过渡元素及其化合物(transition element)
价电子处在两个壳层而不是一个壳层里的任何化学元素。这种结构使它们具有与众不同的能力，能够组成包含多於一个原子的离子（复杂离子，或称配位化合物），有一个中心原子或离子（往往是过渡金属离子），周围是规则排列的配合基。这些离子的键合理论尚在完善中。周期表中从钪到铜（原子序数21～29）、从钇到银（39～47）以及从镧到金（57～79，包括镧系）是经常指的三组主要过渡系列。（锕系及以上原子序数89～111的元素也符合过渡元素的条件。）它们全都是金属，许多具有重要的经济或工业价值（例如铁、金、镍、钛）。大部分密度较高、质地坚硬、脆性、导热和导电性好、熔点高、彼此间以及与其他金属能生成合金。它们的电子结构使它们在化合物中有多种价键。其中许多是有色金属和顺磁体（参阅paramagnetism），往往可用作催化剂。
English version：
transition element
Any chemical element with valence electrons in two shells instead of only one. This structure gives them their outstanding ability to form ions containing more than one atom (complex ions, or coordination compounds), with a central atom or ion (often a transition metal ion) surrounded by ligands in a regular arrangement. Theories on the bonding in these ions are still being refined. The elements in the periodic table from scandium to copper (atomic numbers 21-29), yttrium to silver (39-47), and lanthanum to gold (57-79, including the lanthanide series) are frequently designated the three main transition series. (Those in the actinide series and beyond, 89-111, also qualify.) All are metals, many of major economic or industrial importance (e.g., iron, gold, nickel, titanium). Most are dense, hard, and brittle, conduct heat and electricity well, have high melting points, and form alloys with each other and other metals. Their electronic structure lets them form compounds at various valences. Many of these are colored and paramagnetic (see paramagnetism) and often act as catalysts.