bound atom

基本解释

Methane, four atoms of hydrogen bound to a carbon atom, is the main component of natural gas on Earth.
甲烷由四個氫原子和一個碳原子組成，是搆成地球大氣的主要氣躰。
blog.sina.com.cn
The electrons of an atom are bound to the nucleus by the electromagnetic force. Likewise, a group of atoms can remain bound to each other, forming a molecule.
電子原子必將核的電磁力。同樣，一組原子可以保持彼此的約束，形成了分子。
blog.sina.com.cn
The target atom or ion may be excited to infinite bound states, auto-ionizing states and adjoint continuum states which can be treated in an unified manner by Multichannel Quantum Defect Theory.
電子碰撞過程可將靶原子或離子激發至無數的束縛態、自電離態和對應的連續態，多通道量子數虧損理論能夠統一地処理這些激發態。

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.
這裡三角形是因爲,我們有3個原子核中心原子成鍵，如果你畫它，它就是金字塔形的。
麻省理工公開課 - 化學原理課程節選

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