什麼是化學元素

構成所有物質的基本材料，已知的化學元素約有115種，構成元素最小的單位是原子。一個元素的所有原子在原子核的電荷（即質子數）和電子數（參閱atomic number）是相同的，但如果它們的中子數不一樣，質量（原子量）就可能不同（參閱isotope）。每一種元素有一或兩個字母的化學符號。元素能互相化合形成多種複雜的化合物。原子序數大於83（鉍）的所有元素和一些較輕元素的同位素都是不穩定、放射性的元素。超鈾元素（原子序數大於92）是在1940年以後被發現，以中子或快速運動的帶電粒子轟擊一種元素而產生的。已知的化學元素中有11種是氣體，2種是液體，其餘都是固體。地殼中含量最豐富的元素是氧（46%）、矽（26%）、鋁（8%）、鐵（5%）。

element, chemical

One of the 115 presently known kinds of substances that constitute all matter at and above the level of atoms, the smallest unit of any element. All atoms of an element are identical in nuclear charge (number of protons) and number of electrons (see atomic number), but their mass (atomic weight) may differ if they have different numbers of neutrons (see isotope). Each element has a one- or two-letter chemical symbol. Elements combine to form a wide variety of compounds. All elements with atomic numbers greater than 83 (bismuth), and some isotopes of lighter elements, are unstable and radioactive (see radioactivity). The transuranium elements, with atomic numbers greater than 92 (see uranium), artificially created by bombardment of other elements with neutrons or other heavy particles, have been discovered since 1940. The most common elements (by weight) in the earth's crust are oxygen, 49%; silicon, 26%; aluminum, 8%; and iron, 5%. Of the known elements, 11 (hydrogen, nitrogen, oxygen, fluorine, chlorine, and the six noble gases) are gases, two (bromine and mercury) are liquids (two more, cesium and gallium, melt at about or just above room temperature), and the rest are solid under ordinary conditions. See also periodic table.