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GROUP GROUP 17 ELEMENTS17 ELEMENTS
1.Group 17 Elements
2. Physical Properties of Halogens
3. Reactivity of Group 17 Elements
4. Chemical Properties of Group 17 Elements
5. Safety Precautions in Handling Group 17 Elements
6. Conclusion
7. SPM Checkpoint
8. Check Your Progress
The elements in Group17 are also known as the halogens.
The elements in Group 17 are flourine (F), chlorine (Cl), bromine (Br), iodine (I) and astatine (At).
Halogens are very reactive elements and most of them exist naturally as halide salts.
The halogen molecules exists as diatomic molecules: F2, Cl2, Br2, I2, and At2
All Group 17 elements are non-metals. Hence, they are insulators of heat and electricity.
The atomic radius increase down the group. The reason is that as the number of filled electron shell increase down the group, the distance between the outermost electron shell and the nucleus increase.
The density increases down the group. This is due to the increase in relative molecular mass.
Halogens have low boiling points. The forces of attraction between the molecules are weak.
The melting points and boiling points of the halogens increase down the group. This is because the molecular size increases down the group.
As the size increases, the van der Waal`s forces of attraction between the molecules become stronger. More heat is required to overcome the attractive forces and therefore the melting points and boiling points increase. The first two elements (fluorine and chlorine) are gases at room temperature. Bromine is a liquid whereas iodine and astatine are solids at room temperature.
The colour of the halogen becomes darker down the group. Fluorine is a colourless gas; chlorine is a yellowish green gas; bromine is a dark brown liquid and iodine is a black solid.
All halogens have high electro-negativities. They are electronegative non-metals. Electronegativity is a measurement of the element to attract electrons.
The electronegativity decrease down the group from chlorine to iodine. As the atomic radius becomes larger down the group, the forces of attraction between the nucleus and electrons become weaker and thus electronegativity decreases.
Some Physical Properties of Three Halogens
Halogen Proton number
Electron arrangement
Atomic radius (nm)
Melting point (ºC)
Chlorine 17 2.8.7 0.099 -101
Bromine 36 2.8.18.7 0.114 -7
Iodine 53 2.8.18.18.7 0.133 114
Halogen Boiling point (ºC)
Physical
state at room temperature
Electro-
negativity
Colour
Chlorine -35 Gas 3.0 Yellowish green gas
Bromine 58 Liquid 2.8 Brown liquid
Iodine 183 Solid 2.5 Black solid
All Group 17 elements are very reactive. However, the reactivity decreases down the group.
The chemical reactions of Group 17 elements involve the formation of negative ions of charge –1: Cl -, Br -, I-
The elements in Group 17 have 7 valence electrons each. During a chemical reaction, the atom of a Group 17 element will gain a valence electron to form univalent negative ion to attain the stable octet in its electron arrangement.
X2 + 2e- 2X- ,where by X = Cl, Br, or I
Cl2 + 2e- 2Cl-
2.8.7 2.8.8
Br2 + 2e- 2Br-
2.8.18 2.8.18.8
I2 + 2e- 2I-
2.8.18.18.7 2.8.18.8
The reactivity of Group 17 elements depends on its ability to gain an electron.
The atomic radius of Group 17 elements increases down the group. Thus the forces of attraction between the nucleus and the electrons become weaker.
As the result, the halogen lower in the group has a lower tendency to attract an electron to form negative ion. Therefore, the reactivity of halogens decreases down the group.
Chemical properties of Group 17 elements
The elements in Group 17 have the same chemical properties because each have seven valence electrons
Group 17 elements react with:(i) water to produce acids.(ii) metals such as iron to produce metal halides.(iii) sodium hydroxide to produce salts and water.
The Reaction of Chlorine, Bromine and Iodine with
Water
We notice that chlorine, bromine and iodine have similar chemical properties but differ in reactivity. Halogens react with water to form two acids. For example,
Cl2(aq) + H2O(l) HCl(aq) + HOCl(aq) Chlorine Water Hydrochloric Hypochlorus
acid acid
Result Halogen Observation
Solubility Effect on litmus paper
Chlorine Dissolved quickly in the water to form a light yelllowish solution.
The solution first turned the blue litmus paper red. The red colour of the litmus took a longer time to be turned white.
Bromine Dissolved slowly in water to form a brown solution.
The solution first turned the blue litmus paper red. The red colour of the litmus took a longer time to be turned white.
Halogen Observation
Solubility Effect on litmus paper
Iodine A little of the iodine crystals dissolved slightly in water to form a pale brown solution.
The solution turned the litmus paper from blue to red. The red litmus paper did not change colour.
The Reaction of Halogens with Aqueous Sodium hydroxide
Solution Halogens also react with sodium hydroxide solution,
NaOH to form sodium halate(I) and water. For example,
I2 (s) + 2NaOH (aq) NaI (aq) + NaOI(aq)+H2O(l) Iodine Sodium Sodium Sodium Water
hydroxide iodide iodate(I)
Result
Halogen Observation
Chlorine The greenish chlorine gas dissolved quickly in NaOH solution to form a colourless solution.
Bromine The brownish bromine water dissolved steadly in NaOH solution to form a colourless solution.
Iodine The dark iodine crystal dissolved slowly in NaOH solution to form a colourless solution.
The Reactions of Halogens with Iron Wool
Halogens in gaseous state react with hot iron to form a brown solid, iron (III) halides. For example,
2Fe(s) + 3Br2 (g) 2FeBr3(s) Iron Bromine gas Iron (III) bromide
In general,
2Fe(s) + 3Br2 (g) 2FeBr3(s) Where X is a halogen.
Halogens Observations
Chlorine Hot iron wool glowed brightly when chlorine gas passed over it. A brown solid was formed.
Bromine Hot iron wool glowed moderately bright when bromine gas passed over it. A brown solid was formed.
Iodine Hot iron wool glowed dimly when iodine vapour passed over it. A brown solid was formed.
Safety Precautions in Handling Group 17 Elements
Fluorine is a very dangerously reactive substance whereas astatine is radioactive.
Fluorine gas, chlorine gas, bromine vapour and iodine vapour are poisonous.
Must handle them in a fume chamber. Besides that, must also wear safety goggles and gloves when handling these halogens.
Change when descending the group:
•Melting and boiling point increase
•Electronegativity decreases
•Reactivity decreases
Chemical properties:
•Dissolve in water to produce acidic solutions
•React with NaOH to produce sodium halides, sodium halate(I) and water.
•React with Fe to produce iron (III) halides
All elements in group 17 have seven valence electrons:
•F: 2.7
•Cl : 2.8.18.7
•I : 2.8.18.18.7
Physical properties:
•Low melting and boiling points
• non-conductor of heat or electricity
•Exist as diatomic molecules
When chlorine gas is passed through heated iron wool
I iron (II) chloride is formed.
II the iron wool glows brightly.
III the product is brown in colour.
IV hydrogen chloride gas is produced
A II and III C I,II and III
B II and IV D II,III and IV
Chlorine is a reactive halogen and reacts with iron wool to form brown iron(III) chloride. During the reaction, the iron wool glows brightly.
2Fe(s) + 3Cl2 (g) 2FeCl3 (s)
Answer :
Try This At Home… Explain why an aqueous solution of chlorine is (a)
acidic (b) has blenching properties. Explain why the reactivity of group 17 elements
decreases down the group. Aqueous bromine and iodine solutions are both
brown.
(a)How do you differentiate between the two solutions?
(b)Carry out an experiment to show that bromine is more reactive than iodine.
