structure of atom — BSSC Chemistry

• Key Fact J.J. Thomson discovered the electron (1898), E. Goldstein discovered the proton (1886) via canal rays, James Chadwick discovered the neutron (1932), and Carl Anderson discovered the positron (1932). • Supporting Detail Positron is the anti-particle of an electron, having the same mass but a positive charge. • Related Concept The charge on the electron was measured by Robert Millikan using the Oil Drop Experiment. • Why wrong options are wrong Other options incorrectly swap the discoverers of protons, electrons, or neutrons. • Exam Trick Remember the mnemonic "PEN = RTC" (Proton-Rutherford/Goldstein, Electron-Thomson, Neutron-Chadwick). • Additional Info Rutherford named the proton, but Goldstein is credited with discovering positive canal rays.

• Key Fact Rutherford's 1911 experiment concluded that an atom's mass and positive charge occupy a tiny volume (nucleus). • Supporting Detail He used a gold foil approximately 1000 atoms thick and bombarded it with alpha particles (doubly-charged helium ions). • Related Concept The atomic radius is ~10⁻¹⁰ m, while the nuclear radius is much smaller, ~10⁻¹⁵ m. • Why wrong options are wrong Both statements are factually perfect, and the scattering directly proves the concentrated nucleus. • Exam Trick "Deflection" = Positively charged nucleus; "Straight pass" = Empty space. • Additional Info This experiment led to the downfall of Thomson's "Plum Pudding" model.

• Key Fact The Aufbau principle is named after a German word meaning "building up," meaning electrons fill the lowest energy orbitals first. • Supporting Detail Statements 2 and 3 have their definitions swapped. Pauli's Exclusion Principle is about quantum numbers, and Hund's Rule is about the pairing of electrons. • Related Concept Hund's rule insists on maximizing parallel spins before pairing occurs. • Why wrong options are wrong Options B, C, and D are incorrect because statements 2 and 3 interchange the definitions of Pauli's and Hund's rules. • Exam Trick Remember "Pauli restricts Pair" (No 4 quantum numbers same), "Hund Hunts single first" (Single occupancy before pairing). • Additional Info The order of filling according to Aufbau is 1s → 2s → 2p → 3s → 3p → 4s → 3d.

• Key Fact Atomic number 29 corresponds to Copper (Cu). Its actual configuration is [Ar] 3d¹⁰ 4s¹ instead of the expected [Ar] 3d⁹ 4s². • Supporting Detail Half-filled (d⁵) and fully-filled (d¹⁰) d-subshells provide extra stability. Hence, one 4s electron jumps to the 3d orbital. • Related Concept Chromium (Atomic No. 24) is another exception, having the configuration [Ar] 3d⁵ 4s¹. • Why wrong options are wrong Option A is the theoretical (but incorrect) configuration. Option C uses Krypton ([Kr]), which is wrong since Cu is in the 4th period. • Exam Trick For Cu (29) and Cr (24), always take 1 electron from '4s' and give it to '3d' for stability. • Additional Info The symbol [Ar] represents the Argon core (18 electrons: 1s² 2s² 2p⁶ 3s² 3p⁶).

• Key Fact The common isotope of Hydrogen (Protium, ¹H) contains 1 proton, 1 electron, and 0 neutrons. • Supporting Detail A hydrogen ion (H⁺) has lost its only electron, so it is essentially just a bare proton. • Related Concept Other isotopes of hydrogen do have neutrons: Deuterium (1 neutron) and Tritium (2 neutrons). • Why wrong options are wrong Carbon, Helium, and Lithium all contain neutrons in their nuclei. • Exam Trick Mass Number (A) = Protons + Neutrons. For Hydrogen, A=1 and Z=1, so N = 1 - 1 = 0. • Additional Info Hydrogen is also the only element whose isotopes have completely different names (Protium, Deuterium, Tritium).

• Key Fact The correct sequence is: Dalton (1808) → Thomson (1898) → Rutherford (1911) → Bohr (1913) → Quantum Mechanical Model (post-1926). • Supporting Detail Dalton proposed indivisible atoms; Thomson discovered electrons; Rutherford discovered the nucleus; Bohr introduced quantized energy orbits. • Related Concept The Quantum Mechanical model replaced fixed orbits with 'orbitals' (regions of probability). • Why wrong options are wrong Any sequence placing Rutherford before Thomson or Bohr before Rutherford is historically inaccurate. • Exam Trick Remember the timeline order: D-T-R-B-Q (Dalton, Thomson, Rutherford, Bohr, Quantum). • Additional Info Democritus (400 BCE) gave the ancient philosophical concept of "atomos" before Dalton's scientific theory.

• Key Fact The electron distribution follows K, L, M shells. If M has 7 electrons, the configuration is K=2, L=8, M=7. • Supporting Detail Total electrons = 2 + 8 + 7 = 17. For a neutral atom, number of protons (Z) = number of electrons = 17. • Related Concept Mass number (A) = Number of protons (Z) + Number of neutrons (N) = 17 + 18 = 35. • Why wrong options are wrong Option B is the atomic number (17), not mass number. Option D is just the neutron count. • Exam Trick Always fully write the shell configuration (2, 8, ...) until you reach the outermost shell mentioned in the question to find total electrons. • Additional Info This element is Chlorine (Cl), which has an atomic number of 17 and a mass number of 35.

• Key Fact Cobalt-60 is used for cancer treatment, Iodine-131 for thyroid disorders, Sodium-24 for blood circulation, and Arsenic-74 for studying tumors. • Supporting Detail Radioisotopes emit high-energy radiation (like gamma rays from Co-60) which can destroy cancer cells. • Related Concept Phosphorus-32 is used in biological research. Iron-59 tracks iron in the bloodstream. • Why wrong options are wrong Only option A correctly matches every isotope to its established application in medical science. • Exam Trick "Iodine = Thyroid", "Sodium = Blood Circulation", "Cobalt = Cancer". • Additional Info Uranium-235 is heavily used as fuel in nuclear reactors.

• Key Fact X-rays are electromagnetic waves, hence they carry no electrical charge. Therefore, they are not deflected by electric or magnetic fields. • Supporting Detail Because X-rays are electromagnetic radiation, they travel at the speed of light (3 × 10⁸ m/s) in a vacuum. • Related Concept In contrast, cathode rays (electrons) and canal rays (protons) are deflected by electric/magnetic fields because they possess charge. • Why wrong options are wrong Both statements are true. However, traveling at the speed of light is NOT the reason they are not deflected; the lack of charge is the reason. • Exam Trick If a ray is "Electromagnetic" (like X-rays or Gamma rays), it has zero charge and zero rest mass. • Additional Info Gamma rays from radioactive decay are also uncharged electromagnetic waves.

• Key Fact Statement 2 is incorrect because in ordinary Hydrogen (Protium, ¹H), atomic number (1) equals mass number (1) since it has 0 neutrons. • Supporting Detail Statement 3 is incorrect because the e/m ratio of ANODE (canal) rays DOES depend on the nature of gas, unlike cathode rays. • Related Concept Statement 1 is perfectly correct; in a neutral atom, Protons = Electrons = Atomic Number. • Why wrong options are wrong The question asks for INCORRECT statements. Since 2 and 3 are wrong, option B is the correct answer. • Exam Trick Read carefully if the question asks for "Correct" or "Incorrect". Cathode rays = independent of gas. Anode rays = dependent on gas. • Additional Info The e/m ratio of an electron was calculated by J.J. Thomson as -1.76 × 10¹¹ C/kg.