2.2  Halogens

Halogens are non-metal elements placed in group 7 of the periodic table. They have 7 valence electrons, which means they only need one more electron to get a full shell and become stable. Therefore, they form -1 anions. The anions of halogens are called halides.

'Hal' from halogens means salt and 'gen' means to generate or produce in Greek. So halogen means 'salt producing'. They are so called because they react to form various types of salts by reacting with metals. Since they need to form -1 anions to become stable, they like reacting with the group 1 elements, alkali metals.

Halogens are very reactive elements because of their desperate need for an electron to become stable. Therefore, they do not exist in their pure elemental form. They are found combined to other elements and have to be extracted from those compounds found naturally like NaCl.

 

These elements exist as diatomic molecules in their elemental form. That is chlorine exists as Cl2, fluorine as Fl2, etc.

 

Their reactivity decreases down the group. Halogens form anions which means that for the atoms to be reactive the nucleus needs to have a strong attraction with the valence shell (sort of opposite for cations). But as you go down the group the number of electrons and shells between the valence electrons increases. Therefore, the attraction decreases and the halogens further down the group are not as desperate as the halogens higher up in the group to attract an electron. As a result, reactivity decreases down the group.  

Since halogens are non-metals they can form ionic compounds or covalent compounds. They react with metals to form ionic compounds and they react with non-metals to form covalent compounds.

 

Their diatomic molecules (Cl2,Fl2,etc) are covalent compounds because the bond formed is between two non-metal atoms. The melting and boiling points of halogens increase down the group. But shouldn't it decrease since as the atom size increases the attractive force between the nucleus and shared electrons decreases?

 

Well, this is correct if  the covalent bonds are what's been broken when melting.  You MUST always remember that melting and boiling points are physical properties concerning physical changes and not chemical changes.

 

Therefore, when melting or boiling halogens, the covalent bonds DO NOT break; it's the intermolecular forces that are broken. The bond between the atoms are not broken but rather the weaker bonds between the molecules are broken. Refer to the bonding chapter to quickly review intermolecular forces. 

For intermolecular forces larger the size of the molecule or atoms, greater their strength. We will review the reason behind this in detail later on. But for now know that as a general rule, strength of intermolecular forces (not ionic or covalent bonding!) increases with the size of molecule.

Reaction with Hydrogen

Halogens react with hydrogen to form hydrogen halides. The ions formed by halogens are called halides (F-, Cl-,Br-, I-,etc). Hydrogen halides are poisonous, acidic and colorless gases at room temperature.

 

Lets look at the chemical equation for hydrogen reacting with chlorine shown below.

The first equation is unbalanced and the second one is balanced. As you can see 1 diatomic molecule of hydrogen reacts with 1 diatomic molecule of chlorine to form 2 molecules of hydrogen chloride, which is an acidic gas. 

Hydrogen halides are very soluble in water; therefore, if you expose this gas to water it will dissolve in it to form an acidic aqueous solution. In this case hydrogen chloride will dissolve to form hydrochloric acid, which is very strong and very dangerous if not handled properly.

The other hydrogen halides are hydrogen fluoride, hydrogen bromide and hydrogen iodide. Their bond strength decreases down the group. So HF has the strongest and shortest bonds: therefore, it is very difficult to break apart. HF>HCl>HBr>HI.

Reaction with Alkali Metals

Halogens react with alkali metals to form salts which are ionic compounds. They are also called metal halides. These crystalline salts produced are white in color.  These reactions can be explosive and violent.

 

It is important to note that all the metal halides except for lithium halide is soluble in water. Why?

Fluoride has the smallest size from all the halogens and is the most reactive. Therefore, its bonds are very strong. As a result the lattice structure of lithium fluroide, where lithium also has the smallest size from the alkali metals, is very sturdy and doesn't break away even when exposed to water. As a result, it is insoluble.

Lets look at an example reaction. If you add a piece of sodium to chlorine gas it will burn vigorously with a yellow flame to form sodium chloride. The unbalanced and balanced equations are given below.

Metal halides have a variety of uses. NaCl commontable salt is an essential preservative used by humans for thousands of years. Metals halides are also used in lamps and bulbs.

Note that halogens also react with other metals like silver, alkaline earth metals( group 2 element), etc.

Displacement Reactions

We talked earlier about how the reactivity of halogens decreases down the group. This means that halogens higher up the reactivity ladder will react with halides of halogens that further down the ladder. This type of reaction where a more reactive element takes the place of an element with relatively lower reactivity is known as a displacement reaction. For example if you react KBr (potassium bromide) with Cl, which is more reactive than Br, the bromide ion will be displaced by the chloride ion.

Lets look at the balanced chemical equation for this reaction.

Since KBr is a metal halide it is colorless. But when you bubble Cl2 into a colorless solution containing KBr the solution turns brown. This change in color can be used as proof for the occurrence of a displacement reaction.

So what happens is that since Cl is more reactive than Br, it will rip the Br ion from the ionic molecule and itself combine with K instead to form KCl solution, which is also colorless. The brown color is from the bromide ions that have been pulled apart from their original compound (KBr) and have now dissolved in the solution.

You should also be able to notice that the halogens are basically fighting over potassium. K+ ion is not doing anything but just watching and enjoying the fight between chlorine and bromine. These types of ions in a reaction are called spectator ions. 

 

Depending on the halide that has been displaced the color of the solution will change. If chlorine is displaced - the color will be green, if Br is displaced - color will be brown, if iodine is replaced - color will be purple/violet.

If you add bromine to KCl nothing will happen since Br is too weak to take the place of chlorine.

 

Fluoride cannot be displaced by the other halogens since it is the king in terms of reactivity.

Reactions such as these are also known as redox reactions because there is a transaction of electrons happening here. In the above case, bromine is loosing electrons while chlorine is gaining them. These reduction and oxidation reactions (shortened to redox reactions) will be discussed in detail later on.

End

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