Density is the amount of matter per space or mass/volume.

Explanation:

density is the amount of weight/mass packed into a specific volume of material so i believe that is right

no starving yourself won't it will make you lose more weight what you need to do is find out what foods make you gain weight like bread and meats with fat on

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.