A regular series of lines observed in the visible part of the emission spectrum of a cosmic star has the property that one of its emission lines coincides with the first line of the hydrogen Balmer series, and that every 5th, 10th, 15th, etc. line afterwards coincides with 2nd, 3rd 4th etc. line of the hydrogen Balmer series. Assuming that this particular series in the emission spectrum of the star can be associated with a hydrogen-like ion, what element would be responsible for the observed emission? What is the longest wavelength that can be found in this particular spectral series? Hint: the atomic number should give you a clue about the element in question.​​

the correct option is c.

isotopes refers to different forms of the same element with the same atomic number but different mass number as a result of difference in their neutron numbers. isotopes have different relative atomic mass but share the same chemical properties.

the seeing the electronic configuration we can say that the element is fluorine.

fluorine is the most electronegative element in the periodic table.

but by only seeing the configuration we cannot say about its reactivity because we don't know if its configuration after excitation or by addition of electrons.

but if not then, it is the lightest halogen and exists as a highly toxic pale yellow diatomic gas at standard conditions. as the most electronegative element, it is extremely reactive: almost all other elements, including some noble gases, form compounds with fluorine.