If I had to guess I'd say it has something to do with maybe gasses an/or lights.(just a guess)
here is some info I found though.....
http://en.wikipedia.org/wiki/Group_7_element
A Group 7 element is one in the series of elements in group 7 (IUPAC style) in the Periodic Table, which consists of the transition metals manganese (Mn), technetium (Tc), rhenium (Re), and bohrium (Bh). Like other groups, the members of this family show patterns in its electron configuration, especially the outermost shells resulting in trends in chemical behavior: {| ! Z ! Element ! No. of electrons/shell | 25 manganese 2, 8, 13, 2 43 technetium 2, 8, 18, 13, 2 75 rhenium 2, 8, 18, 32, 13, 2 107 bohrium 2, 8, 18, 32, 32, 13, 2 All of these elements are classed in Group 7 because their valence shells hold seven electrons. Technetium has no stable isotopes. Technetium and promethium are the only two such elements before lead, after which (with bismuth having an extremely long-lived isotope) no known element has a stable isotope.
http://www.chemguide.co.uk/inorganic/group7menu.html
PERIODIC TABLE GROUP 7 (HALOGENS) MENU ;
Atomic and physical properties . . .
: Discusses trends in atomic radius, electronegativity, electron affinity and melting and boiling points of the Group 7 elements. It also looks at the bond strengths of halogen-halogen bonds and of hydrogen-halogen bonds. ;
Halogens as oxidising agents . . .
: Describes and explains the trend in oxidising ability of the Group 7 elements based on the reactions between one halogen and the ions of another one - for example, between Cl2 and I- ions from salts like KI. ;
The acidity of the hydrogen halides . . .
: Discusses the acidity of the hydrogen halides (like hydrogen chloride), and explains why HF is a weak acid. ;
Halide ions as reducing agents . . .
: Describes and explains the trend in reducing ability of the halide ions based on their reactions with concentrated sulphuric acid. ;
Testing for halide ions . . .
: Describes and explains the tests for halide ions using silver nitrate solution followed by ammonia solution. ;
The manufacture of chlorine . . .
: Describes the manufacture of chlorine by the electrolysis of sodium chloride solution using a diaphragm cell and a membrane cell. |}
Bromine and Chlorine both have equal chemical properties. They both have 7 electrons in the outermost energy level. They both gain 1 electron to get the noble gas configuration.Cl and Br have same chemical properties. They have 7 valence electrons. They both are non-metals.
elements in the same group as nitrogen will have similar CHEMICAL properties because they have the same number of electrons in the outer shell. (this is according to Bohrs model of the atom, if you want a real confusing answer about valence shells etc. go look at quantam mechanics) So elements in group 7 are phosphurus, arsenic, selenium, tellurium, polonium, these will react with the same chemicals to from similar compounds however they are all solids unlike nitrogen. Elements with similar PHYSICAL properties are elements that are present as gases at room temperature (like nitrogen is) oxygen, fluorine, chlorine, hydrogen and all of group 8 the noble or inert gasses (although inert isn't really a valid term as they can react in the correct conditions)
The transition metal group contains elements with the most varied properties. This is because transition metals have multiple oxidation states, can form colorful compounds, exhibit catalytic properties, and have high melting points among other characteristics.
There is a total of 17 elements in those groups.
Chlorine is in group 7 because it has 7 electrons in its outermost energy level, making it part of the halogen group. It is in period 3 because it is in the third row of the periodic table based on its increasing atomic number.
Chemists were able to predict the chemical properties of technetium by studying the periodic table and observing trends in the elements around it. Technetium is located in the transition metals group, so it was predicted to have similar properties to other elements in that group with its 7 valence electrons. Additionally, its position in the periodic table suggested that it would exhibit variable oxidation states.
There are 18 periods in the periodic table. The periods are divided according to the chemical properties of elements. Elements in a group have similar properties.
All the elements in the same group as Flourine have similar properties. This group of elements is referred to as the halogens, and is located in group 7.
Iodine appears in group 7 of the periodic table (halogens) because it has 7 valence electrons. This property makes it chemically similar to other elements in the group, such as fluorine, chlorine, and bromine. Grouping elements with similar chemical properties helps in organizing the periodic table.
Bromine and Chlorine both have equal chemical properties. They both have 7 electrons in the outermost energy level. They both gain 1 electron to get the noble gas configuration.Cl and Br have same chemical properties. They have 7 valence electrons. They both are non-metals.
elements in the same group as nitrogen will have similar CHEMICAL properties because they have the same number of electrons in the outer shell. (this is according to Bohrs model of the atom, if you want a real confusing answer about valence shells etc. go look at quantam mechanics) So elements in group 7 are phosphurus, arsenic, selenium, tellurium, polonium, these will react with the same chemicals to from similar compounds however they are all solids unlike nitrogen. Elements with similar PHYSICAL properties are elements that are present as gases at room temperature (like nitrogen is) oxygen, fluorine, chlorine, hydrogen and all of group 8 the noble or inert gasses (although inert isn't really a valid term as they can react in the correct conditions)
number of valence electrons
Elements from group 1 (alkali metals) and group 7 (halogens) are highly reactive due to their electronic configurations. As a result, they tend to form compounds easily to achieve more stable electron configurations. Compounds with these elements often exhibit useful properties in various chemical reactions and industrial applications.
Chlorine belongs in the halogens group because it shares similar chemical properties with other elements in that group, such as fluorine, bromine, and iodine. These elements all have a full outer shell of electrons, making them highly reactive and likely to form compounds with other elements.
Berkelium is in the period 7 of the Mendeleev table of chemical elements and in the group of actinides.
The transition metal group contains elements with the most varied properties. This is because transition metals have multiple oxidation states, can form colorful compounds, exhibit catalytic properties, and have high melting points among other characteristics.
By listing their collective and individual properties, pointing out similarities and differences.