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 of the periodic table they all have one electron in their last shell, so losing this electron will give them a full outer electron shell (stable electronic structure). Losing their outer electron will mean they form ions with a +1 charge e.g. Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺.

The Properties of alkali metals

Physical properties of the alkali metals

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

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

Identifying trends

By studying the information in the table it is fairly clear that the 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 ensures that water is kept well away from these reactive metals.

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

• The melting and boiling point of the alkali metals decreases as you descend the group.
• The densities of the alkali metals increases as you descend the group, though there is a slight anomaly with potassium; whose density does not fit the general pattern. But you can see that lithium, sodium and potassium all have densities of less than 1 g/cm³; the density of water, so these 3 metals will all float on water.

Alkali metals- reaction with water

The alkali metals react very violently with water to form an alkaline solution and the flammable gas hydrogen is also released. The image below shows the typical reactions of lithium, sodium and potassium with water. Enough heat is generated in these reactions with the water 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 potassium 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 brick red.

The image below illustrates the colours of the burning hydrogen flame above the three metals lithium, sodium and potassium. The alkali metals all react violently with water to form alkaline solutions (solutions of metal hydroxides). The reaction of potassium with water is so violent that the hydrogen gas released spontaneously catches fire to give a lilac coloured flame above the metal. Sodium and lithium also release hydrogen but this needs to be lit with a burning splint. The burning hydrogen above sodium is yellow and red with lithium.

If a few drops of universal indicator are 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 denser than water and will sink but they react in a similar but much more violent way. All the alkali metals react with water in a similar way, they all form solutions of metal hydroxides (alkalis) and release the explosive gas hydrogen, this is outlined in the equations below:

alkali metal_(s)   +   water_(l)   →   metal hydroxide_(aq)   +   hydrogen_(g)

e.g. lithium

lithium_(s)   +   water_(l)   →   lithium hydroxide_(aq)   +   hydrogen_(g)

2Li_(s)   +  2H₂O_(l)   →   2LiOH_(aq)   +   H_2(g)

e.g. sodium

sodium_(s)   +   water_(l)   →   sodium hydroxide_(aq)   +   hydrogen_(g)

2Na_(s)   +  2H₂O_(l)   →   2NaOH_(aq)   +   H_2(g)

e.g. potassium

potassium_(s)   +   water_(l)   →   potassium hydroxide_(aq)   +   hydrogen_(g)

2K_(s)   +  2H₂O_(l)   →   2KOH_(aq)   +   H_2(g)

Alkali metals get their name because they react with water to form alkaline solutions, lithium, sodium and potassium hydroxides are all strong alkaline solutions.

Reactions of alkali metals with oxygen and the halogens

Similar violent reactions occur between the alkali metals and oxygen and chlorine gases 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 and as mentioned above less energy is required to remove this outer shell electron the further down group 1 the metal is found. Since the metals 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 a possible set-up for the reaction the alkali metal sodium with chlorine gas. A small piece of sodium is placed ON a mound of sand on the base of the flask. The flask is filled with dry chlorine gas. To start the reaction a drop of water is dropped onto the sodium metal from the pipette. A very violent reaction occurs between the sodium and chlorine; a bright flash is seen as the sodium and chlorine react. The flask fills up with white "smoke"; which is solid sodium chloride.

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) + X_2(g) → 2MX_(s)

So for example when sodium reacts with chlorine to form sodium chloride we have:

sodium_(s)   +   chlorine_(l)   →   sodium chloride_(s)  

2Na_(s)   +  Cl_2(g)   →   2NaCl_(s)

Reaction with oxygen

The products of the reaction of an alkali metal with oxygen depend on its reactivity. Normally when oxygen reacts it gains 2 electrons to form the oxide ion, O^2-. This ion is particularly stable since the oxide ion 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 also formed. One of these ions, the peroxide ion has the formula O₂ ^2- and it forms when sodium and potassium react with oxygen. Reactive alkali metals such as potassium, rubidium and caesium are also able to form an oxide called a superoxide. The superoxide ion has the formula 0₂^-

Lithium

lithium + oxygen → lithium oxide
4Li + 0₂ → 2Li₂O

sodium

sodium + oxygen → sodium oxide
4Na + 0₂ → 2Na₂O
sodium + oxygen → sodium peroxide
2Na + 0₂ → Na₂O₂

potassium

potassium + oxygen → potassium peroxide
2K + 0₂ → K₂O₂

and for potassium superoxide we have:

potassium + oxygen → potassium superoxide
K + 0₂ → KO₂

Key Points

• Alkali metals are soft and reactive metals found in group 1 of the periodic table. They are so soft that they are easily cut with a sharp knife. Lithium is the most difficult to cut but the metals get softer as you descend the group.
• The alkali metals all have one electron in their outer shell and this is lost when they react to form ions with a +1 charge. As you descend the group the metals get more reactive; the reasons for this are:
• The atoms get larger and so the outer electron is further away from the nucleus, this means it requires less energy to remove it as the force of attraction from the nucleus is less. The electrons in the inner shells also help to shield the nucleus from the outer electron and this also reduces the force of attraction between the outer electron and the nucleus.
• The alkali metals all react violently with water to form alkaline solutions and release hydrogen gas. Lithium bubbles and fizzes in water as the hydrogen gas is released. Enough heat is produced by the reaction of sodium and potassium with water that the metal melts to form a ball of molten metal that shoots across the water surface as the hydrogen is released. The reaction with potassium is violent enough for the hydrogen to ignite spontaneously and a lilac flame is seen above the metal.
• The melting and boiling points of the group 1 metals decrease as you descend the group.
• In the reactions with oxygen the metal reactivity determines the products of the reaction.
  • Lithium burns to produce lithium oxide.
  • Sodium burns to produce a mixture of sodium oxide and sodium peroxide.
  • Potassium, rubidium and caesium burn to form a mixture of peroxides and superoxides.

Check your understanding - Questions on the alkali metals

Check your understanding - Additional questions on the alkali metals

Check your understanding - Quick quiz on alkali metals

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