Atomic Structure

It could not account for the observation that most alpha particles passed through the gold foil undeflected, which disproved a uniformly positive sphere

Step 1: Thomson's model said the atom is a uniformly positive sphere with electrons embedded in it like plums in a pudding. Step 2: If this were true, alpha particles (positively charged) would be slowed down or deflected uniformly as they hit the spread-out positive charge. Step 3: But Rutherford's experiment showed most alpha particles passed STRAIGHT THROUGH — impossible if the positive charge were spread throughout. Step 4: Also, a very few bounced back completely — showing the positive charge is concentrated in a tiny, dense nucleus, NOT spread out. Step 5: Option A is wrong because Thoms

Atomic number 17, valency 1, forms an anion by gaining 1 electron

Step 1: Atomic number = total electrons = 2 + 8 + 7 = 17. This is Chlorine (Cl). Step 2: The valence shell (M shell) has 7 electrons. To reach the stable octet (8 electrons), it needs to GAIN 1 electron. Step 3: Valency = number of electrons needed to complete the octet = 8 – 7 = 1. Step 4: When chlorine gains 1 electron, it becomes Cl⁻ — a negatively charged ion called an anion. Step 5: Option A is wrong because gaining electrons forms anions, not cations. Option C is wrong — atomic number is 17, not 18. Option D is wrong — valency is 1 (electrons needed), not 7 (electrons already present).

²⁴₁₂Mg with electronic configuration 2, 8, 2

Step 1: Atomic number 12 corresponds to Magnesium (Mg) with mass number 24. Step 2: In symbolic representation, mass number is written as SUPERSCRIPT (top) and atomic number as SUBSCRIPT (bottom) → ²⁴₁₂Mg. Step 3: Electronic configuration — Total electrons = 12. K shell = 2, L shell = 8, remaining = 12–10 = 2 in M shell. Configuration: 2, 8, 2. Step 4: Option A is wrong — electronic configuration 2, 8, 4 would mean 14 electrons, which is Silicon, not Magnesium. Option C is wrong — the positions of mass number and atomic number are swapped. Option D is wrong — L shell can only hold a maximum of

Electrons revolve in fixed orbits (energy levels) where they neither gain nor lose energy, so the outward force of revolution balances the inward pull of the nucleus

Step 1: The problem with Rutherford's model was that a moving charged particle (electron) should continuously lose energy and spiral into the nucleus. Step 2: Bohr proposed that electrons occupy specific fixed energy levels (shells) called orbits — in these orbits, they do NOT radiate or lose energy. Step 3: As long as no external force acts on them, electrons maintain their position — the centrifugal force (outward) due to revolving exactly balances the electrostatic attraction (inward) by the nucleus. Step 4: This explains atomic stability — electrons stay in their shells. Option C is incorr