Structure of the Atom — Important Questions

X and Y are isobars; X has 12 neutrons, Y has 11 neutrons.

Step 1: Isotopes have the SAME atomic number but DIFFERENT mass numbers. Isobars have DIFFERENT atomic numbers but the SAME mass number. Step 2: X has atomic number 11 and Y has atomic number 12 — different atomic numbers. Both have mass number 23 — same mass number. Step 3: Therefore, X and Y are ISOBARS (same mass number, different atomic numbers). Step 4: Number of neutrons = Mass number − Atomic number. For X: neutrons = 23 − 11 = 12. Step 5: For Y: neutrons = 23 − 12 = 11. So X (Sodium, Na) has 12 neutrons and Y (Magnesium, Mg) has 11 neutrons. Options A and D are wrong because these elem

35.5 u

Step 1: To find average atomic mass, multiply each isotope's mass by its percentage abundance (as a fraction) and add them. Step 2: For Cl-35: mass × fraction = 35 × (75/100) = 35 × 0.75 = 26.25 u. Step 3: For Cl-37: mass × fraction = 37 × (25/100) = 37 × 0.25 = 9.25 u. Step 4: Average atomic mass = 26.25 + 9.25 = 35.5 u. Step 5: This is why the periodic table shows chlorine's atomic mass as 35.5 u — it is a weighted average, not a whole number. No single atom has a mass of 35.5 u, but a bulk sample of chlorine will behave as if each atom has this average mass. Option A (36 u) would be the sim

Thomson proposed electrons are embedded in a positive sphere with equal and opposite charges; rejected because Rutherford's experiment showed the positive charge is concentrated in a tiny nucleus, not spread throughout.

Step 1: Thomson's model (also called the 'plum pudding' or 'watermelon' model) proposed that the atom is a sphere of uniform positive charge with electrons embedded in it like seeds in a watermelon. Step 2: Since the negative charge of electrons exactly equals the positive charge of the sphere, the atom as a whole is electrically neutral — this explained neutrality. Step 3: However, Rutherford's alpha-particle scattering experiment showed that most alpha particles passed straight through AND a very small fraction bounced back almost completely. Step 4: If positive charge were spread uniformly

8

Step 1: The formula 2n² gives the THEORETICAL maximum for each shell: K=2, L=8, M=18, N=32. Step 2: However, the Bohr-Bury rules include a special restriction: the outermost (valence) shell of any atom can hold a MAXIMUM of only 8 electrons, regardless of what 2n² gives for that shell. Step 3: For example, even though the M-shell can theoretically hold 18 electrons, if M is the outermost shell, it will hold no more than 8 electrons. Step 4: This rule is the basis of the 'octet rule' — atoms with 8 electrons in their outermost shell are chemically stable (like noble gases). Step 5: This maximum