Alkali Metals and Alkaline Earth Metals
The alkali metals (group 1: lithium, sodium, potassium, rubidium, cesium and francium) and the alkaline earth metals (group 2: beryllium, magnesium, calcium, strontium, barium and radium) are the two most reactive families of metals, with one and two valence electrons that they readily lose to form 1+ and 2+ ions. In brief: alkali metals are soft, low-density metals that react vigorously with water to make hydrogen and a strongly basic hydroxide, and alkaline earth metals are harder, denser and react less violently.
These s-block metals are common in daily life: sodium and potassium in salt and in the body, calcium in bones and cement, magnesium in alloys and plants. This article describes the properties and trends of both groups, their reactions with water and oxygen, flame test colors, includes data tables, works through reaction calculations and gives the history of their discovery.
What are the alkali metals?
The alkali metals have a single electron in their outer s subshell (ns1). They lose it very easily and always form ions with a charge of 1+. They are so reactive that they are never found as free elements in nature, and they are stored under oil, because they react with oxygen and moisture in the air. Their compounds with oxygen and with the halogens are ionic. They are soft enough to cut with a knife, shiny when freshly cut and tarnish within seconds. Lithium, sodium and potassium are less dense than water.
The name “alkali” comes from an Arabic word for the ashes of plants, from which potassium and sodium carbonates were extracted. Their hydroxides are strong bases, called alkalis.
What are the alkaline earth metals?
The alkaline earth metals have two outer electrons (ns2) and form ions with a charge of 2+. They are less reactive than the alkali metals in the same period, since removing two electrons takes more energy, but they are still reactive and occur in nature only in compounds. They are harder and denser than the alkali metals and have higher melting points. The old term “earths” referred to oxides that are insoluble in water and not easily changed by heat, such as lime (CaO) and magnesia (MgO).
How do properties change down the groups?
| Element | Melting point (°C) | Density (g/cm3) | First ionization energy (kJ/mol) | Flame color |
|---|---|---|---|---|
| Lithium (Li) | 180.5 | 0.53 | 520 | Crimson red |
| Sodium (Na) | 97.8 | 0.97 | 496 | Golden yellow |
| Potassium (K) | 63.5 | 0.86 | 419 | Lilac |
| Rubidium (Rb) | 39.3 | 1.53 | 403 | Red-violet |
| Cesium (Cs) | 28.4 | 1.93 | 376 | Blue-violet |
| Element | Melting point (°C) | Density (g/cm3) | First ionization energy (kJ/mol) | Flame color |
|---|---|---|---|---|
| Beryllium (Be) | 1,287 | 1.85 | 900 | None |
| Magnesium (Mg) | 650 | 1.74 | 738 | None (bright white when burning) |
| Calcium (Ca) | 842 | 1.55 | 590 | Orange-red |
| Strontium (Sr) | 777 | 2.64 | 550 | Crimson |
| Barium (Ba) | 727 | 3.62 | 503 | Pale green |
Down both groups, the atoms get larger, the outer electrons are farther from the nucleus and are lost more easily, so the first ionization energy decreases and the reactivity increases. The melting point decreases down group 1 because the metallic bonding gets weaker, while in group 2 the trend is irregular.
What are their typical reactions?
- With water: 2Na + 2H2O → 2NaOH + H2. Lithium reacts steadily, sodium rapidly, and potassium so fast that the hydrogen can ignite with a lilac flame. Rubidium and cesium react explosively. Calcium reacts with cold water more slowly: Ca + 2H2O → Ca(OH)2 + H2. Magnesium reacts very slowly with cold water but burns in steam: Mg + H2O → MgO + H2.
- With oxygen: 4Li + O2 → 2Li2O. Sodium forms mainly sodium peroxide, Na2O2, and potassium forms superoxide, KO2, when burned in excess oxygen. Magnesium burns with a dazzling white flame: 2Mg + O2 → 2MgO.
- With halogens: 2Na + Cl2 → 2NaCl and Mg + Cl2 → MgCl2.
- With acids: Mg + 2HCl → MgCl2 + H2.
The resulting hydroxides are bases. Their solubility increases down group 2: magnesium hydroxide is only slightly soluble (milk of magnesia), calcium hydroxide moderately soluble (limewater) and barium hydroxide quite soluble.
What are their flame test colors?
When a salt of these metals is heated in a flame, the electrons are excited and then emit light of characteristic colors as they return to lower energy levels. A clean wire loop is dipped in a salt solution and held in the flame. Colors in the table above are used to identify unknown salts. Sodium’s yellow color is so strong that it can hide the other colors, and a blue cobalt glass filter is used to see the lilac of potassium.
Worked examples
- Hydrogen from sodium. A piece of sodium of 2.30 g reacts with water. Moles of Na = 2.30 ÷ 22.99 = 0.100 mol. From 2Na + 2H2O → 2NaOH + H2, 0.100 mol of Na gives 0.050 mol of H2, or 0.050 × 2.016 = 0.101 g. At 0 °C and 1 atm, the volume is 0.050 × 22.4 = 1.12 L.
- Strength of the solution. If the reaction in example 1 is in 500 mL of water, the NaOH concentration is 0.100 mol ÷ 0.500 L = 0.200 mol/L (assuming that the volume does not change), and the pH is 14 + log 0.200 = 13.3.
- Burning magnesium. What mass of magnesium oxide is formed from 2.43 g of magnesium? n(Mg) = 2.43 ÷ 24.31 = 0.100 mol. From 2Mg + O2 → 2MgO, 0.100 mol of MgO forms. The molar mass of MgO is 40.31 g/mol, so the mass is 4.03 g. The mass has increased by 4.03 − 2.43 = 1.60 g, which is the mass of oxygen that combined.
- Calcium and water. 4.00 g of Ca is 4.00 ÷ 40.08 = 0.0998 mol. It gives 0.0998 mol of H2, and 0.0998 mol of Ca(OH)2, which has a mass of 0.0998 × 74.09 = 7.39 g. Only about 1.7 g of calcium hydroxide dissolves in 1 L of water, so most of the product remains as a white suspension.
- Identify the metal. A salt gives a lilac flame, and its chloride has a formula MCl. The metal is potassium (group 1, lilac).
What are they used for?
- Lithium: rechargeable batteries, glass and ceramics, and medicines for mood disorders.
- Sodium: table salt (NaCl), sodium hydroxide, street lamps (yellow light) and a coolant in some nuclear reactors.
- Potassium: fertilizers, which contain potassium chloride and nitrate, and essential for nerves and muscles.
- Cesium: atomic clocks, which define the second.
- Magnesium: light alloys, flares and fireworks, and the central atom of chlorophyll.
- Calcium: bones and teeth, limestone, cement and plaster.
- Barium: barium sulfate as a contrast agent for X-ray imaging of the digestive tract.
How were they discovered?
Humphry Davy isolated sodium and potassium in 1807 by electrolysis of molten hydroxides, and magnesium, calcium, strontium and barium in 1808. Johan August Arfwedson discovered lithium in 1817. Robert Bunsen and Gustav Kirchhoff discovered cesium in 1860 and rubidium in 1861 with the help of the spectroscope, since these elements have bright spectral lines. Francium was discovered by Marguerite Perey in 1939, and radium by Marie and Pierre Curie in 1898.
Which mistakes are common?
Mistake 1: Thinking that all the alkali metals are heavy metals
Lithium, sodium and potassium are lighter than water.
Mistake 2: Putting out a sodium fire with water
Water reacts with the metal and makes the fire worse. Use a dry powder extinguisher designed for metal fires.
Mistake 3: Confusing alkali metals with alkaline earth metals
Alkali metals are group 1 (1+ ions). Alkaline earth metals are group 2 (2+ ions).
Mistake 4: Saying that magnesium does not react with water
It reacts slowly with cold water and quickly with steam. Its reaction with acids is vigorous.