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Halogen Trends and Anomalies
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1
Question
What elements make up Group 17 of the periodic table and what common name is given to them?
Answer
Group 17 consists of the halogens: fluorine (F), chlorine (Cl), bromine (Br), iodine (I) and astatine (At). They are collectively called the halogens.
2
Question
Which Group 17 element is radioactive?
Answer
Astatine (At) is radioactive.
3
Question
Give three important naturally occurring minerals or commercial compounds containing fluoride from the notes.
Page 1
Answer
Fluorite (CaF_2), cryolite (Na_3AlF_6), and fluorapatite (3Ca_3(PO_4)_2·CaF_2).
4
Question
Describe the physical states and colours of fluorine, chlorine, bromine and iodine at room temperature as given in the notes.
Page 1
Answer
Fluorine and chlorine are gases (fluorine pale yellow, chlorine greenish); bromine is a liquid (red-brown); iodine is a solid (violet when sublimed).
5
Question
How do covalent and ionic radii change down Group 17?
Page 1
Answer
Both covalent and ionic radii increase down the group (from F to I).
6
Question
What is the general ionisation process for a halogen atom X and what does ionisation enthalpy describe?
Page 1
Answer
Ionisation: \(X(g) \rightarrow X^+(g) + e^-\). Ionisation enthalpy is the energy required to remove an electron from a gaseous atom (higher values mean harder to remove an electron). All halogens have an ionisation enthalpy (first ionisation energy).
7
Question
State the trend in electronegativity down Group 17.
Page 1
Answer
Electronegativity decreases down the group (fluorine is the most electronegative, iodine the least among the common halogens).
8
Question
How does first electron affinity (electron gain enthalpy) behave for halogens and what sign is it typically?
Page 2
Answer
All halogens have negative electron gain enthalpies (exothermic) because they become more stable on gaining an electron; however, the magnitude varies among halogens.
9
Question
Order the halogens by electron gain enthalpy magnitude given in the notes.
Page 2
Answer
The order given is Cl > F > Br > I (chlorine has the most exothermic electron gain enthalpy in the listed order).
10
Question
Why is chlorine’s electron gain enthalpy more exothermic than fluorine’s, despite fluorine being more electronegative?
Page 2
Answer
Although fluorine is more electronegative, its small size causes strong inter‑electron repulsions in the compact 2p orbitals when an extra electron is added, making it harder to add an electron than to chlorine; thus chlorine has a more exothermic electron gain enthalpy.
11
Question
How does hydration energy depend on ion size for halide ions and what trend results?
Page 2
Answer
Hydration energy increases as ionic size decreases (smaller ions are more strongly hydrated), so the hydration enthalpy magnitude follows F^- > Cl^- > Br^- > I^-.
12
Question
How do melting and boiling points change down Group 17?
Page 3
Answer
Melting points and boiling points increase down the group (from F_2 to I_2) due to increasing van der Waals (dispersion) forces as molecular size and polarizability increase.
13
Question
Order the halogen‑halogen bond lengths for X_2 molecules.
Page 3
Answer
Bond lengths increase down the group: F_2 < Cl_2 < Br_2 < I_2.
14
Question
What is the expected trend for bond dissociation enthalpy of X_2 molecules and what anomaly is observed in reality?
Page 3
Answer
Expected: F_2 > Cl_2 > Br_2 > I_2 (shorter bonds stronger). Reality: Cl_2 > Br_2 > F_2 > I_2. Fluorine’s F–F bond is weaker than expected due to strong inter‑electron repulsion in the small 2p orbitals, making F_2 easier to split.
15
Question
Explain why F_2 has an unexpectedly low bond dissociation enthalpy compared to Cl_2.
Page 3
Answer
In F_2, the small size and compact 2p orbitals lead to strong lone‑pair–lone‑pair repulsion between the fluorine atoms; this repulsion reduces bond strength so F_2 dissociates more easily than expected.
16
Question
List the components discussed under ‘Anomalous Behaviour of F’ in the notes.
Page 4
Answer
The notes list: (1) Hydrides of halogens, (2) Bond lengths, (3) pK_a values (acidity), (4) Bonding/melting point, and (5) reactivity/other anomalies — all illustrating fluorine’s anomalous behaviour compared to other halogens.
17
Question
Order the H–X bond lengths (X = F, Cl, Br, I).
Page 4
Answer
H–F < H–Cl < H–Br < H–I (bond lengths increase down the group).
18
Question
How do acid strengths (pK_a values) of hydrogen halides vary down the group?
Page 4
Answer
Acidity increases down the group (HF is the weakest acid, HI the strongest). Equivalently, pK_a values decrease down the group: HF has the highest pK_a, then HCl, HBr, HI.
19
Question
Provide the acidic order of hydrogen halides given in the notes.
Page 4
Answer
HF < HCl < HBr < HI (from weakest to strongest acid).
20
Question
What trend for pK_a is summarized in the notes for the hydrogen halides?
Page 4
Answer
The notes summarize that pK_a decreases down the group (so acidity increases), giving: HF > HCl > HBr > HI in terms of pK_a (larger pK_a = weaker acid).
21
Question
What reaction between halogens and oxygen produces oxides that can be useful fluorinating agents?
Page 5
Answer
Oxygen difluoride and dioxygen difluoride reactions with fluorine or other elements can produce powerful fluorinating agents. Example given: \(O_2F_2 + Pu \rightarrow PuF_6\) (used to remove plutonium as PuF_6).
22
Question
Why is O_2F_2 useful as a fluorinating agent despite being thermally unstable?
Page 5
Answer
Because O_2F_2 is thermally unstable it readily decomposes and can transfer fluorine to other elements (strong fluorinating ability), enabling reactions like formation of volatile hexafluorides (e.g., PuF_6) for purification processes.
23
Question
Which halogens form oxides with oxidation states from +1 to +7 and how does stability of these oxides change among halogens?
Page 5
Answer
Chlorine, bromine and iodine form oxides spanning oxidation states +1 to +7. The stability of oxides formed increases down the group: I oxides > Cl oxides > Br oxides (per notes). Higher oxidation state oxides are generally more stable than lower ones for halogens.
24
Question
List some chlorine oxides mentioned in the notes and a common use for one of them.
Page 5
Answer
Chlorine oxides mentioned: Cl_2O, ClO_2, Cl_2O_6 and Cl_2O_7. Chlorine dioxide (ClO_2) is used as a bleaching agent for paper pulp.
25
Question
What is said about bromine oxides (like Br_2O, BrO_2, BrO_3) in the notes?
Page 6
Answer
Bromine oxides such as Br_2O, BrO_2, BrO_3 are extremely unstable and exist only at very low temperatures (if at all).
26
Question
What are some iodine oxides listed and what is their stability/behaviour on heating?
Page 6
Answer
Iodine oxides listed: I_2O_4, I_2O_5, I_2O_7. They are described as insoluble solids that decompose on heating; I_2O_5 is noted as a good oxidizing agent.
27
Question
What general reaction occurs between a halogen and a metal?
Page 6
Answer
Halogen + Metal → Metal halide (e.g., X_2 + metal → metal X_n). Metals are oxidized and halogens are reduced to halide ions.
28
Question
Describe the trend in ionic character of metal halides MF, MCl, MBr, MI (where the same metal M combines with different halides).
Page 6
Answer
Ionic character decreases down the halogen group: MF > MCl > MBr > MI. Fluoride gives the most ionic bond (highest ionic character) while iodide gives the least.
29
Question
What types of interhalogen compounds are noted as forming (using X and X')?
Page 6
Answer
Interhalogen compounds of types X'X, X'X_3, X'X_5 and X'X_7 are indicated to form, where X' is normally the smaller halogen and X is the larger halogen.
30
Question
Explain why higher melting/boiling points are observed for iodine compared to chlorine and fluorine.
Page 3
Answer
Melting and boiling points increase down the group because polarizability and molecular size increase, enhancing London dispersion forces. I_2 has the largest polarizability, so it has higher mp/bp than Cl_2 or F_2.