Which of the following group 7a elements is the most reactive?
cl (chlorine)
i (iodine)
f (fluorine)
br (bromine)

which of the following gases emit colors when an electric current is applied?
hydrogen and helium
argon and krypton
fluorine and chlorine
oxygen and nitrogen

1.Fluorine (F)

2.Argon and Krypton

Explanation:

1.Group 7A elements: it is that group of periodic table in which each element has 7 electrons in outermost shell.

The outermost shell can be filled by maximum 8 electrons.Therefore, the elements of this group require 1 electron to full fill their outermost shell.

Fluorine is the most electronegative element because the size of fluorine is small.

The electronegativity of an element is the ability to attract the electrons towards itself.

The reactivity means gain or loose electrons.

The fluorine has high electronegativity therefore, it is most reactive non-metal.

Answer;Fluorine(F)

2.Argon and Krypton are noble gases because their outermost shell is fully filled.They  emit colors when an electric current is applied because when current passes through it then electron are excited  by absorbing energy and they jump from ground state to higher energy level.

When they return from higher to lower energy level  then they release absorbed energy in the form of color light.

Hence,they emit colors.

Argon and Krypton

see earlier answer

an electric circuit