化合价的概念是理解化学键的关键。

我们不是从祖先那里继承了地球；我们是从子孙那里借来的。这个真理指导着我们做出的每一个决定。

Leadership is about making tough decisions today that will benefit generations to come.

To do mathematics is to engage in an act of discovery and conjecture, intuition and inspiration.

We are all searching for something, but we don't know what it is.