Physical and Chemical Properties of Group 17 Elements (Halogens)
The elements that are present in group 17 are fluorine, chlorine, bromine, iodine, and astatine. They are called halogens as they react with metals to produce salts. Halogens are nonmetals with high reactivity. These elements are grouped together as they have similar properties. Group 17 elements, fluorine, chlorine, bromine, iodine, and astatine, are collectively known as halogens (in Greek, halo means salt, and genes mean generating, thus collectively salt-producing).
Electronic Configuration
The electronic configuration of the outermost shell of the elements of group 17 is ns2 and np5. Therefore there are 7 electrons in the outermost shell.
There are 7 electrons in the outermost shell of the elements belonging to group 17. The outermost shell is short by one electron to achieve an octet. These elements require one more electron to achieve an octet or ideal gas configuration.
Halogens are the most reactive non-metals. This is due to their tendency to pick up or share an electron to achieve an octet or closest inert gas configuration.
Occurrence
Halogens do not exist in their free state. Halogens exist in all three different states of matter. Astatine is radioactive in nature and exists in a solid-state at room temperature. Fluorine is the 13th richest element by weight in the crust of the world. It exists in the gaseous state at room temperature. It usually exists as insoluble fluorides, cryolites, fluorspar, and fluorapatite. Fluorine is also found in the soil, plants of stream water, and bones and teeth of creatures. Chlorine is the 20th richest element found by weight in the Earth's crust. It exists in its gaseous state at room temperature. Water in the ocean comprises 1.5% by weight of sodium chloride. The dry bed of the ocean contains sodium chloride. Iodine exists as solid at room temperature. Chlorine, bromine, and iodine are present in the ocean water as chlorides, bromides, and iodides. Bromine exists as a liquid at room temperature.
Atomic Properties
The atomic properties of group 17 which are going to be discussed are:
Ionic and atomic radii
Ionization enthalpy
Electron gain enthalpy
Electronegativity
Trends in Atomic and Ionic Radii
Atomic radii are the measure of the distance from the center of an atom to the outermost shell containing electrons. Ionic radii are the measure of the size of an atom's ion. As we go down the group, the atomic and the ionic radii tend to increase as an extra energy shell is added. The reason the elements belonging to this group have smaller atomic radii compared to other elements is due to high atomic charge.
Ionization Enthalpy
The energy required to remove an electron from its valence shell is known as ionization energy. As we move down group 17, the ionization energy decreases. This is because as we move down the group the size of the atom increases which decreases the attraction of the valence electrons and the valence electrons therefore only little ionization energy is required to remove an electron from the valence shell from any halogens. The ionization energy of fluorine is comparatively higher than any other halogen which is due to its small size because of which greater will be the attraction between the core and the valence shell.
Electron Gain Enthalpy
The energy released when an electron is added to an isolated gaseous atom is known as electron gain enthalpy. Halogens have negative electron gain enthalpy. However, on moving down the group the electron gain enthalpy turns out to be less negative.
Exception: chlorine has a higher gain of enthalpy when compared to fluorine. This is attributed to the small size of fluorine due to which there is higher inter-electronic repulsion in the small 2p orbitals and lesser attraction for the incoming electron. chlorine has more negative electron pick up enthalpy than fluorine. I.e. among all, chlorine has the most extreme negative electron pick up enthalpy. It is a result of the small size and reduced 2p sub-shell of the fluorine atom. Attributable to the small size of the fluorine particle, the approaching electron encounters a more noteworthy measure of repulsion from the electrons that are now present.
Electronegativity
The tendency of an atom to attract electron or a bonding pair of electrons is known as electronegativity. Halogens have high electronegativity. The electronegativity decreases as we move down group 17 because of the increase in nuclear radii. In group 17, fluorine is the most electronegative element.
Physical Properties
The physical properties of elements include:
Physical state
Color
Solubility
Metallic character
Density
Melting and boiling point
Bond dissociation energy
Physical State
Fluorine and chlorine are present in the gaseous state. Bromine is present in the liquid state. Iodine is present in a solid-state. All of these elements exist as diatoms.
Colour
The elements of group 17 exhibit different colours. Fluorine has a pale yellow color. Chlorine has a greenish-yellow color. Bromine has a reddish-brown color. Iodine has a dark violet color.
Solubility
Fluorine and chlorine are soluble in water. Bromine and iodine dissolve in organic solvents.
Metallic Nature
As we move down the group the metallic nature of the elements increases. These elements are nonmetallic in nature due to high ionization enthalpy.
Density
As molecular weight increases down the group, the density decreases.
Melting and Boiling Points
As we move down the group, the melting and boiling point increases. This is because as we go down the group the size of the atoms increases, therefore, the vander walls force of attraction also increases.
Bond Dissociation Energy
Bond dissociation energy is the energy required to break the bond into atoms, each with one electron of the original shared pair. The bond dissociation energy decreases as we go down the group except for fluorine. Fluorine has low bond dissociation energy due to its small atomic radius.
Oxidation States
The general electronic configuration of group 17 is: ns2np5
All the elements of group 17 have 7 electrons in its valence shell. These elements require one electron to finish their octet. They can complete their octet either by picking up an electron or sharing an electron. The oxidation states of all the elements belonging to this group are -1.
Except for fluorine, bromine chlorine and iodine have free d-orbital in their valence shells. Due to this, they display different oxidation states like +1, +3, +5, +7 along with -1. These positive oxidation states are oxoacids, interhalogens, and oxides.
Chemical Properties
Oxidizing Power
Halogens are great oxidizing agents. Fluorine can oxidize all halide particles to halogen in a solution. However, oxidizing power decreases as we move down the group.
Chlorine can oxidize bromide to bromine and iodide to iodine.
Cl₂ + 2Br¯ → Br₂ + 2Cl¯
Cl₂ + 2I¯ → I₂ + 2Cl¯
Bromine can oxidize iodide to iodine.
Br₂ + 2I¯ → I + 2Br¯
Halide particles can also act as reducing agents. The reducing power decreases as we move down the group.
Reaction with Hydrogen
Acidic hydrogen halides are formed when halides react with hydrogen. The reactivity of halogen towards halogen decreases as we move down group 17. Therefore, their acidity also decreases as we move down the group.
In dark
H₂ + F₂→ 2HF
In sunlight
H₂ + Cl₂ → 2HCl
Δ
H + Br₂ → 2HBr
Δ
H₂ + I₂ → 2HI
Reaction with Oxygen
Halogen combines with oxygen to form halogen oxides, but they are not steady. The general formula for oxides is X2O to X2O7.
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Molecular Structure of some Chlorine compounds
Reaction with Metals
Halogens react with metals instantly due to their high reactivity to form metal halides.
Sodium reacts with chlorine to form sodium chloride which releases a large amount of heat energy and yellow light as it is an exothermic reaction.
2Na(s) + Cl₂ (g) → 2NaCl(s)
Metal halides are ionic in nature due to the high electronegativity of halogen and electro positivity of metals. The ionic character decreases down the group.
Reaction with Other Halogens
Halogens form interhalogens when the react with other halogens. The general formula of interhalogens is XYn, where n = 1, 3, 5, or 7. Here X is the less electronegative halogen and Y is the more electronegative halogen.
Anomalous Behaviour of Fluorine
Fluorine illustrates anomalous behavior in properties such as bond dissociation energy, ionization energy, electrode potentials, electro-negativity, electron gain enthalpy, ionic and covalent radii, melting point, and boiling point because of its low bond dissociation energy, small nuclear size. high electronegativity and absence of d-orbital in the valence shell of fluorine.
Applications of Halogens
Fluorine
Fluorine is used in drinking water and toothpaste as it reduces tooth decay
It is present in the clay used in ceramics
They are present in chlorofluorocarbons that are used as refrigerants
They are used to generate nuclear power
Chlorine
Chlorine is used to purify drinking water and swimming pools
It is present in PVC (wire insulation
It is used to sterilize hospital machinery
It is also a key factor of certain pesticides like DDT (dichlorodiphenyltrichloroethane)
Bromine
Bromine has fire resistant properties, so it is used to retard flames like a fire extinguisher
It is used to treat pneumonia and Alzheimer's disease.
Methyl bromide is a pesticide that is used to eliminate the spread of bacteria and allows for crop storage
Iodine
It has a major role in the functioning of the thyroid gland of our body
Solutions used to clean open wounds contain iodine
Silver iodide is used in photography
Astatine
Astatine is radioactive
It has helped in the study of cancer
FAQs on Group 17 Elements
1. What is the origin and meaning of the term 'halogen' for Group 17 elements?
The term 'halogen' is derived from the Greek words 'hals' (meaning salt) and 'genes' (meaning born or producer). Therefore, halogens are literally 'salt-producers' because they readily react with metals to form a wide variety of salts, such as sodium chloride (NaCl) and potassium bromide (KBr).
2. Which elements belong to Group 17, and what is a simple way to remember them?
The elements in Group 17 of the Periodic Table are Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I), and Astatine (At). A simple mnemonic to remember them in order is: "First Class Biryani I Ate."
3. What is the general electronic configuration of Group 17 elements and its significance?
The general outer electronic configuration for Group 17 elements is ns²np⁵. This configuration means they have seven valence electrons, just one electron short of a stable noble gas configuration. This high effective nuclear charge and desire to gain one electron makes them highly electronegative and extremely reactive non-metals.
4. Why do the melting and boiling points of halogens increase as we move down the group?
The melting and boiling points increase down Group 17 because of the strengthening of intermolecular forces. As the atomic size and mass increase from Fluorine to Iodine, the magnitude of the van der Waals forces of attraction between the molecules increases. More energy is required to overcome these stronger forces, resulting in higher melting and boiling points.
5. Why do halogens possess a very high negative electron gain enthalpy?
Halogens have a high tendency to accept an electron to complete their octet and achieve a stable noble gas configuration (ns²np⁶). Their atoms are small with a high effective nuclear charge, which creates a strong attraction for an incoming electron. This process releases a significant amount of energy, resulting in a large negative electron gain enthalpy. Interestingly, Chlorine has a more negative electron gain enthalpy than Fluorine due to the smaller size and higher inter-electronic repulsion in Fluorine's 2p subshell.
6. What are the main reasons for the anomalous behaviour of Fluorine compared to other halogens?
Fluorine exhibits properties that are different from other halogens due to several key factors:
- Small Atomic Size: Its exceptionally small size leads to high charge density.
- High Electronegativity: It is the most electronegative element in the periodic table.
- Low F-F Bond Dissociation Enthalpy: Weak F-F bond is due to repulsion between the lone pairs on the small fluorine atoms.
- Absence of d-orbitals: Fluorine cannot expand its octet, limiting its covalency to one, whereas other halogens can form more bonds.
7. What are some important real-world applications of Group 17 elements?
Halogens have numerous important applications in daily life and industry:
- Fluorine: Used in toothpaste as sodium fluoride (NaF) or stannous fluoride (SnF₂) to prevent tooth decay.
- Chlorine: Widely used as a disinfectant for purifying drinking water and swimming pools, and as a bleaching agent for paper and textiles.
- Bromine: Compounds of bromine are used as flame retardants and in the synthesis of certain pharmaceuticals.
- Iodine: An alcoholic solution of iodine ('tincture of iodine') is used as a common antiseptic for wounds.
8. What are interhalogen compounds? Can you provide some examples?
Interhalogen compounds are molecules formed when two or more different halogen atoms react with each other. The halogen with the larger size and lower electronegativity acts as the central atom. Examples include Chlorine monofluoride (ClF), Bromine trifluoride (BrF₃), and Iodine heptafluoride (IF₇).
9. How does the oxidising power of halogens change down Group 17, and what is the reason for this trend?
The oxidising power of halogens decreases down the group, with Fluorine being the strongest oxidising agent (F₂ > Cl₂ > Br₂ > I₂). This is because oxidising power is the ability to accept electrons, which is directly related to the standard reduction potential (E°). The high E° of Fluorine is a combined result of its low bond dissociation enthalpy, high electronegativity, and very high hydration enthalpy of the F⁻ ion, which more than compensates for its less negative electron gain enthalpy compared to Chlorine.
10. What are the different types of oxoacids formed by halogens?
Halogens (except Fluorine) form several types of oxoacids, which are acids containing hydrogen, oxygen, and a halogen atom. The main series of oxoacids are:
- Hypohalous acid (HXO), e.g., Hypochlorous acid (HClO)
- Halous acid (HXO₂), e.g., Chlorous acid (HClO₂)
- Halic acid (HXO₃), e.g., Chloric acid (HClO₃)
- Perhalic acid (HXO₄), e.g., Perchloric acid (HClO₄)
The acidic strength of these oxoacids increases with the number of oxygen atoms attached to the halogen.
