alkali metals image

Higher and foundation tier

The alkali metals

alkali metal reactivity list When we think of metals we usually think of a shiny, hard material which is strong and fairly unreactive. However not all metals fit this description, some metals are soft and very reactive . Group 1 in the periodic table is called the alkali metals, it contains the metals lithium, sodium , potassium, rubidium, caesium and francium. These metals are all very reactive and must be handled with great care. Lithium at the top of group 1 is the least reactive and as we go down the group the metals react more and more violently. Since they are in group 1 they all have one electron in their last shell, so losing this electron will give them full shell (stable electronic structure). Losing one electron will mean they form ions with a +1 charge e.g. Li+, Na+, K+, Rb+ , Cs+.

Properties of alkali metals

The alkali metals are stored in jars filled with oil or paraffin to keep them away from contact with water or the air. They are soft and can all be cut with a knife, lithium is the most difficult to cut but the metals become softer as you go down the group. When cut the metal is silvery and shiny but if left exposed to air the freshly cut surface will react with oxygen and moisture in the air and quickly tarnish.

akali metals in jars

Physical properties of alkali metals

The physical properties of the alkali metals are also very different from what you expect of metals. The table below lists the melting and boiling points of the alkali metals as well as their densities.

alkali metal melting point/0C boiling point/0C density(g/cm3) at 250C
lithium 180 1347 0.53
sodium 98 883 0.97
potassium 63.5 774 0.86
rudidium 39 696 1.53
caesium 28 669 1.87

To compare alkali metals with some other everyday metals compare the values in the table below with the table above.

metal melting point/0C boiling point/0C density(g/cm3) at 250C
aluminium 660 2519 2.7
tungsten 3422 5555 19.3
iron 1535 2861 7.8
titanium 1668 3287 4.5

Identifying trends

By studying the information in the table it is fairly clear that alkali metals have unusually low melting and boiling points when compared to other metals, their densities are also low when compared to other metals. The density of water is 1 g/cm3, so any metal with a density less than this will float in water. However the alkali metals react violently with water, this is one of the reasons why they are stored in jars containing oil or paraffin, this ensure water is kept well away from these metals.

The information in the table for the alkali metals shows other trends or patterns:

The alkali metals, are all group 1 metals so they will have 1 electron 1 their outer electron energy level. Since the atoms get larger as you descend the group, this outer electron is further away from the attraction of the positive nucleus and so is more easily lost. This means that the reactivity of the alkali metals will increase as you decend the group.
Having just one electron in their outer shell means that when they react, the alkali metals will lose this electron and form ions with a 1+ charge.

Alkali metals- reaction with water

The alkali metals react very violently with water to form an alkaline solution and the flammable gas hydrogen is released. The image above show typical reactions of lithium, sodium and potassium with water. Enough heat is generated in these reactions that the metal may actually melt and form a ball of molten liquid metal that shoots across the surface of the water. The reaction of water with potssium is violent enough that the hydrogen gas will ignite by itself and burn with a mauve flame (lilac or pale purple). Sodium and lithium react less violently and the hydrogen produced here will not ignite and burn by itself but will have to be lit by a burning splint, however once lit the gas will continue to burn on its own. Sodium colours the hydrogen flame yellow and lithium will turn the burning hydrogen gas red.

alkali metals reacting with water

If a few drops of universal indicator is added to the water in the glass trough above then it will quickly turn purple once the alkali metals start reacting, showing that a strong alkali has been formed. Rubidium and caesium being at the bottom of group 1 are even more reactive, they are more dense than water and will sink but they react in a similar way, e.g.

alkali metal equation for reaction with water

Reactions of alkali metals with oxygen and the halogens

Similar violent reactions occur with oxygen and chlorine to form oxides, peroxides, superoxides and chlorides, the same trends are always seen, the reactions become more violent the lower the metal is in group 1. The reason for this is that all alkali metals have 1 electron in their outer (last electron shell), therefore if they can lose this one electron they will end up with a stable or full last shell. Since they are losing 1 electron, the atom will end up with 1 more positively charged proton in the nucleus than negatively charged electrons in its shells, so it will end up forming a positive ion with a charge of +1.

The image below shows the set-up needed to react the alkali metal sodium with chlorine gas. This reaction is very violent. A bright flash is seen as the sodium and chlorine react. The flask fills up with white "smoke", which is solid sodium chloride.

sodium reacting with chlorine gas

Reaction with halogens

The alkali metals react with the halogens (F,Cl,Br,I) to form colourless ionic solids. The reactions are very exothermic and can be violent. The reactions follow the expected trends, the more reactive the alkali metals and the more reactive the halogen the more violent and explosive the reaction. The reaction can be summarised as:

2M(s) + X2(g) → 2MX(s)

Where M= any of the alklai metals and X= F,Cl,Br,I

Reaction with oxygen

The products of the reaction of an alkali metal with oxygen depends on its reactivity. Normally when oxygen reacts it gains 2 electrons to form the oxide ion, O2-. This ion is particularly stable since it has full octet or 8 electrons in its outer shell. However in the reactions of oxygen with the alkali metals other ions of oxygen are formed. One of these ions, the peroxide ion has the formula O2 2- and it forms when sodium and potassium react with oxygen.

Lithium

If a piece of lithium is placed on a burning spoon and heated strongly in air it burns with a red flame to form lithium oxide:
lithium + oxygen → lithium oxide
4Li + 02 → 2Li2O

sodium

If a piece of sodium is placed on a burning spoon and heated strongly in air it burns with a yellow flame to form a mixture of two products, sodium oxide and sodium peroxide:
sodium + oxygen → sodium oxide
4Na + 02 → 2Na2O
sodium + oxygen → sodium peroxide
2Na + 02 → Na2O2

potassium

Potassium is the most reactive alkali metal studied in GCSE chemistry. The reactive alkali metals are able to form an oxide called a superoxide. The formula for the superoxide ion is O2-.
So when a piece of potassium metal is burned in air a lilac coloured flame is seen and a mixture of two oxides are produced, these are potassium peroxide and potassium superoxide.
potassium + oxygen → potassium peroxide
2K + 02 → K2O2
and for potassium superoxide we have:
potassium + oxygen → potassium superoxide
K + 02 → KO2

Key Points


Practice questions

Check your understanding - Questions on the alkali metals


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