Q.In modern periodic table, the period number indicates the:
a. atomic number
b. atomic mass
c. principal quantum number
d. azimuthal quantum number

Step 1. Recall section 7.3.1. 'The period number is same as the principal quantum number n of the outermost or valence shell of the elements.'

Step 2. Rule out the distractors. Atomic number and atomic mass both increase steadily WITHIN a period (not fixed at a single value for the whole period), so neither can 'indicate' the period number itself. Azimuthal quantum number l varies by subshell within a period (s, p, and for later periods also d, f), so it is not fixed per period either.

Step 3. Confirm with an example. Every element of period 3 has its valence shell at n=3 (e.g. Na: $3s^1$, Cl: $3s^23p^5$) regardless of atomic number or mass.

Recalling the direct period-number-equals-n statement from section 7.3.1

• Selecting atomic number, confusing 'the number used to order elements overall' with 'the number that identifies which period an element sits in'.
• Selecting azimuthal quantum number l, confusing it with the quantum number that identifies the PERIOD rather than the subshell/orbital shape.