You may be aware of the periodic table and its numbering system from left to the right. As you go from left to right, the 17th
group is second last before the 18th
, the last group, and next to the noble gases. Looking further into the 17th
group from top to bottom you would find elements like fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I) and Astatine (At). These elements are termed as ‘halogens’.
Meaning of Halogen: It’s a Greek word meaning ‘salt producing’.
Characteristics of halogens
• These are non-metals• They react with metals to form compounds forming salts. • These need one more electron to fill their electron shell that has 7 valence electrons• They form diatomic molecules• They form covalent bondsElectronic configuration
The electronic configuration of valence shell is ns2 np5
which means that there are 7 electrons in the outermost shell and it requires one more electron to complete its octet. It can either lose one electron to form a covalent bond or it can gain one electron and form an ionic bond. Due to this reason it is said to be very reactive in nature.
Different elements of Group 17
1) Fluorine• Found abundantly on earth’s surface• Occurs as yellow gas at room temperature• Has atomic number of 9• Found first in group and as gas at room temperature• Has small radius and is most electronegative• Is very reactive having oxidation state of -1.• Reaction with hydrogen produces weak acid.• Has configuration as 1s2, 2s2, 2p52) Chlorine• Found abundantly as dissolved ions in ocean• Occurs as light green gas• Found in diatomic state• Has atomic number of 17• Exhibits oxidation state of -1, +1, 3,5, 7• Very reactive forming chlorides with metals• Has configuration as [Ne]3s23p53) Bromine• Found as bromide salt in the sea• Has atomic number 35• Occurs as reddish brown liquid • Exhibits oxidation state of -1, +1, 3, 4 and 5.• Third in the group is less reactive than chlorine and fluorine• Is very toxic and is oxidizing agent• Has configuration as [Ar]3d104s24p54) Iodine• Occurs in sea water• Has atomic number of 53• Occurs as violet solid at room temperature• Exhibits oxidation state of -1, +1, 5 and 7• Has configuration as [Kr]4d10 5s25p55) Astatine• Occurs as radioactive element• Has atomic number of 85• Occurs as metallic solid and is black colored.• Exhibits oxidations state of -1, +1, 5,7Summary
|Elements||Color||State||Oxidation state||Atomic number|
As we move down the group, the electronegative values decrease
since it depends on the strength of the bonding energy between the outermost electrons and nucleus. While going down the group, the nuclei radius increases indicating that the outermost electrons gets farther from the nucleus, thus bonding gets weaker. Fluorine being the first element has small radius indicating that the electrons are closely attached to the nucleus, thereby the electronegativity is high. Iodine has more shells keeping the outermost electrons far away from nucleus reducing the bond energy between them and thereby the electronegativity is low.
The arrangement of electronegativity is F> Cl> Br> I
2) Atomic and ionic radii
As we go down the group, the atomic radii and ionic radii increase. This is because there is an increase in the number of shells as we go down the group. There is another reason being that the atomic charge decreases as we go down the group. The series is F< Cl< Br< I
3) Ionization energy
As we move down the group, the number of shells increases wherein the diameter or size of the nucleus increases. Due to this, the ionization enthalpy decreases
as we move down. In other words, it means, that the bonding energy between the nucleus and the outermost electrons decrease, wherein less energy is needed to pull out the outermost electrons. But fluorine has highest ionization enthalpy as its size is small wherein the electrons are held strongly to the nucleus. The series is F> C> Br> I
4) Electronic gain enthalpy
As we go down the group, the electron gain enthalpy becomes less negative
. For example, Fluorine at the top of the group has lesser enthalpy than chlorine, though fluorine is more electronegative. This is due to the reason that the size of the atom of fluorine is small and the size of 2p shell is small. Here, the addition of electron is not favored as electron repulsion is more intense.
The series is F< Cl< Br< I
5) Electron affinity
As we go down the group, the atomic radius increases and the outermost electrons have less attraction with the nucleus. Hence, the electron affinity decreases as electronegativity decreases since the forces of electron pull is lessened. The series is Cl> F> Br> I
6) Reactivity of elements
As we go down the group, the reactivity decreases
as there is an increase in the atomic radii with an increase in the number of energy shells. Due to this, there is decreased forced of attraction between the outermost electrons and nucleus.
The series is F> Cl> Br > I
1) Colour: These elements exhibit a variety of colors like fluorine exhibits pale yellow color while iodine exhibits dark violet color. 2) Physical nature: Fluorine and Chlorine are found to be in gaseous form while bromine is in liquid form and iodine is found in the solid form.3) Solubility in water: Fluorine and Chlorine are soluble in water while bromine and iodine are less soluble in water. 4) Boiling point: The boiling point of these elements increases as we move down the group due to the Van der Waal forces. Going down the group, the molecular size increases, indicating that the Van der Waals forces also increase thereby making it difficult to break down the molecules. When considering the elements, fluorine has the lowest boiling point.
|Elements||Melting point(° C)|
The series is F< Cl< Br< I
5) Melting point: The melting point of group 17 elements increases as we go down the group. This is due to the increased molecular size wherein the Van der Waals forces become stronger. Thus, making it difficult to break the bonds. Hence, Fluorine has the lowest melting point.
|Elements||Boiling point(° C)|
The series is F< Cl< Br< I
1) Action on Hydrogen: The halogens produce hydrogen halides when it reacts with hydrogen. The reactivity of these elements is such that fluorine reacts violently while chlorine reacts in the presence of sunlight. Bromine reacts only on heating while iodine requires a catalyst to produce halides.
The compounds formed are HF, HBr, HCl, HI.
2) Action on oxygen: The halogens react with oxygen to form oxides
3) Oxidizing nature: The halogens are said to be good oxidizing agents with first element fluorine being the most powerful that can oxidize all other halides. But the oxidizing power reduces as we go down the group.
4) Action on metals: As other properties, halogens react instantly with metals forming its halides. Example: Reaction of sodium with chlorine gives sodium chloride giving out bright yellow light and heat.
2Na (s) + Cl2(g) 2NaCl (s) + Heat
As the halogens are electronegative and metals are electropositive, the metal halides are ionic in nature. Fluorine being more electronegative shows strong ionic bonds and the ionic nature reduces as we go down the group.
Uses of the halogens:
1) Fluorine is used predominantly in toothpaste to prevent caries and to take care of the enamel. 2) Chlorine is used mainly in bleaching in the form of Sodium hypochlorite to disinfect drinking water and swimming pools. It is also used in metallurgy of platinum and gold. It is used for treating infections, allergies and is a useful component in the pharmaceutical industry. It is also a main component in polyvinyl chloride that is used for making wire insulation, pipes and electronics. Chlorine finds its way in neutralizing hospital equipment. Moreover, it is found in pesticides like DDT that is an agricultural insecticide. 3) Bromine has the property of being a flame retardant and hence finds its applications in fire resistant products. It is also used in pesticide like methyl bromide that helps in storage of crops and prevents the growth of bacteria. It is widely used in production of photographic films. It is also used in fire extinguishers. In pharmaceutical industry, bromine is used for making drugs that are necessary for the treatment of pneumonia and Alzheimer’s disease.4) Iodine is used as an antiseptic. It is used for cleaning wounds and also is also found in disinfectant. Besides, it is a common element required for the proper functioning of the thyroid gland of human body. Hence, iodine is an important ingredient in table salt. Iodine is also an important element in photographic development chemicals in the form of silver iodide.