Topic 6 - groups in the periodic table

explain why some elements can be classified as alkali metals (group 1), halogens (group 7) or noble gases (group 0), based on their position in the periodic table

• groups (columns) in the periodic table can be classified in specific groups

• eg. alkali metals, halogens or noble gases, because they have the same number of electrons in their outer shell (position in the periodic table determines this), therefore they have the same chemical properties

alkali metals

they are:

• soft

• have relatively low melting points

describe the reactions of lithium, sodium and potassium with water

• lithium, sodium and potassium in group one react vigorously with water to create an alkaline metal hydroxide and hydrogen

describe the pattern in reactivity of the alkali metals, lithium, sodium and potassium, with water; and use this pattern to predict the reactivity of other metals

lithium = fizzes steadily

sodium = melts into a ball then fizzes quickly

potassium = gives off sparks and hydrogen burns with a lilac flame

• as shown by the way the reactions with water become more vigorous down the group, the reactivity of all alkali metals increases down the group

explain this pattern in reactivity in terms of electronic configuration

• down the group - easier to lose electrons and form positive metal ions (cations) (these are formed when metals react)

• it is easier to lose electrons due to the increase in electron shells as you go down the group. this means there is more electron shielding and so decreases in attraction between the positively charged nucleus and the negatively charged outer shell electrons, which can then be lost more easily

recall the colours and physical states of chlorine, bromine and iodine at room temperature

• chlorine is a yellow-green gas

• bromine is a red-brown liquid

• iodine is a purple solid

describe the pattern in the physical properties of the halogens, and use this pattern to predict the physical properties of other halogens

• there is a trend in state from gas to liquid to solid down the group

• this is because the melting and boiling points increase as you go down the group

• from this, you can predict that any halogens above chlorine will be gases (their boiling points will be even lower), and any below iodine will be solids (their melting points will be even greater)

describe the chemical test for chlorine

• when damp litmus paper is put into chlorine gas the litmus paper is bleached and turns white

describe the reactions of the halogens with metals to form metal halides, and use this pattern to predict the reactions of other halogens

• they react with metals to form ionic compounds in which the halide ion carries a -1 charge eg. NaCl or MgBr₂ (as Mg has a +2 charge so you need two Br^- to cancel this out)

• reaction is less vigorous as you move down group 7, but they still all react to form metal halides

what do halogens form

the halogens, chlorine, bromine and iodine, form hydrogen halides which dissolve in water to form acidic solutions

• halogen + hydrogen → hydrogen halide (HCl, HBr, Hl)

• reaction becomes less vigorous down the group: chlorine reacts in sunlight, but bromine will react in a flame (higher temperature)

• hydrogen halides dissolve in water to produce acidic solutions - in solution the hydrogen halide will fully dissociate into H⁺ and halide^- ions

describe the relative reactivity of the halogens as shown by their displacement reactions with halide ions in aqueous solution, and use this pattern to predict the reactions of astatine

• a more reactive halogen can displace a less reactive in an aqueous solution of its salt

• eg. chlorine will displace bromine if we bubble the gas through a solution of potassium bromide:

  • chlorine + potassium bromide → potassium chloride + bromine

• chlorine will displace bromine and iodine

• bromine will displace iodine but not chlorine

• iodine can replace neither chlorine or iodine

• this happens because as you go down the group, the reactivity of halogens decreases

• the halogens react by gaining an electron in their outer shell, as you go down the group:

  • outer shell becomes further from the nucleus

  • electron shielding increases

  • attraction decreases between nucleus and outer electrons

  • electrons are gained less easily

  • halogens become less reactive

explain why these displacement reactions are redox reactions in terms of gain and loss of electrons, identifying which of these are oxidised and which are reduced

• OILRIG - oxidation is loss, reduction is gain (of electrons)

• more reactive halogen which displaces the less reactive one, forms a negative ion itself, therefore being reduced as it has gained electrons

• the less reactive halogen that is displaced is oxidised as it loses these electrons to go from a negative ion to an atom with 0 charge

• eg. for the equation: chlorine + potassium bromide → potassium chloride + bromide

  • the symbol equation without potassium is: Cl₂ + 2Br^- → 2Cl^- + Br₂

  • so for chlorine the half equation is: Cl₂ + 2e^- → 2Cl^-, chlorine has gained electrons, so it has been reduced

  • for bromine the half equation is: 2Br^- → Br₂ + 2e^-, bromine has lost electrons, so it has been oxidised

explain the relative reactivity of the halogens in terms of electronic configurations

• electronic configurations of the halogens:

  • fluorine: 2,7

  • chlorine: 2,8,7

• these show clearly the extra shell of electrons gained as you move down group 7, which lead to greater shielding and weaker attraction, leading to reduced reactivity

explain why the noble gases are chemically inert

• they have 8 electrons in their outer shell (except helium, which has 2- but this shell is still full)

• their electronic configurations demonstrate their full outer shells and this makes them unreactive because they are very stable

explain how the uses of noble gases depend on their inertness, low density and/or non-flammability

• low density = helium used in balloons and airships since it is much less dense than air, so balloons filled with it float upwards

• inertness & non-flammability = argon, krypton and xenon inside light bulbs, stops the filament burning away

• inertness = argon used as a shield gas during welding, it is also denser than air which keeps it away from the metal

describe the pattern in physical properties of noble gases

• the boiling points of the noble gases increase with increasing relative atomic mass (going down the group)

• the densities of the noble gases increase as you go down the group