Question

Fluorine, chlorine and Bromine are put in one group of their similar properties. What are those similar properties?

Answer 1

Periodic table is the representation of chemical elements and arranged in periods and groups. Each group consists of chemical elements containing similar properties. Halogens are the elements of group $17$ and mostly non-metallic. The valence electrons of halogens are seven.

Complete answer:
Periodic table is the representation of chemical elements arranged in the order of increasing atomic numbers. These elements are arranged in the vertical columns and horizontal rows. The vertical columns are called groups and the horizontal rows are called periods. There are a total of $118$ elements in the periodic table.
The group $17$ elements are called halogens. The elements that belong to halogens are Fluorine, chlorine, bromine, iodine and astatine.
The fluorine, chlorine and bromine have $7$ valence electrons and are deficient of one electron to attain inert gas configuration.
Similar properties of fluorine, chlorine and Bromine have the same valence electrons, non-metallic character. These all three atoms exist as diatomic molecules.
The valence electrons are $7$ , and these all atoms try to gain one electron to attain the nearest noble or inert gas configuration. So, the valency will be $+ 1$ .

Note:
The halogens are placed on the extreme right side of the periodic table. These are the most electronegative atoms. Fluorine is the most electronegative element in the periodic table. Chlorine is the most electron affinity atom. As these are electronegative, these atoms attract electrons from other atoms.