结构
这不是环上点阵的示意图，而是对每个已占据子层实际概率密度|ψ|²进行蒙特卡洛采样的结果。拖动即可旋转。蓝色和紫色表示波函数的相反符号，这正是成键成为可能的原因。
测量值
每个条形图都显示氦在全部118种元素中该性质的排名位置。
来源：IUPAC 2021 标准原子量 ·《CRC 化学与物理手册》· NIST。标有 ~ 的数值是预测值，而非测量值。
按比例显示的大小
Radius 31 pm — that is 0.031 nm, so about 16129 million of them side by side would span a millimetre.
热范围
氦 is liquid over a 3 K window, from 1 K to 4 K.
故事
氦以希腊太阳神赫利俄斯命名，是唯一一种先在太空中、后在地球上发现的元素。1868年日食期间，人们首次在太阳光谱中探测到氦。氦是宇宙中丰度第二高的元素，却是地球上最稀有的元素之一。
氦具有令人惊叹、超出日常经验的性质。在正常压力下，氦是唯一无法冻结成固态的元素——即使在绝对零度（-273.15°C）下，除非压力超过25个大气压，氦仍保持液态。在2.17K时，氦-4变成黏度为零的超流体，能够向上流动并穿过分子大小的孔隙。
作为元素周期表中的第一种稀有气体，氦在所有已知条件下都完全化学惰性。其电子排布（1s²）代表一个已填满的电子层，使其成为最稳定的元素。自然界中不存在氦化合物，不过人们已在实验室条件下制备出奇特的氦氢化物离子（HeH⁺）。
氦-4（⁴He）：丰度为99.999863%——由放射性元素的α衰变产生
氦-3（³He）：丰度为0.000137%——极其稀有，具有潜力的聚变燃料，每克价值$15,000
应用
处理
单纯窒息性气体——氧气置换危害
气体泄漏：
吸入事故：
低温暴露：
快速回答
氦 (symbol He) is element 2 on the periodic table, a 稀有气体 in period 1, group 18. Inert, weightless and permanently escaping: helium is the only element that will not freeze at ordinary pressure, no matter how cold you make it. At room temperature it is a gas.
1s², giving 1 occupied shell holding 2 electrons respectively. Its outer shell holds 8 electrons, which is what sets its bonding behaviour.氦 melts at 1 K (-272.2 °C) and boils at 4.2 K (-268.9 °C).
The standard atomic weight of 氦 is 4.003 u. That is a weighted average across its naturally occurring isotopes, which is why it is rarely a whole number.
氦 is a gas at room temperature; its density is about 0.178 g/L at 0 °C and 1 atm — roughly lighter than air.
氦 was discovered in 1868 by Pierre Janssen & Norman Lockyer. It is named after greek helios, the Sun — found there before Earth.
氦 makes up about 0.52 mg/kg of the Earth's crust — genuinely rare, which is why it is expensive.