And, furthermore, they are characteristic of the target gas element.

更进一步的说，它们是目标气体元素的特征。

麻省理工公开课 - 固态化学导论课程节选

Second thing, we started talking about the periodic table and I believe that it's a hallmark of any educated person in the 21st century who is technically literate to know the periodic table by heart.

第二点，我们之前讨论过元素周期表,我相信这是个标志性的特征,对于那些在21世纪里,用心去学习,元素周期表的人来说。

麻省理工公开课 - 固态化学导论课程节选