1-bromodecane (C10H21Br) produces a mass spectrum with a parent molecular ion (M ) peak at m/z 220 (100%). A peak at m/z 222 (98%) also appears in the spectrum. What causes this signal in the mass spectrum

The presence of Br atom produces 2 ions (M and M+2) with the same abundance.

Explanation:

Mass spectrometry is an instrumental technique in which chemical substances are ionized and fragmented. The nature of the substances produce different fragmentations.

A way to know if a substance contains bromine is seeing the parent molecular ion (M, Subtance with ⁷⁹Br) and M+2 (Substance with ⁸¹Br) with the same relative abundance. Where parent molecular ion is the substance without fragmentations

That is because in nature, the isotope abundance of bromine is ⁷⁹Br with 51% and ⁸¹Br with 49%, producing the 49% of ions having  2 units of m/z higher than its consecutive ion.

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.