Group I, Group VII and Group O Elements

Group I Elements aka The Alkali Metals

– Some of their physical properties:

  • Shiny, soft solids which can be cut by a blade
  • Low melting points. Melting points decrease down the group
  • Low densities. Densities increase down the group
  • Good conductors of heat and electricity

– Some of their chemical properties:

  • Form +1 ions as they lose 1 valence electron to obtain the “stable” octet state
  • Very reactive (as they only need to lose one valence electron)
    Reactivity of Group I elements increases down the group. This is because as atomic size increases down the group, the lone valence electron is further away from the nucleus and thus, less attracted by it. Hence, it is easier to lose that electron and form ions.
  • React with oxygen easily (even violently) and hence are kept under oil eg. $$4Li \, (s) + O_{2} \, (g) \rightarrow 2Li_{2}O \, (s)$$ and $$K \, (s) + O_{2} \, (g) \rightarrow KO_{2} \, (s)$$ Lithium, sodium and potassium burn with a red, orange and lilac flame respectively with oxygen.
  • React with water vigourously to form alkaline hydroxides (and hence, Group I elements are also known as Alkali Metals) eg. $$2Na \, (s) + 2H_{2}O \, (l) \rightarrow 2NaOH \, (aq) + H_{2} \, (g)$$ This reaction with water gives out a lot of heat (ie. exothermic reactions. See Chemical Energy), which may ignite the formed hydrogen gas and cause a fire or explosion.
  • React with Group VII elements vigourously to form ionic salts eg. $$2K \, (s) + Cl_{2} \, (g) \rightarrow 2KCl \, (s)$$
  • In summary, all compounds of Group I Elements are 1) colourless, 2) soluble in water and 3) stable to heating (as these alkali metals form strong bonds which do not break down easily on heating).


Group VII Elements aka The Halogens

– Exist as diatomic molecules eg. $F_{2}$, $Cl_{2}$

– Are poisonous

– Some of their physical properties:

  • Low melting and boiling points. These increase down the group
  • Low densities. Densities increase down the group
  • Poor conductors of heat and electricity
  • Are coloured atoms. Colours intensify and darken down the group
    eg. At room temperatures, fluorine gas is pale yellow, chlorine gas is yellowish green, bromine liquid is dark red/ brown, iodine solid is bluish black

– Some of their chemical properties:

  • Form -1 ions as they gain 1 electron to reach the octet state
  • Very reactive
    Reactivity of Group VII elements decreases down the group. This is because as atomic size increases down the group, it becomes harder for the nucleus to attract an additional electron to form ions.
  • Can have displacement reactions, where a more reactive halogen will displace a less reactive halogen from its own salt in its solution. In general, what one will see during displacement reactions will be 1) the coloured halogen disappearing as it displaces the less reactive halogen salt and 2) the colour of the solution being tainted by the colour of the displaced halogen. eg. $$Cl_{2} \, (g) + 2NaBr \, (aq) \rightarrow 2NaCl \, (aq) + Br_{2} \, (aq)$$ In the above reaction, the pale yellowish green chlorine gas will disappear as it is bubbled through the colourless sodium bromide solution. The solution will be coloured reddish brown by the bromine ions which had been displaced. If there are no colour change, no displacement reaction has occurred. A displacement reaction is an example of a redox reaction (See Oxidation and Reduction).


Group O Elements aka The Noble Gases

  • Chemically stable and do not usually react with other elements ie. they are inert
  • With the exception of helium, all the noble gases have 8 electrons in their outer most shell
  • Exist as monatomic atoms
  • Are colourless gases with low melting and boiling points
  • Low densities which increases down the group
  • Widely used due to their chemical stability eg. Helium is used as a fuel to power lightweight aircraft, neon is used in television tubes, advertising lightings, argon is used in arc welding etc.


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