Answer:
Fluorine, chlorine, bromine and iodine are grouped together because they have the same number of valence electrons.
Explanation:
Fluorine, chlorine, bromine and iodine are all found in group 7, which is the second-last column from the right. Group 7 elements are also called the "Halogens" family.
The group number also tells you the number of valence electrons that the elements have in that group. Valence electrons mean the outermost electrons (See picture).
For example, fluorine has two shells (the circles with dots on them). The outermost electrons, or valence electrons, are the dots on the biggest circle. There are 7 dots, so there are 7 valence electrons, which corresponds with Group "7".
A full shell (except the for first shell) is when there are 8 dots. Since 7 is so close to 8, Halogens are very reactive.
Fluorine, chlorine, bromine and iodine all have 7 valence electrons and are in the Halogens family, which are very reactive.
