Neutrons and protons are bound together in the nucleus of an atom.
在一个原子的核内，中子和质子聚合在一起。
《柯林斯英汉双解大词典》
2
So it's trigonal because we have these three atoms that are bound to the central atom here, and if you picture it, it's actually shaped like a pyramid.
这里三角形是因为,我们有3个原子核中心原子成键，如果你画它，它就是金字塔形的。
3
This is partly explained because the bigger an atom is, the more weakly its outer electrons are bound to it (and hence the further those electrons are from the nucleus).