Video Transcript
In this video, we will learn how to
identify and name the halogens, describe their physical properties, and explain
their chemical activity.
The halogens are a group of
nonmetal elements located towards the right-hand side of the periodic table. The halogens form group seven A of
the periodic table. This group is also often referred
to as group 7 or group 17 of the periodic table. Group seven A is found within what
is known as the p-block in the periodic table. The halogens are composed of
fluorine, chlorine, bromine, iodine, and astatine. Although the halogens have the
chemical symbols F, Cl, Br, I, and At, they ordinarily exist as diatomic molecules
in their elemental form. Molecular astatine has not been
observed or studied by scientists.
A diatomic molecule is a molecule
made of two atoms chemically bonded together. So, the chemical formulae of the
halogen elements are F2, Cl2, Br2, and I2. Following this pattern, the
presumed molecular formula for astatine could be At2, although this has not been
discovered yet. Astatine is a radioactive element,
and it is very rare. Not much of it exists on Earth at
any given time. So, we will not mention astatine
for the rest of this video.
In the next section of this video,
we shall look at the physical properties of the halogens. Firstly, we’ll take a look at the
appearance of the halogens as elements. Elemental fluorine is found as a
pale-yellow gas. Chlorine is a green gas at room
temperature, whilst bromine is a dark reddish-brown colored liquid. Bromine is a very dense liquid, but
it gives off a red-to-orange vapor with a very strong and unpleasant smell. Iodine is found as an almost black
solid with a slightly shiny appearance. Overall, we can see that the colors
of the elements darken as we move down the group. When iodine is heated, it sublimes,
and a purple vapor is released.
Sublimation is the change of matter
from solid to gas state with no intermediate liquid stage. We can see that as we move down the
group, the physical states of the elements change from gases to a liquid to a
solid. In fact, as we move down the group,
the molecules get larger and the melting points increase.
In the next section of this video,
we shall look at the chemical properties of the halogens. The halogens are all very reactive,
nonmetal elements. Because of this, they are
ordinarily not found as pure elements in nature. Fluorine to iodine react very
readily with metals to make salts by direct combination of the elements. Fluorine reacts rapidly with iron
wool simply by coming into contact with the metal. This is a very fast reaction that
happens at room temperature.
Chlorine reacts with iron wool when
iron wool is heated very strongly. In this reaction, brown fumes of
iron(III) chloride are formed. This reaction happens at a fairly
fast rate, but only at high temperature. We can see by comparing these two
reactions that fluorine is clearly more reactive than chlorine. Similar reactions happen with iron
wool and liquid bromine and with iron wool and solid iodine. In both cases, the reactants must
be heated very strongly. These reactions are slower
reactions, and they only happen at high temperature.
In the reaction of iron with
bromine, iron(III) bromide is formed. This can be represented in a symbol
equation where two atoms of iron react with three molecules of bromine to produce
two units of iron(III) bromide. The reaction of iodine with iron is
similar. The product is iron(III) iodide in
this case.
From the descriptions of the
reactions of iron wool with the halogens, we can see that chemical reactivity
decreases as we move down the group. This is the opposite to the trend
in reactivity seen in group one A, the alkali metals. In this group, the most reactive
metals are found at the bottom of the group. In terms of electronic
configurations within the atoms, all of the halogens have seven outer shell
electrons. For example, an atom of chlorine
has atomic number 17. It has two electrons in the first
shell, eight electrons in the second shell, and seven electrons in the outer
shell.
When halogens react, they gain one
more electron from another atom and form an ion with a negative one charge
state. Halogens ordinarily get eight outer
shell electrons and the same electronic configuration at a noble gas atom. In this example, a chlorine atom
has changed into a chloride ion. The negative ions of halogen atoms
are called halide ions. Halogen atoms gain one electron and
form a halide ion. A halide ion is a single halogen
atom that has gained one electron and a net negative charge state. Halide ions are found in metal
halide salts. Metal halide salts are easily
formed by direct combination of a halogen with a metal.
Potassium will react rapidly with
bromine to form potassium bromide. Group one A metal halides are
usually white salts, and they are often very soluble in water, giving solutions
containing anions and cations. Whilst solutions of halide ions are
usually colorless, solutions of the halogens in water are very different in
appearance. The halogens chlorine, bromine, and
iodine are all soluble in water to varying extents. The aqueous solutions have
noticeable colors.
Chlorine forms a very pale-green
solution in water. It is reasonably soluble, and it’s
used to kill bacteria in drinking water in this way. Bromine forms an orange solution,
whilst iodine forms a brown solution. It is often used as an antiseptic
in this form.
When the halogen solutions are
added to halide ion solutions, sometimes displacement reactions can occur. We will look at some of these in
the next section of this video.
A displacement reaction is a
chemical reaction in which one reactant displaces part of another reactant. A more reactive halogen can
displace a less reactive halogen from its halide salt during a displacement
reaction. An example of this is the reaction
of aqueous chlorine with aqueous bromide ions. The chlorine solution is a
pale-green color, and the potassium bromide solution is colorless.
Chlorine is more reactive than
bromine and bromine will be displaced. The green color caused by the
chlorine disappears as it displaces bromide ions from potassium bromide. An orange color appears as bromine
molecules are formed. Bromine is more reactive than
iodine. Bromine solution is orange. So, bromine can displace iodide
ions from a solution of potassium iodide. Iodine is formed in this
reaction. There is a color change in this
reaction. The orange color of the bromine
water disappears and is replaced by the brown color of the iodine solution. This reaction can be represented by
a symbol equation where one molecule of bromine reacts with two units of potassium
iodide. This reaction produces one molecule
of iodine and two units of potassium bromide.
Now that we have learned about the
physical and chemical properties of the halogens, let’s take a look at some
questions.
Based on their position in the
periodic table, which of the following elemental types can the majority of halogens
be described as? (A) Metals, (B) metalloids, (C)
inert gases, (D) nonmetals.
The halogens are located in group
seven A of the periodic table towards the right-hand side. This part of the periodic table is
where the majority of the nonmetals are located. In fact, the halogens are all
nonmetals. Answer (A) is therefore
incorrect. Answer (B) is also incorrect as
metalloids form the boundary between metals and nonmetals. Elements like silicon and germanium
form the boundary between conventional metal and nonmetal elements. Silicon and germanium are
metalloids.
At room temperature, fluorine and
chlorine are both found as gases. So, answer (C) could be
correct. However, fluorine and chlorine, as
with the other halogens, are both highly reactive elements. This means they are not inert,
which means unreactive. Answer (C) is therefore an
incorrect description of the halogens in general.
The best answer to the question
“Which of the following elemental types can the majority of halogens be described
as?” is nonmetals.
Which of the halogens is the most
reactive?
The halogens are located in group
seven A of the periodic table. From the top of the group to the
bottom of the group, they are composed of fluorine, chlorine, bromine, iodine, and
astatine. All of these elements are highly
reactive nonmetals, and they react with metals like iron. If fluorine comes into contact with
iron wool at room temperature, a fierce reaction starts immediately.
A similar reaction happens with
iodine and iron wool, but the iron wool must be heated very strongly before it will
react. This is a much slower reaction
compared with the reaction between fluorine and iron wool. In fact, we see the reactivity of
the halogens steadily decrease as we move down the group. So, fluorine is the most reactive
of the halogens, whilst astatine, although it’s extremely rare, could be predicted
to be the least reactive of the halogens.
To answer the question “Which of
the halogens is the most reactive?” fluorine is the correct answer.
Now, it’s time to review the key
points that we have learned in this video. The halogens are a group of
reactive nonmetals that form group seven A of the periodic table. Reactivity decreases down the
group. The colors of the halogens darken,
and the melting points increase moving down the group. Halogens ordinarily exist as
diatomic molecules. Halogen atoms contain seven
electrons in their outer shell. Halogen atoms gain one electron
when they react and become one minus charge state halide ions. A more reactive halogen can
displace a less reactive halide ion from its solution.
