Question

An element 'M' has 25% of the electrons filled in the third shell as in the second shell. The element 'M' is :

• A. Sodium
• B. Magnesium
• C. Aluminium
• D. Calcium

Answer 1

The Correct Option is B

The L-shell holds a maximum of 8 electrons. For an element entering the third shell with a filled second shell, the second shell contains 8 electrons. The third shell has 25% of the second shell's electrons.Electrons in third shell = 0.25 \(\times\) 8 = 2 electrons.The electronic configuration of element 'M':K-shell: 2 electronsL-shell: 8 electronsM-shell: 2 electronsTherefore, the electronic configuration is 2, 8, 2.The atomic number of element 'M' = Total electrons = 2 + 8 + 2 = 12.Element with atomic number 12 is Magnesium (Mg).Electronic configurations of given options:Option (a) Sodium (Na, Z=11): 2, 8, 1Option (b) Magnesium (Mg, Z=12): 2, 8, 2Option (c) Aluminium (Al, Z=13): 2, 8, 3Option (d) Calcium (Ca, Z=20): 2, 8, 8, 2Element 'M' is Magnesium.\[ \boxed{\text{Magnesium}} \]