noble gases in Group 0. Individual groups show certain ‘group characteristics’. These properties follow a trend in particular groups.
Questions 3.1 What is the name of the most reactive non-metal? 3.2 What is the similarity in the electron arrangement in the noble gases? 3.3 How many elements are there in Period 1? 3.4 Where, in the Periodic Table, will the largest atom be found? 3.5 Sort the following properties into those characteristic of a metal, and those typical of a non-metal. is an insulator can be beaten into sheets gives a ringing sound when hit conducts heat has a dull surface conducts electricity
3.2 Trends in groups Group I – the alkali metals
to explosive in the case of caesium. You might predict that francium, at the bottom of Group I, would be the most reactive of all the metals. However, it is highly radioactive and very rare because it decays with a halflife of 5 minutes. It has been estimated that there are only 17 atoms of francium in existence on Earth at any one moment in time. The physical properties of the alkali metals also change as we go down the group. The melting points become lower while the density of the metals increases. The alkali metals (Group I) are the most reactive metals that occur. They are known as the alkali metals because they react vigorously with water to produce hydrogen and an alkaline solution.
Group VII – the halogens The most reactive non-metals are the halogens in Group VII of the table (Figure 3.8). In contrast with Group I, here reactivity decreases down the group. For example, fluorine is a dangerously reactive, pale yellow gas at room temperature. There is a steady increase in melting points and boiling points as we go
The metals in Group I are often called the alkali metals. They are soft solids with relatively low melting points and low densities (Figure 3.7). They are highly reactive and are stored in oil to prevent them reacting with the oxygen and water vapour in the air. When freshly cut with a knife, all these metals have a lightgrey, silvery surface, which quickly tarnishes (becomes dull). Reactivity increases as we go down the group. All Group I metals react with water to form hydrogen and an alkaline solution of the metal hydroxide. The reactions range from vigorous in the case of lithium
Chlorine (Cll2) • dense pale-green gas • smelly and poisonous • occurs as chlorides, especially sodium chloride in the sea • relative atomic mass 35.5
Bromine (Br2) • deep-red liquid with red-brown vapour • smelly and poisonous • occurs as bromides, especially magnesium bromide in the sea • relative atomic mass 80
Iodine (I2) • grey solid with purple vapour • smelly and poisonous • occurs as iodides and iodates in some rocks and in seaweed • relative atomic mass 127 Figure 3.7 The alkali metals are all soft and can be cut with a knife. This is a sample of lithium.
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Cambridge IGCSE Chemistry
Figure 3.8
The general properties of some of the halogens (Group VII).
Original material © Cambridge University Press 2014