7/27/2019 Assignment FKAX 0111.docx

http://slidepdf.com/reader/full/assignment-fkax-0111docx 1/2

Use the periodic table to predict the characteristic valence-electron configuration of the halogens.

  In each of the halogens' outer 'shells', they have a full s-orbital, and a nearly full p subshell, with

one of the p orbitals missing one electron from a stable configuration. This is what makes the

halogens (especially fluorine and chlorine) so reactive. 

  At room temperature all the halogens exist as diatomic molecules. The melting points, boiling

points, atomic radii and ionic radii all increase on descending the Group. The shapes of the

covalent molecules and ions are readily explained by VSEPR (valence shell electron pair

repulsion) theory and these compounds are often used to illustrate the theory. Fluorine is never

surrounded by more than 8 electrons, whereas the other halogens may be surrounded by up to

14 electrons.

  The most characteristic chemical feature of the halogens is their ability to oxidise. Fluorine has

the strongest oxidising ability, so other elements which combine with fluorine have their highest

possible oxidation number. Fluorine is such a strong oxidising agent that it must be prepared by

electrolysis. Chlorine is the next strongest oxidising agent, but it can be prepared by chemical

oxidation. Most elements react directly with chlorine, bromine and iodine, with decreasing

reactivity going down the Group, but often the reaction must be activated by heat or UV light.

The oxidation of thiosulfate ions, S2O 32-

, by the halogens is quantitative. This means that

oxidising agents can be estimated accurately; the oxidising agent is reacted with excess I-ions,

and the liberated I2 titrated with standard thiosulfate solution. The end point is detected with

starch as indicator, which forms a dark blue complex with iodine. Chlorine, bromine and iodine

disproportionate in the presence of water and alkalis.

  The melting and boiling points increase down the group because of the van der Waals forces. 

The size of the molecules increases down the group. This increase in size means an increase in

the strength of the van der Waals forces.

  The size of the nucleus increases down a group because there is a higher number of protons and

neutrons. Also, more energy levels are added on after passing each period. This results in

a bigger orbital, and therefore a bigger radius.

  If the outer valence electrons are not near the nucleus, it will not take as much energy to

remove them. Therefore, the energy required to pull off the outermost electron will not be as

high for the elements at the bottom of the group since there are more energy levels. Also, the

high ionization energy makes the element appear non-metallic. Iodine and astatine display

metallic properties so ionization energy should decrease down the group.

7/27/2019 Assignment FKAX 0111.docx

http://slidepdf.com/reader/full/assignment-fkax-0111docx 2/2

 

  The number of valence electrons increases due to the increase in energy levels as the elements

progress down the group. The electrons are not as near to the nucleus anymore. Therefore, the

nucleus and the electrons are not as attracted to each other as much. An increase in shielding is

observed. Electronegativity will therefore decrease down the group.

  Since the atomic size increases down the group, electron affinity will decrease. An electron will

not be as attracted to the nucleus, resulting in a low electron affinity. However, fluorine has a

lower electron affinity than chlorine. This can be explained by the small size of fluorine,

compared to chlorine.

  Down a group, the atomic radius gets bigger, and there is an increase in the amount of energy

levels. This results in less attraction of the valence electrons. It also decreases because

electronegativity decreases down a group, which means that there will be less interactions with

the electrons in terms of "pulling". Also, since there is a decrease in oxidizing ability down a

group, the reactivity of the elements will decrease as well.