钠Sodium

钠原子最外层只有一个 3s 电子，第一电离能低（496 kJ mol⁻¹），极易失去该电子形成 Na⁺。它的第二电离能高达 4562 kJ mol⁻¹，因此钠在化合物中只呈 +1 价。在第三周期中，钠的原子半径最大、金属性最强。

Sodium has a single 3s outer electron and a low first ionization energy (496 kJ mol⁻¹), so it readily forms Na⁺. Its second ionization energy is 4562 kJ mol⁻¹, so sodium is always +1 in compounds. It has the largest atoms and is the most metallic element of period 3.

1807 年戴维电解熔融的氢氧化钠，首次制得金属钠。英文名来自 soda（苏打），元素符号 Na 来自拉丁文 natrium。

In 1807 Davy obtained metallic sodium for the first time by electrolysing molten sodium hydroxide. The English name comes from soda, and the symbol Na from Latin natrium.

钠质软，可用小刀切割，新切面有银白色光泽，很快因氧化而变暗，所以保存在煤油或液体石蜡中。钠与冷水剧烈反应，浮在水面上熔成小球四处游动：2Na + 2H₂O → 2NaOH + H₂↑。在空气中燃烧发出黄色火焰，生成淡黄色的过氧化钠 Na₂O₂。钠的焰色为黄色，来自 589 nm 附近的发射谱线。

Sodium is soft enough to cut with a knife; the fresh surface is silvery but quickly tarnishes, so it is stored under paraffin oil. It reacts vigorously with cold water, floating, melting into a ball and darting about: 2Na + 2H₂O → 2NaOH + H₂↑. It burns in air with a yellow flame to form pale yellow sodium peroxide, Na₂O₂. Its yellow flame colour comes from emission lines near 589 nm.

工业上电解熔融氯化钠制取金属钠（唐斯电解槽）：2NaCl → 2Na + Cl₂。钠用于钠光灯、作还原剂制取钛等金属；液态钠和钠钾合金用作快中子反应堆的冷却剂。钠的化合物如 NaCl、NaOH、Na₂CO₃、NaHCO₃ 在生活和工业中极为重要。

Sodium metal is made industrially by electrolysing molten sodium chloride in the Downs cell: 2NaCl → 2Na + Cl₂. It is used in sodium-vapour lamps and as a reducing agent to make metals such as titanium; liquid sodium and sodium–potassium alloy cool fast-breeder reactors. Sodium compounds such as NaCl, NaOH, Na₂CO₃ and NaHCO₃ are vital in daily life and industry.