What are the Alkali Metals?
The word “alkali” comes from Arabic, meaning “ashes of the saltwort”. In chemistry, it refers to a base that can dissolve in water. The alkali metals are all members of group 1 on the periodic table, minus hydrogen. This is the first column of the periodic table. They include lithium, sodium, potassium, etc. They all have one electron in their outer shell, which results in weak metallic bonding. These metals have very similar properties and group trends.
Alkali metals have been known for a long time. In 1807, Sir Humphry Davy, soon after Volta first invented a battery, isolated sodium and potassium metal by electrolysis of molten sodium hydroxide and potassium hydroxide.
Properties of the Alkali Metals
The group 1 elements are all shiny, soft, and highly reactive. They can easily be cut with a knife. Because of their weak metallic bonding, they have lower melting points than other metal groups. They react quickly with air, and violently with water. They are usually stored under argon, or in mineral oil.
The heavier elements react more violently with water, but even sodium can cause a large explosion if sufficient quantity is used. Most textbooks explain the violent reaction as hydrogen catching on fire. However, recent research shows that it is in fact not hydrogen, but a Coulomb explosion responsible for the explosive behavior in water.
All of them have a very low density, and in fact lithium, sodium, and potassium float on water. They also have low electronegativities, low first ionization energies, but higher second ionization energies.
Because the outer s electron is easily excited, these metals tend to have very distinctive flame colors. Rubidium has a striking red-violet flame color.
Of the 5 non-radioactive alkali metals, cesium has the lowest melting and boiling point, the highest density, the largest atomic radius, and the highest reactivity. It has the lowest electronegativity and the lowest first ionization energy.
List of all Alkali Metals
The alkali metals include lithium, sodium, potassium, rubidium, cesium, and francium.
Alkali Metal Chemistry
Alkali metals form compounds almost exclusively in the +1 oxidation state.
The reaction of these metals with water produce hydrogen and the corresponding hydroxide in solution. When exposed to air, these metals, particularly potassium, can form unstable peroxides and superoxides over time, which can explode in the right circumstances. Lithium gets a layer of lithium nitride when exposed to air.
They react easily with most non-metals, including the halogens.
The oxides and hydroxides form extremely basic solutions in water, which are highly corrosive. Most of their compounds are soluble, with lithium fluoride being an exception.
Why are the Alkali Metals Reactive?
The alkali metals are so reactive, due to a number of factors. Their low first ionization energy, combined with the fact they have just 1 electron to donate to get to a stable full shell state, makes them so reactive – even more so as their atomic radius gets larger.
Fun or interesting facts about the Alkali Metals
- Cesium is one of the only metals that is not silvery, but a yellow-gold color.
- Sodium and potassium form an alloy called NaK, which is a highly reactive liquid at room temperature
- Rubidium and cesium are used in atomic clocks.
- One of the few ways to separate sodium compounds from potassium compounds is by converting to the chlorate; potassium chlorate is mostly insoluble in very cold water.
- Francium is highly radioactive, and the bulk element has never been observed
- Alkali metals can dissolve in liquid ammonia to form solvated electrons – basically an electron, surrounded by an envelope of solvent molecules
- Alkali metals can react with certain organic compounds and form a cation in the -1 oxidation state
- Sodium loses its metallic character at ultra-high pressures, and become transparent insulators
Further Reading
Halogens
Transition Metals
Alkaline Earth Metals
Noble Gases
Periodic Table with Names