This group includes the elements fluorine, chlorine, bromine and iodine. They all possess 7 valence electrons (7 electrons in their outer shell).
The Electronic configuration is shown below:
Elements in this series react by gaining a single electron when they react with metal atoms or share a pair of electrons when they bond with another non-metal. They do this to achieve the stability of closest Nobel gas. For e.g. Fluorine will gain a single electron from a metal atom to achieve a complete outer shell. You may be wondering why they don’t lose electrons similarly to metals in group 1, 2 or 3. The answer is that it is more energetically feasible for them to gain 1 electron vs. lose the 7 electrons in their outer shell.
The ability to gain electrons is called electronegativity. This ability decreases as you move down the group, because electrons are attracted by protons. As the atom gets larger, the further the electrons being attracted are from the nucleus. Since attraction is inversely proportional to distance then the greater the distance, the smaller the attraction. This is why fluorine is more reactive than chlorine.
The order of reactivity increases with reduced size for non metal elements, the opposite is true for metals in group 1-3.
The order of reaction for the group 7 elements is shown below:
Fluorine> Chlorine > Bromine > Iodine
As stated before non-metals may form bonds with other non-metals. This is done by means of a covalent bond. Covalent bonds are formed by the sharing of electrons by non-metal atoms. This is similar to what happens in group 7 elements since these elements are non-metal. They form covalent bonds with other non-metals atoms or with themselves (forming diatoms). You may be wondering how these atoms are able to bond with themselves. We are aware that covalent bonding is a concept that many students have a problem in understanding as such, we will take special consideration when explaining this. For now keep in mind that many non metal elements including group 7 elements are found naturally as diatoms. E.g. Fl2, Cl2, Br2 and I2