题目

The huge explosion at Krakatau was the result of two different factors．One was that the volcano had not broken out for some time and so at the very center of the volcano there was a huge plug of lava, which acted like a cork（塞子）in a bottle, stopping the pressure below from being released（释放）．The second factor was that ocean water entered the very centre of the volcano as the explosions became more violent．This caused superheated steam to build up enormous pressure and this was finally released in the largest explosion of all on August 27, 1883．The sudden release of the 某原子的核外有两个电子层．第一层电子数为第二层电子数的1/3，该原子是________元素的原子．