom highest energy first CIn order of increasing energy: 1s, 2s, 2p, 3s, 3p, 4s, 3d... DPairing in same orbital first before filling empty ones

Aufbau (building up): lowest energy orbitals fill first. Order: 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p. The (n+l) rule: lower (n+l) value fills first; if equal, lower n fills first.

Question 13 of 20

Hund's rule states that within a subshell, electrons:

APair up immediately in available orbitals BEnter orbitals singly with parallel spins before any pairing CAlways have paired antiparallel spins DFill alternating spin directions from the start

Hund's rule: each orbital in a subshell is singly occupied (same spin) before any orbital receives a second electron. This minimises electron repulsion. Example: nitrogen (2p3) has 3 unpaired electrons in 3 separate 2p orbitals.

Question 14 of 20

Charge on a proton is:

A-1.6 x 10^-19 C BZero C+ 1.6 x 10^-19 C DVaries with element

Proton charge = +1.6 x 10^-19 C. Electron = -1.6 x 10^-19 C. Neutron = 0. The magnitudes of proton and electron charges are equal — hence neutral atoms have equal numbers of each.

Question 15 of 20

The electronic configuration of Na+ ion (sodium loses 1 electron) is:

A2, 8, 1 (same as neutral Na) B2, 8 (same as neon) C2, 8, 2 D2, 7

Na (Z=11): 2,8,1. Na+ loses 1 electron: 10 electrons remain -> 2,8 = same as neon. Cations are smaller than neutral atoms; anions are larger.

Question 16 of 20

When an electron jumps from a higher to a lower energy level in Bohr's model:

AEnergy is absorbed and the electron moves away from nucleus BEnergy is emitted as a photon of specific frequency (E = hv) CThe atom becomes an ion DNo energy change occurs

Bohr: electron drops from E2 to E1 -> emits photon: E = hv = E2 - E1. This explains atomic emission spectra (unique spectral lines for each element). Absorption = electron jumps up; emission = electron falls down.

Question 17 of 20

In the atom 238/92 U (Uranium), the number of neutrons is:

A92 B146 C238 D184

Neutrons = A - Z = 238 - 92 = 146. Z=92 protons, A=238 mass number. Neutrons = 146. U-235 (fissile) has 235-92 = 143 neutrons; U-238 (fertile) has 238-92 = 146 neutrons.

Question 18 of 20

Millikan's oil drop experiment determined:

AThe e/m ratio of electrons BThe absolute charge of the electron CThe mass of the proton DThe existence of the nucleus

Millikan (1909): balanced gravitational and electric forces on charged oil droplets -> measured elementary charge e = 1.6 x 10^-19 C. Combined with Thomson's e/m ratio -> mass of electron = 9.1 x 10^-31 kg.

Question 19 of 20

For 40/18 Ar (argon), the number of protons, neutrons, and electrons is:

A18, 40, 18 B18, 22, 18 C22, 18, 22 D40, 18, 40

Z=18 (protons=18, electrons=18 for neutral Ar). A=40. Neutrons = 40-18 = 22. So: p=18, n=22, e=18. Always: Z = protons = electrons (neutral); neutrons = A-Z.

Question 20 of 20

The number of electrons in the 3rd shell (M shell) of a silicon atom (Z=14) is:

A2 B4 C8 D14

Si (Z=14) configuration: K(2) + L(8) + M(4) = 2,8,4. The 3rd shell (M) has 4 electrons. This gives Si 4 valence electrons, making it tetravalent and central to semiconductor technology.