Alkali Metal
Alkali metals are the six elements of Group 1 of the periodic table — lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr) — each having a single valence electron that is readily lost to form a +1 cation. They are soft, shiny, highly reactive metals that react vigorously with water to produce a metal hydroxide and hydrogen gas, with reactivity increasing down the group. Alkali metals are found widely in nature as salts and are essential to biological processes; sodium and potassium ions, for example, regulate nerve impulse transmission in living organisms.
Key Formula
2M(s) + 2H2O(l) → 2MOH(aq) + H2(g)
LaTeX: \text{2M}(s) + \text{2H}_2\text{O}(l) \rightarrow \text{2MOH}(aq) + \text{H}_2(g)
| Symbol | Meaning | Unit |
|---|---|---|
| M | Alkali metal (Li, Na, K, etc.) | — |
| MOH | Metal hydroxide formed | — |
| H2 | Hydrogen gas released | — |
Worked Example
Problem
Sodium (Na) reacts with water. Write the balanced equation and calculate the volume of hydrogen gas (at STP) produced when 4.6 g of Na reacts completely.
Solution
Step 1 – Balanced equation: 2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g) Step 2 – Moles of Na: Molar mass of Na = 23 g/mol n(Na) = 4.6 g ÷ 23 g/mol = 0.2 mol Step 3 – Moles of H2: From stoichiometry, 2 mol Na produces 1 mol H2 n(H2) = 0.2 ÷ 2 = 0.1 mol Step 4 – Volume at STP (1 mol gas = 22.4 L): V = 0.1 mol × 22.4 L/mol = 2.24 L
Answer
2.24 L of hydrogen gas at STP
Properties of alkali metals
| Element | Symbol | Atomic Number | Melting Point (°C) | Reactivity with Water |
|---|---|---|---|---|
| Lithium | Li | 3 | 180.5 | Slow, steady fizzing |
| Sodium | Na | 11 | 97.8 | Vigorous, floats and melts |
| Potassium | K | 19 | 63.5 | Very vigorous, lilac flame |
| Rubidium | Rb | 37 | 39.3 | Explosive |
| Caesium | Cs | 55 | 28.4 | Extremely explosive |
Interactive Tools
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Related Terms
Chemical Group
A chemical group (also called a family) is a vertical column in the periodic table containing elements that share the same number of valence electrons, and therefore exhibit similar chemical properties and reactivity patterns. The modern IUPAC system numbers groups 1 through 18 from left to right. Elements within a group show predictable trends: for example, reactivity increases down Group 1 (alkali metals) because the outermost electron is progressively further from the nucleus and more easily lost.
Alkaline Earth Metal
Alkaline earth metals are the six elements of Group 2 of the periodic table — beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra) — each with two valence electrons that are lost to form 2+ cations. They are harder and denser than alkali metals, with higher melting points, and they react with water (though less vigorously than Group 1) to form alkaline hydroxide solutions. Calcium and magnesium are biologically essential; calcium is the primary mineral in bones and teeth, while magnesium is a cofactor for hundreds of enzymatic reactions.
Periodic Trend
Periodic trends are systematic patterns in elemental properties that arise from the regular variation in nuclear charge and electron configuration across periods and down groups of the periodic table. Key periodic trends include atomic radius, ionisation energy, electron affinity, electronegativity, and metallic character, all of which change predictably as atomic number increases. Understanding periodic trends allows chemists to predict chemical reactivity, bond types, and physical properties of elements and their compounds without needing to memorise individual data for every element.
From Arabic "al-qaly" (the ashes of saltwort plant), referring to the alkaline ash residues from which sodium and potassium salts were historically extracted. The word "alkali" entered European chemistry via medieval alchemical texts, and Humphry Davy isolated the first pure alkali metals in 1807.