The gas phase decomposition of phosphine at 120 °c ph3(g) 1/4 p4(g) + 3/2 h2(g) is first order in ph3 with a rate constant of 1.80×10-2 s-1. if the initial concentration of ph3 is 3.16×10-2 m, the concentration of ph3 will be m after 99 s have passed

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.