What Are the Key Chemical and Physical Trends in Halogens?
Group 17 elements of the periodic table comprises Halogens. They come under the p-block elements of the modern periodic table. There are five Halogens and in order of valency in the periodic table they are as follows: Chlorine, Fluorine, Iodine, Bromine and Astatine. The word “Halogen” is a Greek word which means salt producer due to the properties of elements like Bromine, Chlorine and Iodine. Halogens exhibit high electronegativity and the anions formed become the anionic part of the salts which are widely found in seawater. Electronegativity is defined as the ability of an atom to accept an electron and form an octet. The last element of the group, Astatine, is radioactive in nature. In this article, students will learn about Halogens and their properties.
What are Halogens?
Halogens are highly reactive nonmetals belonging to the 17th group of the periodic table. They have seven electrons in their outermost shell with the electronic configuration ns2np5. They are highly electronegative in nature as they are only one electron short of the nearest noble gas configuration. Out of the five Halogens, Astatine is the only radioactive element. The Physical and Chemical properties of Halogens are greatly similar which is not observed in other groups.
Properties of Halogens
Physical Properties
Halogens exhibit smooth transitions in their Physical state. Chlorine and fluorine are glasses naturally, bromine is liquid in its natural state and iodine is solid in nature.
With change in atomic number, the melting and the boiling points of the elements change.
Halogens are usually coloured due to the absorption of radiation in their visible regions. Fluorine exhibits yellow color, bromine shows red color, Chlorine is greenish-yellow in color and iodine is violet.
Chlorine and fluorine dissolve in water completely, iodine and bromine are partially soluble in water but completely soluble in organic solvents such as chloroform disulphide.
Chemical Properties
Halogens show a -1 oxidation state, however, bromine, Chlorine and fluorine can also exhibit +1, +3, +5 and +7 oxidation states as well. The higher oxidation states are exhibited only when Halogens combine with highly electronegative atoms like oxygen and fluorine. They are usually highly reactive and can react with metals and nonmetals alike, thus, forming halides. They can readily accept an electron to complete electronic configuration and form an octet. Out of all the Halogens, fluorine is the strongest Oxidizing agent.
Trends Among Halogens
Reactivity of Halogens decreases down the group.
Halogens have a small atomic size.
They have a high nuclear charge.
Group 17 Trends Properties
The modern periodic table's group 17 consists of the following elements.
Chlorine
Fluorine
Iodine
Bromine
Astatine
These elements are called halogens. Halogen is a Greek word, which means a salt producer. These elements are known as salt producers due to the properties exhibited by bromine, chlorine, and iodine. They are highly electronegative in nature and form anions, which constitute the anionic part of salts found in the seawater. Astatine is the last element of the group and is radioactive in nature. These elements belong to the p block of the modern periodic table. The halogen family constitutes the most homogenous group after the alkali group in the modern periodic table.
Did You know?
Why is the electron affinity of chlorine more than that of fluorine?
Answer: This is due to the compact structure of fluorine. Chlorine has a larger atomic size hence it can accommodate more electrons, whereas the electrons in fluorine are closely-spaced. So, the crowded electrons in fluorine screen its effective nuclear charge, and thus, it exhibits a lower electron affinity.
FAQs on Group 17 (Halogens): Trends in Properties Explained
1. What are the Group 17 elements commonly known as, and why?
The elements in Group 17 of the periodic table are called halogens. This name originates from the Greek words 'hals' (meaning salt) and 'genes' (meaning born or producer). Therefore, 'halogen' literally means 'salt-producer' because these elements readily react with metals to form a wide variety of salts, such as sodium chloride (NaCl) and potassium bromide (KBr).
2. What are the major trends observed in the physical properties of Group 17 elements as you move down the group?
As we move down Group 17 from Fluorine to Astatine, several key trends in physical properties are observed:
- Physical State: The elements transition from gas to solid. Fluorine (F₂) and Chlorine (Cl₂) are gases, Bromine (Br₂) is a volatile liquid, and Iodine (I₂) is a solid at room temperature.
- Atomic and Ionic Radii: Both atomic and ionic radii increase down the group due to the addition of a new electron shell with each successive element.
- Melting and Boiling Points: These points steadily increase down the group. This is because the strength of the intermolecular van der Waals forces increases with the increasing size and mass of the atoms.
- Colour: All halogens are coloured. The colour deepens down the group, from pale yellow (F₂) to greenish-yellow (Cl₂), reddish-brown (Br₂), and violet-black (I₂).
3. How does electronegativity change for halogens, and what is the consequence?
Electronegativity, the ability of an atom to attract shared electrons, decreases as you move down Group 17. Fluorine is the most electronegative element in the entire periodic table. This high electronegativity is due to its small atomic size and high effective nuclear charge. The consequence of this trend is that halogens are highly reactive non-metals and act as strong oxidising agents, with the oxidising power decreasing from fluorine to iodine.
4. Why is the electron gain enthalpy of chlorine more negative than that of fluorine, despite fluorine's higher electronegativity?
This is a key exception to the general periodic trends. Although fluorine is more electronegative, its electron gain enthalpy is less negative than that of chlorine. This is because the fluorine atom is extremely small and its valence electrons are tightly packed in the 2p subshell. The strong inter-electronic repulsion in this compact shell makes it slightly less favourable for an incoming electron to be added. Chlorine, with its larger 3p subshell, can accommodate the new electron with less repulsion, resulting in a greater release of energy and a more negative electron gain enthalpy.
5. Explain why all halogens exhibit distinct colours.
Halogens are coloured because their molecules absorb radiation from the visible region of the electromagnetic spectrum. This absorbed energy causes the outer (valence) electrons to get excited and jump from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The colour we see is the complementary colour of the light that is absorbed. The amount of energy required for this excitation decreases down the group, causing the colour to deepen from fluorine (absorbs violet light, appears pale yellow) to iodine (absorbs yellow light, appears violet).
6. What is the trend in the oxidising power of halogens, and which is the strongest?
The oxidising power of halogens decreases as we move down the group from fluorine to iodine (F₂ > Cl₂ > Br₂ > I₂). An oxidising agent is a substance that readily accepts electrons. Halogens have a strong tendency to accept one electron to achieve a stable noble gas configuration. Fluorine (F₂) is the strongest oxidising agent because it has the highest electronegativity and a high standard reduction potential, allowing it to easily oxidise other elements and even other halide ions.
7. Why is the bond dissociation enthalpy of F₂ an exception and lower than that of Cl₂?
Generally, bond dissociation enthalpy should decrease down the group as the atomic size increases. However, the bond dissociation enthalpy of the F-F bond in F₂ is exceptionally low, and lower than that of the Cl-Cl bond. This anomaly is due to the very small size of the fluorine atoms. In the F₂ molecule, the lone pairs of electrons on the two small fluorine atoms are very close to each other, leading to significant lone pair-lone pair repulsion. This repulsion weakens the F-F covalent bond, making it easier to break.
8. How do the common oxidation states of halogens vary within the group?
All halogens exhibit a -1 oxidation state as they need only one electron to complete their octet. However, halogens other than fluorine (i.e., Chlorine, Bromine, and Iodine) can also show positive oxidation states of +1, +3, +5, and +7. This is possible because they have vacant d-orbitals in their valence shell, allowing them to expand their octet by unpairing and promoting electrons to these empty orbitals when forming compounds with more electronegative elements like oxygen or fluorine itself. Fluorine, having no d-orbitals, is restricted to only a -1 oxidation state.
9. How does the ionic character of metal halides (M-X) change for a fixed metal down Group 17?
For a given metal (M), the ionic character of its halides decreases down the group. The order is M-F > M-Cl > M-Br > M-I. According to Fajan's rules, the ionic character is highest when the electronegativity difference between the metal and the halogen is largest. Since electronegativity decreases down the group from F to I, the electronegativity difference with the metal also decreases. Consequently, fluorides are the most ionic, while iodides are the most covalent in nature.
