C1a.5.7 Group 7 – the Halogens By the end of this lesson you should be able to: Recall some of the properties of the halogens Interpret data describing the properties of chlorine and iodine and explain their uses Properties of the Group 0 elements The Group 7 elements have the following properties: ● Non-metals – low melting and boiling points, poor conductors of heat and electricity ● Two are gases, one is a liquid and two are solids (at room temperature) ● Very reactive – hence the name ‘Noble’ but some do form compounds under extreme conditions ● Exist as molecules (diatomic). The chemical formulae for halogen molecules has a small 2 after the symbol, e.g. chlorine molecules, Cl2 Their reactivity decreases as you go down the group. The halogens that are gases at room temperature are coloured. Those that are solids or liquids form coloured vapours when heated: Symbol and Name State and colour at room temperature, colour of vapour when heated F Fluorine Cl Chlorine Br Bromine I Iodine At Astatine Trend in colour* Trend in state** pale yellow gas green gas dark red liquid, brown vapour dark crumbly solid, purple vapour black solid, dark vapour * The halogens become darker as you go down the group. ** The halogens change from gas to liquid to solid as you go down the group. The halogens trend in state is due to their melting points and boiling points – these both increase as you go down the group. Uses of the halogens All of the halogens are toxic and must be treated very carefully. However, this means that they can be used for killing germs and bacteria, e.g. chlorine is used to kill germs in drinking water during its treatment before being piped into homes. Chlorine is also used to kill germs in water in swimming pools. Iodine is used in antiseptic washes and to treat wounds. Questions 1. Write the chemical symbols for fluorine, chlorine, bromine and iodine 2. Astatine is at the bottom of group 7. Suggest its colour and reasons for your answer. 3. (a) What is the trend in the boiling points of the halogens? (b) What would you expect the physical state of astatine to be at room temperature? Explain your answer. 4. What does toxic mean? 5. What is a diatomic molecule? 6.The following table shows the melting and boiling points of each halogen: Halogen Melting Boiling Point (°C) Point (°C) Fluorine Chlorine Bromine Iodine Astatine 327 445 539 660 848 358 512 605 730 883 Copy the table and add an extra column ‘State at 120°C’ Complete the column for each halogen. 7. Why would it be dangerous to put too much chlorine into the water in a swimming pool? Homework 1. The halogens are in group __________ of the periodic table. They are __________ conductors of __________ and electricity. The halogens are coloured and they exist as pairs of __________ called __________ molecules. The boiling points of the halogens __________ as you go down the group, and __________ is the only liquid non-metal. __________ is used to sterilise __________ water and the water in __________ pools. atoms bromine chlorine diatomic drinking heat increase poor swimming 7 2. At what temperature will all the halogens be solid? 3. What are the trends in colour, state and boiling point of the halogens as the atomic number increases? 4. Table salt is sodium chloride, NaCl. It usually contains added potassium iodide or potassium iodate. What are the chemical formulae for potassium iodide and potassium iodate?