Video Transcript
In this video, we will learn about
the halogens and the trends in their physical and chemical properties. We will learn how to explain the
trend in the reactivity of the halogens using examples of their reactions with
metals to form salts and also by looking at halogen displacement reactions.
The halogens are the six
nonmetallic elements that are found in group 17 or seven of the periodic table. This is the second column from the
right-hand side of the periodic table. These elements are fluorine,
chlorine, bromine, iodine, astatine, and tennessine. Astatine and tennessine are both
radioactive elements, and their isotopes are not stable. Indeed, the Greek word “astatos”
translates as unstable. Although astatine isotopes have
been discovered in minute amounts in natural radioactive decay chains, astatine can
be considered to be a synthetic element. In 2016, the discovery of the
synthetic element with the atomic number 117 was recognized, and it was named
tennessine after the state of Tennessee where research centers were located. Astatine and tennessine are both
considered to be synthetic elements. And because their isotopes are very
short-lived, you will not find an atomic mass value in the modern periodic table for
these two elements.
The halogens are all very reactive
elements, and they’re not encountered in nature as pure elements. Fluorine, chlorine, bromine, and
iodine are all found combined with other elements in the Earth’s crust. Fluorine and chlorine are more
common than bromine and iodine in this location. None of the halogens make the top
10 list for the most abundant elements found on planet Earth. As halogens are never found in
nature as pure elements, they have to be manufactured. So, let’s take a look at each
halogen in turn so that we can describe their physical appearance accurately.
Fluorine is the most reactive
element of all known elements. It appears as a
pale-yellow-to-green gas. Fluorine is very hard to contain
due to its high reactivity. Iron wool bursts into flames when
simply exposed to fluorine at room temperature. Iron three fluoride is formed in
this very exothermic reaction. Although in its pure form fluorine
exists as diatomic covalently bonded molecules, fluorine is more commonly found as a
compound in useful products. Fluorine is found as the halide ion
in toothpaste to help prevent tooth decay. And fluorine is also found in the
compound PTFE, known under the brand name of Teflon, which is found in the nonstick
coating on frying pans.
Chlorine is a dense yellow-to-green
gas. It is very toxic, and it was used
as a weapon during World War One. It is also found as diatomic
covalently bonded molecules. Chlorine is a little less reactive
than fluorine. And iron wool reacts rapidly when
heated and placed into chlorine to produce dense brown fumes of iron three
chloride. Chlorine is extracted via the
electrolysis of brine, a concentrated solution of sodium chloride, the salt that we
commonly call table salt. Much of the chlorine produced in
this way is used directly as the element to kill bacteria to provide drinking
water. Chlorine is also found in the
compound hypochlorite, also known as bleach, which is often used to kill bacteria in
swimming pools. Chlorine is also found in the
compound PVC, a tough, flexible plastic often used as an electrical insulator.
Bromine is a dense reddish-brown
liquid that gives off a dense red vapor. If we were to pick up a bottle of
bromine liquid, we’d be surprised how heavy it were compared to a bottle of water of
equal volume. Bromine is extracted from
seawater. Like chlorine, it’s very toxic. The name bromine is derived from
the Greek word “bromos,” meaning stench. Liquid bromine is best handled in
the fume hood. Iron wool reacts when heated
strongly and placed into liquid bromine but less violently than it does with
chlorine. Aluminum foil will react very
exothermically with liquid bromine, but the reaction takes a long time to get
going. In both reactions, the metal salts
iron three bromide and aluminum three bromide are produced. Bromine compounds are found widely
used in pesticides and in drug molecules.
Iodine can be a difficult element
to describe. Under normal conditions, it exists
as a gray-to-black solid with a shiny crystalline structure. When heated, it sublimes to a
purple vapor. When dissolved in water, it forms a
brown solution. And dissolved in hexene, a purple
solution is seen. Iodine can be extracted readily
from seaweed. Like all the other halogens, iodine
exists as covalently bonded diatomic molecules. Iodine has an important role as an
antiseptic as the element. And it has an important function in
the human body in small amounts in the thyroid gland located in your neck. The thyroid gland produces an
important hormone called thyroxine. This is a complicated molecule that
contains iodine atoms. If we mix powdered aluminum with
powdered iodine, a reaction does occur after a long induction period. The product is aluminum three
iodide, and the reaction is quite exothermic.
It’s clear that as we descend group
17, these halogens get darker in appearance. They also turn from gas to liquid
to solid. By comparing their reactions with
iron wool, we can see that the reactions get progressively less violent. If we had to make predictions about
astatine based upon the trends and patterns emerging here, we would probably say
that astatine is a black solid and it’s less reactive then iodine. If we could safely lay our hands on
enough of this element, it might react with iron wool or aluminum only when heated
very strongly.
From the descriptions of these
halogens, we can see that there are some trends in their physical appearance as we
move down group 17. The physical appearance, and in
particular the color of these elements, is a physical property. As we progress down group 17, the
colors progressively darken. We have also seen that as we move
down group 17, the elements turn from gas to liquid to solid states at room
temperature. This indicates that the melting and
boiling points of these elements are progressively increasing. As elements, these halogens are all
covalently bonded simple molecules.
In the solid state, these halogen
molecules are all arranged in a regular way. If we were to melt these halogens
to turn them from a solid state to a liquid state, we would have to overcome
intermolecular forces that operate between these halogen molecules. Likewise, if we were to boil a
liquid halogen to turn it into a halogen in the gas state, we also need to overcome
intermolecular forces. Remember that when we melt or boil
a simple molecular substance, we’re just separating the molecules apart from each
other, and no covalent bonds are broken at all. This is also the case with these
halogens. Melting or boiling a halogen sample
does not involve breaking the covalent bonds within the halogen molecules at
all.
As we move down group 17, the
melting points and boiling points of these halogen molecules increase as the
intermolecular forces that operate between the halogen molecules get increasingly
stronger. It is also evident from the
descriptions of how iron wool reacts with these halogens that the chemical
reactivity of each halogen decreases as we move down the group. Fluorine reacts with iron wool when
cold; it needs very little encouragement for the reaction to get started. The reaction is very exothermic,
and the iron wool bursts into flames. Iodine and bromine, however, only
react with iron wool when the iron wool is strongly heated first.
Fluorine is clearly the most
reactive halogen, and iodine, the least. This trend in chemical reactivity
can be explained by considering what is happening when halogens react with
metals. Atoms of halogens each have seven
valent shell electrons. When halogen atoms react with
metals, they gain one electron to form a stable halide ion with a single negative
charge. This gives the halogen the
electronic configuration of the corresponding noble gas in the same period. Let us look at these electronic
structures in more detail to see if we can link the electronic structure of each
halogen to its chemical reactivity.
When halogens react with metals,
the halogen atom gains one electron to form a stable halide ion. In the case of fluorine reacting
with lithium, lithium loses one electron from its valence shell and fluorine gains
one electron into its valence shell. A stable ionic compound, a salt
called lithium fluoride, is formed. In the case of fluorine, the
fluorine atom gains electrons very readily indeed. This is because it’s a very small
atom. In the case of fluorine, the
incoming electron is strongly attracted to the fluorine nucleus as it enters a shell
close to the fluorine nucleus. It’s the protons in the fluorine
nucleus that are attracting the incoming electron in the first place.
Another consequence of fluorine
being a very small atom with only two shells occupied by electrons is that fluorine
has less shielding. The concept of shielding is
described as the effects of reduced attraction for valent shell electrons caused by
inner shell electrons. The more occupied shells an atom
contains, the more shielded the valent shell electrons are from the attractive
effect of the nucleus by the inner shell electrons. So, as we move down group 17, the
halogen atoms get larger as we add shells to the atoms. The outer shell is, therefore,
further from the nucleus and shielded or screened from the nucleus by the inner
shells present.
This means that there’s a reduction
in the force of attraction between the nucleus and the valent shell or outermost
electrons. It becomes harder for the halogens
to attract an electron into the outer shell as we move down the group. Iodine is observed to be the least
reactive halogen because it’s the largest atom with the most shielding. And it doesn’t gain electrons very
easily.
Another important set of reactions
that halogens take part in are the displacement reactions involving a more reactive
halogen with the halide ion of a correspondingly less reactive halogen. When a more reactive halogen is
placed into a solution of a less reactive halide ion, the more reactive halogen
displaces the less reactive halide ion as the corresponding halogen. It’s important to note here that
halide ion solutions are colorless. And when the halide ion is
displaced to form the halogen, a color change is often observed.
An example of such a reaction would
be a solution of chlorine dissolved in water being added to a solution of potassium
iodide. In this reaction, chlorine reacts
with potassium iodide to produce potassium chloride and iodine. As chlorine is more reactive than
iodine, it displaces the iodide ions, and a brown solution develops.
If solutions of chlorine gas,
bromine, and iodine are available, these reactions can easily be performed in a
school or college lab. Fluorine is practically
unobtainable in a school or college lab and reacts with water so will be omitted
from these reaction descriptions. These reactions can be summarized
in a table. Since halogens and halide ions of
equal reactivity will not react with each other at all, these have been eliminated
from the table.
If a solution of chlorine is added
to a solution of bromide ions, bromine is displaced. An orange-to-red solution may be
observed, or orange-to-red fumes may be evolved. If chlorine is added to a solution
of iodide ions, iodine is displaced and a brown solution is formed. Both of these reactions happen
because chlorine is more reactive than the respective halide ions concerned.
If a solution of bromine were added
to a solution of aqueous chloride ions, there would be no reaction at all. Bromine is not reactive enough to
displace chlorine from the chloride ions. The bromine solution would,
therefore, stay orange to red. If bromine were added to aqueous
iodide ions, iodine would be displaced. The solution would, therefore, turn
brown.
Lastly, if an aqueous solution of
iodine were added to aqueous chloride or bromide ions, there would be no reaction at
all. Iodine is brown, and the solution
will stay brown. We will now look a question to test
your understanding of these reactions and any observations that you would make.
A halogen displacement reaction is
shown in the following equation: X2 plus 2Br− aqueous makes Br2 gas plus 2X−
aqueous. What is the color change that
occurs when the bromide solution is converted to bromine gas? What halogen could X2 be? (A) Colorless to purple, chlorine;
(B) colorless to brown, iodine; (C) colorless to brown, chlorine; (D) brown to
colorless, iodine; and (E) brown to colorless, chlorine.
In this question, we see that we
are starting with a solution of bromide ions. Bromide ions are halide ions. And all halide ion solutions are
colorless unless there’s a positively charged cation present that gives the solution
a color. So, the bromide ions solution
starts off as a colorless solution. In the reaction, bromine gas is
being displaced from bromide ions and another halide ion X− is being formed. The bromine gas displaced will have
an orange-to-brown color, and the X− ions will be colorless.
So, the color change expected would
be colorless to orange or brown. We can, therefore, eliminate
responses (D), (E), and (A) as these do not contain the correct color changes
expected. The halogen or X2 that displaced
the bromine must be more reactive than the bromine itself. This is the halogen that was added
to the bromide ions in the reaction at the start. The only halogen commonly found in
the lab that is more reactive than bromine is chlorine. Response (B) suggests that iodine
was used to displace the bromine, and this will not work as it’s less reactive than
bromine. We can, therefore, reject response
(B). The only response remaining is (C),
which describes the correct color change and suggests chlorine a halogen that is
more reactive than bromine as the displacing agents. This is the correct answer.
To finish off, let us look at the
key points. As we move down group 17, there is
a decrease in reactivity. There is an increase in atomic
radius or the size of the atom. There is an increase in the number
of filled electron shells. There is a decrease in the power of
the halogen to attract an electron to form a negative ion. There is an increase in melting and
boiling points, and the color of the elements tends to get darker.
