Group 17 are collectively called as halogens (In Greek: halo means salt and genes mean producing, so collectively salt producing) and it consists of fluorine, chlorine, bromine, iodine, and astatine. Halogens are highly reactive non-metals. These elements greatly resemble in property with each other. The similarity to this extent is not found in other groups of the periodic table. They have a regular gradation in the physical and chemical properties. Astatine is the only radioactive element in the group. They have seven electrons in their outermost shell (ns2 np5) and are short of one electron from the configuration of nearest noble gas. The chemical properties and reactivity of an element are determined by the oxidation state exhibited by them.
Chemical Properties and Oxidation State
All the elements of halogen family exhibit -1 oxidation state. However, elements such as chlorine, bromine, and iodine also show +1, +3, +5 and +7 state. This higher oxidation state of chlorine, bromine, and iodine is realized when these halogens are in combination with small and highly electronegative atoms of fluorine and oxygen. The oxides and oxoacids of chlorine and bromine have +4 and +6 states. There are no valence shell d orbitals in fluorine atom and therefore it cannot expand its octet. Fluorine being the most electronegative element exhibits only -1 oxidation state.
Halogens are highly reactive, they react with metals and non-metals in order to form halides. Their reactivity decreases as we move down the group. Halogens have strong oxidizing properties. F2 is the strongest oxidizing halogen. It easily oxidizes other halide ions present in solution or in the solid phase. In general, a halogen oxidizes halide ion which is of higher atomic number. For example:
F2 + 2X– → 2F– + X2 (X = Cl, Br or I)
From standard electrode potential, the decreasing oxidizing ability of halogen can be easily observed.
The relative oxidizing nature of halogens can be illustrated by their reactions with water. Fluorine oxidizes water to oxygen. Whereas chlorine and bromine react with water in order to form respective hydrohalic and hypohalous acids. Iodine reacts with water in a non-spontaneous way. I– can be oxidized by water in the acidic medium. For example:
4 I– (aq) + 4H+ (aq) + O2 (g) → 2I2(s) + 2H2O (l)
General Characteristics of Halogen Family:
Electronic Configuration of halogen family:
Members of the halogen family have seven valence electrons, that is, they have seven electrons in their outermost orbit. Thus, they are one electron short of the nearest noble gas configuration. The general configuration of halogen family is given as ns2np5.
Atomic and Ionic Radii of halogen family:
The members of group 17 have the smallest atomic radii in their respective periods. This is attributed to the fact that they have maximum effective nuclear charge. Atomic and ionic radii increase from top to bottom in a group thus, it increases from fluorine to iodine due to increasing number of quantum shells.
Ionisation Enthalpy of halogen family:
Members of group 17 have very little or no tendency to lose an electron. Thus, they have a very high value of ionization enthalpy. Ionisation enthalpy decreases from top to bottom in the group due to the increase in atomic size.
Electron Gain Enthalpy of halogen family:
The atoms of group 17 elements are only one electron short of attaining stable noble gas configurations. Thus, these elements have maximum negative electron gain enthalpy in the corresponding periods. Electron gain enthalpy of these elements becomes less negative as we move down the group due to the increase in atomic size. However, for chlorine has a more negative value of electron gain enthalpy with respect to fluorine.
Electronegativity of halogen family:
The elements of group 17 have a very high value of electronegativity. The electronegativity decreases down the group due to the decrease in effective nuclear charge. Thus, fluorine is the most electronegative element.