For Aluminium (Al) (atomic number : 13, mass number : 27), which one among the following statements is NOT correct ?

Based on the atomic structure of Aluminium (Al):

• Atomic Number (Z) = 13: This defines the number of protons in the nucleus. In a neutral atom, the number of electrons also equals the atomic number. Thus, Statements A and B are correct.
• Mass Number (A) = 27: The number of neutrons is calculated as Mass Number minus Atomic Number (27 - 13 = 14). Thus, Statement C is correct.
• Electronic Configuration: With 13 electrons, the configuration is 2, 8, 3. Aluminium has 3 electrons in its outermost (valence) shell. To achieve a stable octet, it typically loses these 3 electrons, forming an Al3+ ion.

Therefore, the valency of Aluminium is 3, making Option D the incorrect statement.