Trends, exceptions, Mendeleev MCQs

Q1. Which of the following is the correct order of atomic radii?

• A.Li > Be > B > C
• B.Be > Li > B > C
• C.C > B > Be > Li
• D.Li > C > Be > B

Answer: A. Li > Be > B > C

Asked for the unknown; data is laid out in the stem. By Across the period 2 (Li → C), Z increases with electrons added to n=2. Across the period 2 (Li → C), Z increases with electrons added to n=2. atomic radius monotonically decreases: Li > Be > B > C. Thus option A) Li > Be > B > C.

Q2. Which property generally INCREASES across a period (left to right) in the modern periodic table?

• A.Electronegativity
• B.Atomic radius
• C.Metallic character
• D.Number of shells

Answer: A. Electronegativity

NCERT fact (chemistry, chapter 'Periodic Classification of Elements'): Across a period, the effective nuclear charge increases while electrons enter the same shell. Electronegativity, ionisation enthalpy and electron affinity rise; atomic radius and metallic character fall. Final answer - A) Electronegativity.

Q3. Which of the following anomalies is INCORRECTLY attributed?

• A.Cu placed before Ni in Mendeleev's table because Cu has higher mass
• B.K before Ar in modern table despite K having lower atomic mass
• C.IE(O) < IE(N) due to pairing repulsion in O's 2p⁴
• D.Atomic radius of Ga (135 pm) slightly LESS than Al (143 pm) due to poor shielding of 3d

Answer: A. Cu placed before Ni in Mendeleev's table because Cu has higher mass

The problem states: the data stated in the problem. What we must find: the unknown asked in the stem. Key equation - (K=39.1 < Ar=39.9 by mass but K (Z=19) > Ar (Z=18). This is the equation that links the given quantities to the unknown (chemistry, chapter 'Periodic Classification of Elements'). Substituting: (K=39.1 < Ar=39.9 by mass but K (Z=19) > Ar (Z=18). The intuition - Mendeleev had Ni (58.7) BEFORE Cu (63.5) - i.e. Therefore the answer is A) Cu placed before Ni in Mendeleev's table because Cu has higher mass.