Science — CBSE Class 10 board question

(a) Two advantages of using atomic number as the basis of classification:

1. Atomic number is a unique, fixed property of each element, leaving no ambiguity in an element's position in the table.
2. It removes anomalies of Mendeleev's table (e.g., cobalt and nickel are placed correctly based on atomic number, not atomic mass).

(b) Electronic configurations:

• Element X (atomic number 13): 2, 8, 3
• Element Y (atomic number 20): 2, 8, 8, 2

• For part (a), examiners expect any two valid advantages — uniqueness of atomic number and removal of Mendeleev's anomalies are the most standard ones. Avoid vague answers.
• For part (b), simply fill shells in order (2, 8, 8, 2 for 20 electrons). These are straightforward but students often make errors with Y — remember the 3rd shell can hold up to 18, but it stabilises at 8 before the 4th shell begins filling.