Which one of the following sets of ions represents a collection of isoelectronic species? (Atomic nos.: F = 9, Cl = 17, Na = 11, Mg = 12, Al = 13, K = 19, Ca = 20, Sc = 21)

Sure, I can help with that. Isoelectronic species are atoms, ions, or molecules that have the same number of electrons.

Here are the atomic numbers and the number of electrons in the neutral state:

• F (Fluorine) = 9 electrons
• Cl (Chlorine) = 17 electrons
• Na (Sodium) = 11 electrons
• Mg (Magnesium) = 12 electrons
• Al (Aluminium) = 13 electrons
• K (Potassium) = 19 electrons
• Ca (Calcium) = 20 electrons
• Sc (Scandium) = 21 electrons

Now, let's consider the ions:

• F-: Fluorine gains one electron to become stable, so it has 10 electrons.
• Cl-: Chlorine gains one electron to become stable, so it has 18 electrons.
• Na+: Sodium loses one electron to become stable, so it has 10 electrons.
• Mg2+: Magnesium loses two electrons to become stable, so it has 10 electrons.
• Al3+: Aluminium loses three electrons to become stable, so it has 10 electrons.
• K+: Potassium loses one electron to become stable, so it has 18 electrons.
• Ca2+: Calcium loses two electrons to become stable, so it has 18 electrons.
• Sc3+: Scandium loses three electrons to become stable, so it has 18 electrons.

From the above, we can see that F-, Na+, Mg2+, and Al3+ are isoelectronic because they all have 10 electrons. Similarly, Cl-, K+, Ca2+, and Sc3+ are isoelectronic because they all have 18 electrons.