Today 18 million tourists visit Venice every year, but in 20 years this number risks doubling.

VOA: standard.2009.10.27

So to figure out bonding electrons, -- what we take is that number 18, which is our total number of electrons we need to fill valence shells, and we subtract it from our number of valence electrons, which is 10.

那么为了找出成键电子，我们将十八，也就是填满所有价壳层,所需要电子的总个数，减去我们所有的价电子的个数，也就是十。

麻省理工公开课 - 化学原理课程节选

Now, I don't have to fill my head 6 with 2.18 times 10 to the minus 18. 13.6, that's a cool number. I can remember that.

现在，我不需要记住,2。18乘10到负18，13。,那数字太给力了，我可以记住它。

麻省理工公开课 - 固态化学导论课程节选