Which of the following atoms and ions are isoelectronic,i.e.,have the same number of electrons as the neon atom?

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

An atom which has lost one electron would be

Which of the following is correct?

The atomic number of an element is always equal to

An atom has a mass number of $25$ and contains $13$ neutrons in its nucleus. What is the charge of its ion if it forms a stable octet?