Group I Properties
Section: 8. The Periodic Table | Syllabus: Cambridge AS Level Physics 9702
The Alkali Metals Group I elements are called the alkali metals and include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). They are among the most reactive metals in the Periodic Table and share similar chemical properties.
Alkali Metals The elements in Group I of the Periodic Table (excluding hydrogen), characterized by having one electron in their outer shell and forming alkaline solutions when reacting with water. Group I Elements Element Symbol Atomic Number Electron Configuration Lithium Li 3 2,1 Sodium Na 11 2,8,1 Potassium K 19 2,8,8,1 Rubidium Rb 37 2,8,18,8,1 Cesium Cs 55 2,8,18,18,8,1 Why Exclude Hydrogen?
Although hydrogen (H) is in Group I, it is NOT an alkali metal. It's a non-metal gas with very different properties from the other Group I elements. Common Feature All alkali metals have one electron in their outer shell This single outer electron is easily lost to form 1+ ions Similar electron configuration leads to similar chemical properties Electronic structure: [previous noble gas] + one outer electron Physical Properties Alkali metals have distinctive physical properties that make them different from typical metals.
Appearance and Texture Soft metals - can be cut easily with a knife Shiny silver surface when freshly cut Surface quickly tarnishes (becomes dull) in air due to oxidation Become softer as you go down the group Density and Melting Points Element Density (g/cm³) Melting Point (°C) Boiling Point (°C) Lithium 0.53 181 1342 Sodium 0.97 98 883 Potassium 0.86 63 759 Rubidium 1.53 39 688 Cesium 1.93 28 671 Low density - Li, Na, and K float on water Low melting points - for metals (can melt in your hand for cesium) Melting point decreases down the group Density generally increases down the group (exception: K is less dense than Na) Why Low Melting Points?
Alkali metals have weak metallic bonding because they only have one outer electron to contribute to the metallic bond. As atoms get larger down the group, the bonding becomes even weaker. Conductivity Good conductors of heat and electricity Conduct electricity because of delocalized electrons Less conductive than transition metals Diagram Images showing alkali metals being cut with a knife, revealing shiny surfaces that quickly tarnish, with comparative softness demonstration Chemical Properties Alkali metals are highly reactive and share similar chemical behavior due to their single outer electron.
General Reactivity Very reactive metals - among the most reactive in the Periodic Table React by losing their single outer electron to form 1+ ions Reactivity increases down the group (Cs > Rb > K > Na > Li) Must be stored under oil to prevent reaction with air and moisture Form ionic compounds with non-metals Why Does Reactivity Increase Down the Group?
The outer electron is further from the nucleus and more shielded by inner electrons, making it easier to remove. Lower ionization energy = higher reactivity. Ion Formation All alkali metals form 1+ ions by losing their single outer electron: Li → Li⁺ + e⁻ Na → Na⁺ + e⁻ K → K⁺ + e⁻ These ions have a stable noble gas configuration All compounds are ionic and contain the metal as a 1+ ion Reactions with Water One of the most characteristic reactions of alkali metals is their vigorous reaction with water.
General Reaction All alkali metals react with water to produce a metal hydroxide and hydrogen gas : General equation: 2M(s) + 2H₂O(l) → 2MOH(aq) + H₂(g) Where M = alkali metal (Li, Na, K, etc.) Observations for Each Metal Metal Observations Vigor of Reaction Lithium Floats, fizzes steadily, moves around slowly Least vigorous Sodium Floats, melts into a ball, fizzes rapidly, moves quickly on surface More vigorous Potassium Floats, melts immediately, burns with lilac flame, moves very quickly, may ignite hydrogen produced Very vigorous Rubidium Explodes on contact with water Explosive Cesium Explodes violently on contact with water Most explosive Specific Equations Lithium + water: 2Li(s) + 2H₂O(l) → 2LiOH(aq) + H₂(g) Sodium + water: 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g) Potassium + water: 2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g) Products Metal hydroxide - dissolves to form alkaline solution (pH 12-14) Hydrogen gas - can be tested with a lit splint (pops) Solution becomes alkaline - turns red litmus blue All metal hydroxides are soluble in water Safety Warning Never handle alkali metals with bare hands or allow them to contact water without proper safety precautions.
The reactions are highly exothermic and can cause burns or explosions. Diagram Series of images showing lithium, sodium, and potassium reacting with water in sequence, highlighting increasing vigor and the lilac flame of potassium Reactions with Oxygen Alkali metals react readily with oxygen in the air, which is why they tarnish quickly and must be stored under oil.
Reaction with Oxygen When heated in air or oxygen, alkali metals burn to form metal oxides . General equation: 4M(s) + O₂(g) → 2M₂O…
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