Physical properties of the Group 17 elements

a. What is the molecular formula of bromine?

b. Put the elements bromine, chlorine and iodine in order of boiling point, starting with the lowest.

c. Explain the reasons for the trend described in part b.

a. The molecular formula of bromine is Br₂.

b. The order of boiling point starting with the lowest is chlorine  < bromine < iodine

c. The boiling points of halogen increase from chloride to iodine because iodine has most electrons or biggest molecules, leading it to have the strongest instantaneous dipole–induced dipole forces or London dispersion forces compared to others.

A. State the full electronic configuration for bromine.

B. Explain why bromine is a liquid at room temperature, but fluorine and chlorine are gases.

A. The full electronic configuration for bromine is `1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^5`

B. Because bromine has a higher boiling point than fluorine and chlorine due to having more electrons in molecules of bromine. That makes bromine have stronger intermolecular forces of attraction between molecules that others. Thus, more energy is required to overcome the forces of attraction between bromine molecules compared to others.

The bond enthalpy of the halogen molecules generally decreases down Group 17. Explain why the bond strength of fluorine is lower than that of chlorine.

Remark: The bond enthalpy is the heat needed to break one mole of a covalent bond.

Because fluorine is smaller than chlorine, as such, the lone pairs are closer together, making them cause repulsion. This counteracts the attraction between the bonding pair of electrons and two nuclei. Therefore, the bond strength of fluorine is lower than that of chlorine.

a. Chlorine is a greenish-yellow gas, bromine is a dark red liquid, and iodine is a dark grey solid. 

State and explain the property which most directly causes these differences in volatility.

b. Explain why Cl₂ rather than Br₂ would react more vigorously with a solution of I^-.

a. The property which most directly causes these differences in volatility could be mentioned to 

(1) Halogens are diatomic

(2) The number of electrons in the molecule increases when we move down the group, resulting in a decrease in volatility and stronger instantaneous dipole–induced dipole forces in the molecule

b. Because outer electrons of chlorine are closer to the nucleus than bromine, as such, Cl₂ has a greater attraction for electron, making it react with I^- more vigorously.

Chlorine gas is reacted with aqueous sodium hydroxide. The oxidation number of chlorine changes from 0 to −1 and also from 0 to +1. 

Under which conditions does this reaction occur and what is the colour of the solid silver salt with chlorine in the oxidation state −1?

The answer is A

The reaction between chlorine and cold dilute sodium hydroxide solution is: 

`2NaOH+Cl_2 ​ → NaCl+NaClO + H_2O`

Thus, the oxidation number of chlorine changes from 0 to −1 in NaCl and changes from 0 to +1 in NaOCl.

Silver salt with NaCl to give white AgCl precipitate

`NaCl + AgNO_3 ​ → AgCl + NaNO_3`

B is incorrect because AgCl is white

C & D are incorrect because when chlorine gas is reacted with hot, concentrated alkali, it will create NaClO₃ with the oxidation state of Cl is +5 instead

The halogens exist as diatomic molecules, X₂ . 

Descending through Group 17 from chlorine to iodine the boiling points of the elements increase. 

Which statement explains this trend? 

A. Of the permanent dipole in the X₂ molecule increases as the group is descended.

B. The X–X bond strength increases as the group is descended. 

C. The electronegativity of X decreases as the group is descended. 

D. The number of electrons in each X₂ molecule increases as the group is descended.

The answer is D

A is incorrect because the dipoles are not permanent

B is incorrect because the bond energy will decrease when we move down Group 17

C is incorrect because the electronegativity affects the reactivity of the Group 17 elements

How do the strengths of the forces between molecules, and the bonds within molecules, vary going down Group 17 from chlorine to bromine to iodine? (With strength of van der Waals’ forces and strength of covalent bonds, respectively)

A. increase - increase

B. decrease - increase

C. increase - decrease

D. decrease - decrease

The answer is C

In diatomic molecules, there are only weak van der Waals’ forces between them, which are caused by instantaneous dipole–induced dipole forces. Thus, the larger the molecules, the stronger the van der Waals’ forces.

The strength of a covalent bond is identified by the strength of the attractions between the bonding pair of electrons and the two nuclei. Therefore, the bond energy decreases when we move down the group.

Which properties of hydrogen halides steadily increase in the sequence HCl, HBr and HI?

A. 1 only

B. 1 and 2

C. 2 and 3

D. 1, 2 and 3

The answer is B

The bond length increase when we go down the Group 17 because it is proportional to the ionic radius

The larger the halide ion, the further the outer electrons are from the nucleus and the more they are shielded by inner electrons, leading to get easier for the halide ions to lose electrons when we move down the group 

The statement 3 is incorrect because the thermal stability will decrease when we move down the Group 17

A scientist compares the properties of chlorine, iodine and their compounds. 

Property X for iodine is bigger than for chlorine. 

What is property X? 

A. Solubility of the silver halide in NH₃ (aq) 

B. Oxidising ability of the element 

C. Thermal stability of the hydrogen halide 

D. Strength of van der Waals’ forces between the molecules of the element

The answer is D

A is incorrect because the solubility of silver halide in NH₃ (aq) decrease when we are going down the group

B is incorrect because the halogen atoms get larger, making them accept electrons less easily. Thus, the oxidising power becomes weaker

C is incorrect the thermal stability will decrease when we move down the Group 17, as such, the hydrogen halide bond is weaker.

A molecule of chlorine has a relative molecular mass of 72. 

Which properties of the atoms in this molecule are the same?

A. 1 only

B. 1 and 2

C. 2 and 3

D. 1, 2 and 3

The answer is A

It is easily noticed that it’s related to the combinations of isotopes because the molecule of chlorine has a relative molecular mass of 72. Therefore, it will have `""^35Cl + ""^37Cl` atoms.

B is incorrect because the isotopic mass is different between the two atoms.

C and D are not incorrect because the number of protons and neutrons is known as the nucleon number. The nucleon number will differ between isotopes because of the differences in neutron counts.