Answer: The correct answer is Option D.
Explanation:
Number of electrons that are present in an atom is determined by the electronic configuration of that atom.
If an ion is carrying a positive charge, it means that the atom has lost electrons and if an ion is carrying a negative charge, it means that the atom has gained electrons.
For the given options:
Option A: The atomic number of hydrogen atom is 1 and the electronic configuration for [tex]H^+[/tex] ion will be: [tex]1s^0[/tex]
Thus, this atom does not have any electrons.
Option B: The atomic number of bromine atom is 35 and the electronic configuration for [tex]Br^-[/tex] ion will be: [tex]1s^22s^22p^63s^23p^64s^23d^{10}4p^6[/tex]
Thus, this atom has 36 electrons.
Option C: The atomic number of aluminium atom is 13 and the electronic configuration for [tex]Al^{3+}[/tex] ion will be: [tex]1s^22s^22p^6[/tex]
Thus, this atom has 10 electrons.
Option D: The atomic number of calcium atom is 20 and the electronic configuration for [tex]Ca^{2+}[/tex] ion will be: [tex]1s^22s^22p^63s^23p^6[/tex]
Thus, this atom has 18 electrons.
Hence, the correct answer is option D.
