why do the halogens all end in "ine"?

Answer 1

Halogens all end with "ine" because they have similar properties. They are all highly reactive, they are all monovalent and readily form negative ions.

Before the 1800s, they did not exist in their elemental form. This allowed the chemists who discovered them in the 1800s to name them based on their similarities. (The periodic table was thought up about the same time).

At this time, there is no element 117, the name Ununseptium is just a placeholder name and this yet to be created element is also referred to as eka-astatine. (See http://en.wikipedia.org/wiki/Ununseptium ). The placeholder name is required by international convention to end in "ium." (See http://www.apsidium.com/theory/naming.htm ).

If and when it is created, it will get a new name with either an "ine" on the end referring to its Halogen grouping or an "ium" referring to its trans-Uranium status (all elements so far created that are heavier than Uranium are named "ium," but they are also all metals which Ununseptium will not be).

At least for now, all Halogens end with "ine."

Answer 2

According to wikipedia, "The undiscovered element 117, temporarily named ununseptium, may also be considered a halogen." so they don't ;-)

I guess the rest end in -ine for the same reasons any nomenclature exists in chemistry- it tells you about the element without having to know its position in the table.