ZON PENA

Introduction

• Group 17 elements are typical non-metals and also known as halogens.
• As shown in the diagram on the right, elements in this group (pink colour) are fluorine. chlorine, bromine, iodine and astatine.
• In nature, all halogens exist as diatomic molecules. They are written as F₂, Cl₂, Br₂, I₂and As₂.
• The word 'Halogens' means 'salt formers' because they can form salt easily with metals.
• For example, chlorine reacts with sodium forms sodium chloride. Sodium chloride is one of the most abundant salt found in the nature
• Most of the halogens exist in the nature as halide salts.
• Halide is the name given to the ion of halogens. Table below shows the corresponding halide of the halogen.

Halogen	Halide
Fluorine	Fluoride
Chlorine	CHloride
Bromine	Bromide
Iodine	Iodide

Physical Properties

• All group 17 elements are non-metals. Therefore they are heat and electricity insulator.
• Table below shows the electron arrangement and physical properties of group 17 elements.

Name	Proton Number	Electron arrangement	colour	Melting point	Boiling point
Fluorine	9	2.7	pale yellow gas	-220oC	-188oC 85K
Chlorine	17	2.8.7	Yellowish green gas	-102oC	-34oC
Bromine	35	2.8.18.7	dark red liquid, brown vapour	-7oC	59oC
Iodine	53	2.8.18.18.7	black solid, purple vapour	114oC	184oC
Astatine	85	2.8.18.32.18.7	black solid, dark vapour	302oC	380oC

Size of Atom and Density

• As shown in the diagram above, the atomic size of group 17 elements increases down the group.
• This is due to the increase of number of electron shell down the group.
• The density of group 17 elements is also increases down the group.
• This is because the rate of increment of the atomic mass is higher than the rate of increment of the volume.
• When solid of group 17 elements (Iodine and Astatine)they are brittle and crumbly.

Colour

• All halogens are coloured non-metallic elements.
• The colour of the halogen gets darker down the group.
• Diagram above shows the picture of the halogens.
• More high quality pictures of the elements can be found in [www.theodoregray.com www.theodoregray.com]. Click on the links provided below to access to the particular images.

Pictures of the Halogen: theodoregray.com: Fluorine theodoregray.com-wooden periodic table: Fluorine theodoregray.com: Chlorine theodoregray.com-wooden periodic table: Chlorine theodoregray.com: Bromine theodoregray.com-wooden periodic table: Bromine theodoregray.com: Iodine theodoregray.com-wooden periodic table: Iodine

Melting Point and Boiling Point

• As shown in the graph above, the melting points and boiling increase steadily down the group.
• The physical state t room temperature also change from gas to liquid and then to solid.
• This is because the intermolecular attractive force (van der Waals force) increase with increasing size of atom or molecule.

Chemical Properties

• Group 17 elements are very reactive non-metals.
• The atoms all have 7 valence electrons, makes them have very similar chemical properties.
• During chemical reaction, the atom gain one electron to form an ion with charge of -1.

• For example (as shown in the diagram above), the electron arrangement of fluorine is 2.7. In a chemical reaction, it will receive 1 electron from other atom and form fluoride ion, F^-. The diagram below shows the formation of chloride (Cl^-)ion when it receives 1 electron from other atom.
• The bromine and iodine will react in the same way to form bromide and iodide ions.
  

• In nature, the elements all exist as diatomic molecules, X₂ where X represents the halogen atom. For example F₂, Cl₂, Br₂, I₂ and At₂.
• The reactivity of group 1 decreases down the group.
• All group 17 elements are poisonous.
• Astatine is very radioactive.

• In the SPM chemistry syllabus, we discuss 3 reaction of group 17 elements:

1. Reaction with water.
2. Reaction with metal.
3. reaction with alkali solution, for example sodium hydroxide.

• The discussion of this 3 reaction are as follow.

React with water

Reaction of Chlorine with Water

---

Observation:

• As shown in the diagram above, chlorine gas dissolves in water to form a pale yellow solution.
• When the solution is tested with blue litmus paper, the blue litmus paper turns red before it is bleached.
• This shown that the solution is acidic and contain bleaching agent.

Discussion:

• Chlorine gas dissolves in water produces hydrochloric acid (HCl) and hypochlorous(I) acid (HOCl).
• Hypochlorous(I) acid is a strong bleaching agent. It decolourises the colour of litmus paper.
• The equation of the reaction is shown below.

Chlorine + Water Hydrochloric acid + Hypochlorus acid

Reaction of Bromine with Water

---

Observation:

• Bromine liquid dissolves slowly in water to form a yellowish-brown solution.
• When the solution is tested with blue litmus paper, the blue litmus paper turns red and then bleached slowly.

Discussion:

• Bromine liquid dissolves in water produces hydrobromic acid (HBr) and hypobromous(I) acid (HOBr).
• Hypobromous(I) acid is a weak bleaching agent.
• The equation of the reaction is shown below.

Bromine + Water Hydrobromic acid + Hypobromous(I) acid

Reaction of Iodine with Water

---

Observation:

• Only a little iodine dissolves in water to form a yellowish solution.
• When the solution is tested with blue litmus paper, the blue litmus paper turns red but it is not bleached .

Discussion:

• Iodine solid slightly dissolves in water produces hydroiodic acid (HI) and hypoiodous(I) acid (HOI).
• Hypoiodous(I) acid has very weak bleaching characteristic..
• The equation of the reaction is shown below.

Iodine + Water Hydroiodic acid + Hypoiodous(I) acid

Chlorine, bromine and iodine are soluble in water to form an acidic solution. The solubility decreases down the group. Aqueous chlorine and bromine are bleaching agent. Aqueous iodine does not act as bleaching agent.

React with iron

• Halogen is a reactive non-metal. It form salt when react with metal.
• The experiments below show the reaction of chlorine, bromine and iodine with iron.
• Iron wool rather than iron piece is used to increase the rate of reaction.

Reaction of Chlorine with Iron

---

Observation The iron wool burns vogorously with bright flame, forming a brown solid after reaction. Discussion The reaction of potassium with concentrated hydrochloric acid produces chlorine gas. The equation of the reaction is shown below. The chlorine gas flow into the combustion tube and react with iron wool when heated. Chlorine react with iron to form brown iron(III) chloride. Chlorine gas is a poisonous. Excess chlorine gas is absorbed by the soda lime (sodium hydroxide) so that it does not escape to the surrounding. Equation The equation of the reaction is as below: Chlorine + Iron iron(III) chloride

Reaction of Bromine with Iron

---

Observation The iron wool glows brightly but less vigorously. A brown solid is formed. Discussion The bromine liquid evaporates to form bromine vapour when it is warmed. The bromine vapour flow into the combustion tube and react with iron wool when heated. Bromine react with iron to form brown iron(III) bromide. The reaction is less reactive compare with chlorine. Bromine gas is a poisonous. Excess bromine gas is absorbed by the soda lime (sodium hydroxide) so that it does not escape to the surrounding. Equation Bromine+ Iron iron(III) bromide

Reaction of Iodine with Iron

---

Observation The iron wool glows slowly with dim light. A brown solid is formed after reaction. Discussion The iodine solid sublimates to form purplish iodine vapour when it is warmed. The iodine vapour flow into the combustion tube and react with iron wool when heated. Iodine react with iron to form brown iron(III) iodide. The reaction is slow compare with chlorine and bromine. Excess iodine vapour is absorbed by the soda lime (sodium hydroxide) so that it does not escape to the surrounding. Equation Iodine+ Iron iron(III) Iodide

React with Alkali Solution

Reaction of Chlorine with Sodium Hydroxide

---

Observation:

• The greenish chlorine gas dissolves quickly in sodium hydroxide, forming a colourless solution.

Discussion:

• Chlorine gas react with sodium hydroxide to form salt of sodium chloride, sodium chlorate(I) and water.
• The equation of the reaction is shown below.

Chlorine + Sodium Hydroxide Sodium Chloride + Sodium Chlorate(I) + Water.

Reaction of Bromine with Sodium Hydroxide

---

Observation:

• The reddish-brown liquid bromine dissolves in sodium hydroxide, forming a colourless solution.

Discussion:

• Bromine liquid react with sodium hydroxide to form salt of sodium bromide, sodium bromate(I) and water.

The reaction is less reactive compare with chlorine.

• The equation of the reaction is shown below.

Bromine+ Sodium Hydroxide Sodium Bromide + Sodium Bromate(I) + Water.

Reaction of Iodine with Sodium Hydroxide

---

Observation:

• The black iodine crystals dissolve slowly in sodium hydroxide, forming a colourless solution.

Discussion:

• The reddish-brown liquid bromine dissolves in sodium hydroxide, forming a colourless solution.

Discussion:

• Solid iodine react slowly with sodium hydroxide to form salt of sodium iodide, sodium iodate(I) and water.

The reaction is leaet reactive among the three.

• The equation of the reaction is shown below.

Iodine + Sodium Hydroxide Sodium Iodide + Sodium Iodate(I) + Water.

Explaining the Reactivity Trend of the Group 17 Halogen

• The three experiments above shows that the reactiveness of halogens decreases down the group.
• This can be explained as below:

1. When a halogen atom reacts, it gains an electron to form a singly negative charged ion.
2. As we go down the group from F => Cl => Br => I, the size of the atom increases due to an extra filled electron shell.
3. The valence electrons are further and further from the nucleus, the attraction force between the electrons and the nucleus become weaker and weaker.
4. Therefore the ability of the atom to attract electron to fill the outermost shell reduces., which means the reactiveness of the atom reduces.

Safety Precaution

• Fluorine, chlorine and bromine gases are poisonous.
• Therefore all the experiments involving these gases should be carried out in a fume chamber.
• The experiments involve fluorine are nor done in school.
• This is because fluorine is so reactive that it will react with most of the substance it comes into contact with.
• It is very difficult to conduct experiments involving fluorine.