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》

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。

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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).

原子越大，其外层电子受原子束缚力越弱，这是解释铅和锡两者差别的一部分原因。

youdao

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。

麻省理工公开课 - 化学原理课程节选

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).

ECONOMIST: Einstein and car batteries