South Korean officials have demanded a formal apology from the North and say they have not yet decided whether and how to pursue the matter further.

VOA: standard.2009.09.11

We haven't actually learned how to calculate the formal charge yet, we'll learn it soon.

实际上我们还没学如何计算形式电荷，我们很快就会学到了。

麻省理工公开课 - 化学原理课程节选