An Introduction to Group 17 Elements
The Group 17 elements have an oxidation state of -1 when they combine with the left of their position and below elements of the periodic table. The elements of Group 17 of the periodic table are known as Halogens. Halogens are reactive nonmetals and include fluorine, chlorine, bromine, and iodine. The oxidation state of oxygen is usually -2 except in compounds with fluorine, oxygen has a positive oxidation number. All the elements of Group 17 form compounds in odd oxidation states (-1, +1, +3, +5, +7) but down the group importance of the higher oxidation states generally decreases. Group 17 elements only required one additional electron to form a full octet. This characteristic makes them more reactive than the other non-metal groups.
What are Halogens?
The Group 17 elements of the periodic table are known as Halogen, in greek: Halo means salt and genes mean producing, so collectively halogens means salt producing. They are highly reactive nonmetals. Fluorine, chlorine, bromine, iodine, and astatine are the elements. Halogens react with metals to form compounds called salt. The halogen elements have seven valence electrons, that's why they are located on the left of the noble gases on the periodic table. Halogens have seven electrons in their outermost shell (ns2np5) and one electron is short from the configuration of the nearest noble gas.
Oxidation State
1. All elements of the halogen family exhibit a -1 oxidation state.
2. Elements such as chlorine, bromine, and iodine also show +1, +3, +5, and +7 states.
3. When chlorine, bromine, and iodine, halogens in combination with small and highly electronegative atoms of fluorine and oxygen, the higher oxidation state is realized.
4. The oxoacids and oxides of bromine and chlorine have +6 and +4 states. Fluorine atoms can not expand their octet, because there are no valence shell d orbitals in fluorine.
5. Fluorine is the most electronegative element and exhibits only -1 oxidation state.
Physical Properties
Physical state: Fluorine and chlorine are gases on the other hand bromine is a liquid and iodine is a solid.
Colour: Group 17 elements have a variety of colours. For example, iodine is dark voilet in colour and Fluorine is pale yellow in colour.
Solubility: Chlorine and Fluorine are soluble in the water on the other hand Iodine and Bromine are less soluble in water.
Chemical Properties of Halogens
1. Being highly reactive, halogens react with metals and non-metals in order to form halides. As we move down the group reactivity of halogens decreases. Halogens have strong oxidizing properties among the halogen element, F2 is the strongest oxidizing halogen, it easily oxidizes the other halide ions present in the solid phase, or in the solution. Generally, halogen oxidizes the halide ions which are of higher atomic number. For Example:
\[F_{2} + 2X^{-} \rightarrow + X_{2}\] (where , X = cl, Br or l )
2. With the help of reaction of halogens with water, the relative oxidizing nature can be illustrated. Where chlorine and bromine react with water and form hydrohalic and hypohalous acid. In a non-spontaneous way, iodine reacts with water. With water in the acidic medium I- can be oxidized. For example:
\[4l^{-} (aq) + 4H^{+} (aq) + O_{2} (g) \rightarrow 2I_{2} (s) + 2H_{2} O (I)\]
General characteristics of Group 17 elements
1. Characteristics of Halogen Family
2. Electronic Configuration
The halogen family members have seven valence electrons, while halogens have seven electrons in their outermost orbit. From the nearest noble gas configuration halogens have one electron short. The configuration of the halogen family is given as ns2np5.
3. Electronegativity of Halogen Family
Fluorine: 4.0
Chlorine: 3.0
Bromine: 2.8
Iodine: 2.5
Astatine: 2.2
Explain Relative Reactivity
Among the halogen family, fluorine is considered to have the most reactivity. This is mainly because of the low dissociation energy it possesses when compared to other elements in the family. And with most of the other elements fluorine can create strong and stable bonds.
While meeting the elements in the halogen family. Both chlorine and fluorine are gases seen at room temperature. While bromine is a reddish-brown liquid with approximately 20 degrees Celsius. Under such temperature, iodine can create crystals of dark violet.
Do you know what electron affinity is? From a free atom or ion energy is released in the form of an ion. And that rate is called electron affinity. Almost all of the elements in the halogen family do have the same electron affinity. And they differ only slightly by their values. In the periodic table, halogens do have a higher electron affinity. A similar concept is what electronegativity is. It is the property of elements to attract electrons. And it is the highest for fluorine. In other words, we can say the urge of the element to become stable by completing the octet.
Fun Facts
Group 17 elements are called “halogens” because they give salts when they react with metals.
Group 17 elements are very reactive nonmetals. The electronic configuration of the valence shells of these electrons is ns2np5. Thus, in the outermost shell of these elements, the number of electrons is 7. These elements have various colours. Fluorine and chlorine are soluble in water, bromine and iodine are less soluble in water. Fluorine and chlorine are gases, on the other hand, bromine is liquid and iodine is solid.
In our day to day life, we use fluoride in water and also in toothpaste. It helps us to prevent tooth decay.
Bromine, on the other hand, has a bad odour and is also very strong. The name originates from the Greek word ‘bromos’ meaning stench.
FAQs on Oxidation State of Group 17 Elements
1. What are the common oxidation states of Group 17 elements (halogens)?
All halogens consistently show a -1 oxidation state due to their high electronegativity and strong tendency to gain one electron to achieve a stable noble gas configuration. However, elements like chlorine, bromine, and iodine can also exhibit positive oxidation states of +1, +3, +5, and +7 when they form compounds with more electronegative elements like oxygen or fluorine.
2. How does the electronic configuration of halogens influence their oxidation states?
The valence shell electronic configuration of halogens is ns²np⁵. This means they are just one electron short of a stable octet, which explains their most common oxidation state of -1. For positive oxidation states (in Cl, Br, I), electrons can be promoted from the p and s orbitals to vacant d orbitals in the same valence shell. This allows them to form more than one bond and exhibit higher oxidation states like +3, +5, and +7.
3. Why does fluorine exhibit only a -1 oxidation state, while other halogens show positive oxidation states?
Fluorine shows only a -1 oxidation state for two primary reasons based on its atomic structure:
- It is the most electronegative element in the periodic table, so it will always attract electrons in a bond and cannot be oxidised by any other element to show a positive state.
- It lacks vacant d-orbitals in its valence shell (n=2). This prevents the promotion of electrons to higher energy levels, making it impossible to expand its octet and exhibit positive oxidation states.
4. Under what conditions do halogens like chlorine, bromine, and iodine show positive oxidation states?
Halogens other than fluorine show positive oxidation states only when they are bonded to an element that is more electronegative than they are. The two most common examples are:
- Interhalogen compounds: For example, in chlorine trifluoride (ClF₃), chlorine is in a +3 oxidation state because it is bonded to the more electronegative fluorine atoms.
- Oxoacids and Oxides: For example, in perchloric acid (HClO₄), chlorine reaches its highest oxidation state of +7 as it is bonded to highly electronegative oxygen atoms.
5. What is the trend of oxidising power for Group 17 elements and why?
The oxidising power of halogens decreases down the group, following the order: F₂ > Cl₂ > Br₂ > I₂. This trend is determined by their standard electrode potentials (E°). Fluorine is the strongest oxidising agent because of its high electronegativity, low F-F bond dissociation enthalpy, and high hydration enthalpy of the F⁻ ion, which collectively make its reduction potential the highest.
6. Explain the disproportionation reaction of halogens with an example.
A disproportionation reaction is a redox reaction where the same element is simultaneously oxidised and reduced. Halogens (except fluorine) undergo disproportionation in an alkaline medium. For instance, when chlorine reacts with hot, concentrated sodium hydroxide (NaOH), its oxidation state changes from 0 to both -1 and +5.
3Cl₂(0) + 6NaOH → 5NaCl(-1) + NaClO₃(+5) + 3H₂O
In this reaction, chlorine at oxidation state 0 is reduced to -1 in NaCl and oxidised to +5 in NaClO₃.
7. Why are the positive oxidation states of halogens generally odd numbers like +1, +3, +5, and +7?
The positive oxidation states result from the excitation of electrons from filled p and s orbitals to vacant d orbitals. The ground state configuration is ns²np⁵. Unpairing of electrons to form bonds happens systematically:
- +1 state: One unpaired p-electron is used.
- +3 state: One electron from a paired p-orbital is promoted to a d-orbital, resulting in three unpaired electrons.
- +5 state: A second p-orbital electron pair is broken, resulting in five unpaired electrons.
- +7 state: The s-orbital electron pair is also promoted, resulting in seven unpaired electrons.
8. How does the oxidation state of the central halogen atom in its oxoacids affect the acidic strength?
The acidic strength of the oxoacids of a specific halogen increases as the oxidation state of the halogen increases. For example, in the oxoacids of chlorine, the order of acidic strength is:
HClO (+1) < HClO₂ (+3) < HClO₃ (+5) < HClO₄ (+7)
This occurs because a higher positive oxidation state on the central halogen atom increases its ability to pull electron density from the O-H bond. This weakens the O-H bond, making the release of a proton (H⁺) easier and thus increasing the overall acidic strength.
