Halogen (compare and contrast between Fluorine and Bromine)

HALOGEN

As we know the element of group 7A produce salts with metals, so they are called halogen, from a greak word halos- genes, meaning  “salt former”. This group contains fluorine (F), chlorine (Cl), bromine (Br), iodine (I) and astatine (At). Astatine is radioactive element. Halogen have 7 valence electrons. Since their atomic number are less than that of the following noble gases by one, they have -1 oxidation state in their stable compounds. Florine has only -1 oxidation state in its compounds. Florine is the most electronegative element of the periodic table. Within this group the electronegativity decreases from fluorine to iodine. Halogen are the element with the highest electronegativity in each period, so halogens are the most active nonmetals of each period.

Figure 1. group 7A. halogens

Physical properties

Halogen are found as diatomic molecules in nature, such as F₂, Cl₂, Br₂, I₂. They are unstable in their  monoatomic forms. In the diatomic structure of the molecules, each atom shares its single electron to achieve a noble gas electron configuration. Among the halogen, as the atomic number increases, the melting and biolling points, atomic radius , atomic weight and density also increase, whereas the ionization energy and non-metallic property decrease from fluorine to iodine. They are found in nature in the form of halides, except iodine . it’s found as iodates, such as sodium iodate, NaIO₃. At room temperature, fluorine and chlorine are gases, bromine is liquid and iodine is a solid. Increasing intermolecular force, van der waals force, because of increasing mass, is responsible for this gradual change from gas to solid.

Chemical properties

As already mentioned, since the halogens are very active elements, they occure either as diatomic molecules or in the form of their compound in nature. They are good oxidizing agents. They have -1 oxidation state in their stable compound. All halogen, except fluorine (it takes only -1 oxidation), can take several oxidation number from -1 to +7. Halogen, after the noble gases, have the second greatest ionization energies. The activity order of halogen is F>Cl>Br>I. A more active halogen eliminates a less active halogen from its compounds. That means an active halogen oxidize the other -1 charged halogen ion into elemental form. We will discuss about flourine and bromine.

1.      Fluorine

The word fluorine is devided from the latin word “fluere”, meaning flowing. At room temperature, fluorine is a gas with pale yellow color. It is heavier than air. Since fluorine has the smallest atomic radius among the halogen, it is the best oxidizing agent. Fluorine is a very reactive element. It can react vigourously with a lot of substance at room conditions. It has only -1 oxidation number in its compounds. Fluorine gas and all fluoride compounds are toxic in large amounts. But fluoride are essential to life and beneficial in preventing dental cavities.

In nature, fluorine is founds as the minerals such as fluorite (CaF₂), kryolite (Na₃AlF₆) and apatite (Ca₅F(PO₄)₃). Additionally, fluorine is also found in sea water in trace amounts.

Fluorine was first discovered by Henry Moissan in 1886 during the electrolysis of KHF₂. Fluorine gas was formed at the anode.

KHF₂(l)  ^{electrolysis   >}    H₂ (g) + F₂ (g) + 2KF (l)

The atomic number of fluorine is 9 and it has 1s²2s²2p⁵ electron configuration. It has only-1 oxidation state in compounds.

₉F: 1s²2s²2p⁵

Fluorine oxidizes all elements except He, Ne, and Ar. That means it can react with other noble gases: Kr, Xe, and Rn.

Xe  +  2F_{2   --------------->}  XeF₄

It is an extremely reactive element. For example, it reacts with hydrogen explosively, even at -250^oC.

H₂ + F_{2  ------------------>}  2HF

It oxidizes many elements, even in their compounds.

2H₂O  + 2F₂   ------------>  4HF  +  O₂

2HNO₃  + 4F_{2 --------------------->} 2HF  +  2NF₃  + 3O₂

            The most important compound of flourine is hydrofluoric acid, HF. It attacks SiO₂ the main constituent of glass, so it can not be stored in glass containers.

HF is used to decorate glassware.

^{HF molecule}

SiO₂(s)  +  4HF(aq) --------------> SiF₄(g)  +  2H₂O(l)

Na₂SiO₃(s)  + 6HF(aq) ---------->2NaF(aq)  +  SiF₄(g) +  3H₂O(l)

Ca₂SiO₃(s)  +  6HF(aq) ----------> CaF2(aq)  +  SiF₄(g)  +  3H2O(l)

            2. Bromine

            The name bromine is derived from the latin word “bromos”, meaning “dirty odor”. Bromine has a dark red color. It is the only liquid nonmental at room temperature. It is quite dense (3.1 g?cm³) and can be easily vaporized.

            Bromine is found in the bodles of some sea creatures and in sea water. It also exixts as the salts of magnesium and sodium in nature, but the main source is sea water. Bromine occurs in the sea as bromides. The concentration of Br^- ion in sea water is about 10^-3 mol/L.

            When chlorine gas is passed through a solution of salts of bromine, elemental bromine is obtained.

2Br^-(aq)  +  Cl₂(g) ------------> 2Cl^-(aq)  + Br₂(l)

Bromine is very reactive and toxic. It is a good oxidizing agent. It can react with a lot of substance at room temperature.

2Al  +  3Br₂ -----------> 2AlBr₃

                The organic compounds of bromine with ethylene are used in agriculture as drugs. They are also used to protect engines from knocking. When they are added to fuel oil, bromine combines with the lead in the fuel-oil.

            The bromide ion, like chloride and iodide, reacts directly with Ag⁺ and yields a dirty yellow precipitate.

Ag⁺(aq)  +  Br^-(aq) ---------------> AgBr(s) (dirty-yellow)

            Since the silver halides, especially silver bromine, are very sensitive to light. When they are axposed to sunlight, they are aesily reduced to the metallic silver. As a result of this property, they are used to produse photographic films.

           

            There are compare and contrast between fluorine and bromine. Compare is to examine the character or qualities of especially in order to discover resemblances or differences or  to represent as similar. While contrast is to compare in order to show unlikeness or differencess, note the opposite natures, purposes, etc.

Character	F	Br
Atomic number	9	35
Configuration of electrons	[He] 2s2 2p5	[Ar] 3d10 4s2 4p5
The covalent radius (Ao)	0,64	1,14
The radius of X- ion (Ao)	1,19	1,82
Ionation energy level I (kJ/mol)	1681	1140
Electron affinity	-328	-325
Standard reduction potential, Eo (volt)	2,87	1,06
X-X bonding energy (kJ/mol)	155	193
H-X bond energy (kJ/mol)	562	366
Keelektronegatifan	4,0	2,8
Boiling point (oC)	-233	-7,2
Freezing point (oC)	-188	58,8
Beings at25oC	Gas (pale yellow)	Liquid (red)

Duble bubble maps:

            Compare between fluorine and beomine are halogen. The halogen group is a highly reactive group that captures electrons (oxidizers). While contrast between fluorine and bromine as in the table above. At room temperature, fluorine is a colorless gas or rather yellowish. Bromine at room temperature is a dark red oil liquid and has a very high vapor pressure. Liquid bromine is one of the most dangerous general laboratory reagents, because of its effects on the eyes and nasal passages. Halogen elements can be identified from the smell and color because it smells stimulating. Fluor is light yellow and brom is brown. Being at 25^oC, fluor is yellow and brom is red.