9701 · 11.1
Physical properties of the Group 17 elements flashcards
Revision flashcards for Cambridge 9701 Physical properties of the Group 17 elements (syllabus 11.1). Flip, recall, then mark a real past-paper question.
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What are the elements in Group 17 commonly called?
The halogens, which means 'salt-formers'.
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Describe the trend in colour down Group 17.
The colour becomes darker and more intense. Fluorine ($F_2$) is a pale yellow gas, Chlorine ($Cl_2$) is a green-yellow gas, Bromine ($Br_2$) is a red-brown liquid, and Iodine ($I_2$) is a grey-black solid.
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What is volatility?
A measure of how easily a substance vaporises (turns into a gas). High volatility corresponds to a low boiling point.
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What is the trend in volatility for the halogens?
Volatility decreases down the group. Fluorine is the most volatile and iodine is the least volatile.
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Why do the boiling points of halogens increase down the group?
The number of electrons per molecule increases. This leads to stronger instantaneous dipole-induced dipole (id-id) forces between molecules, which require more energy to overcome.
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What type of intermolecular force exists between halogen molecules?
Instantaneous dipole-induced dipole (id-id) forces, also known as London dispersion forces.
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What is the general trend in X-X bond enthalpy down Group 17?
It decreases from chlorine to iodine ($Cl-Cl > Br-Br > I-I$). Fluorine is an exception.
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Why is the F-F bond enthalpy anomalously low (+158 kJ mol⁻¹)?
The fluorine atom is very small, causing the lone pairs on adjacent atoms in a $F_2$ molecule to be very close. This results in significant lone pair-lone pair repulsion, which weakens the covalent bond.
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What is electronegativity?
The ability of an atom to attract the pair of electrons in a covalent bond.
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Describe and explain the trend in electronegativity down Group 17.
Electronegativity decreases. The atomic radius and number of inner shells (shielding) increase, so the nucleus has a weaker attraction for the bonding electrons.
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What is the physical state of iodine at room temperature and pressure?
A grey-black solid which sublimes to form a purple vapour.
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Why are halogens non-polar molecules?
They exist as diatomic molecules ($X_2$) where the two atoms are identical. Therefore, there is an equal sharing of electrons and no permanent dipole.