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 second shell (L-shell) can hold a maximum of 8 electrons. Assuming the second shell is completely filled for an element progressing to the third shell, it has 8 electrons. Number of electrons in the third shell = 25% of electrons in the second shell Number of electrons in the third shell = 0.25 \(\times\) 8 = 2 electrons. The electronic configuration of element 'M' is: First shell (K-shell): 2 electrons Second shell (L-shell): 8 electrons Third shell (M-shell): 2 electrons So, the electronic configuration is 2, 8, 2. The atomic number of element 'M' = Total number of electrons = 2 + 8 + 2 = 12. The element with atomic number 12 is Magnesium (Mg). Electronic configurations of the given options: % Option (a) Sodium (Na, Z=11): 2, 8, 1 % Option (b) Magnesium (Mg, Z=12): 2, 8, 2 % Option (c) Aluminium (Al, Z=13): 2, 8, 3 % Option (d) Calcium (Ca, Z=20): 2, 8, 8, 2 Element 'M' is Magnesium. \[ \boxed{\text{Magnesium}} \]