In the text below some words are missing. Drag words from the box below to the appropriate place in the text. To undo an answer choice, drag the word back to the box below the text.
下面的文本缺了一些词。请把词框里的词拖到文中合适的位置；要撤销选择，把词拖回文本下方的词框即可。
The electrons that orbit closest to the nucleus are strongly .They are called bound electrons.The electrons that are farther away from the pull of nucleus can be out of their . These are called free electrons. Free electrons can move from one atom to another. This phenomenon is known as electron flow. Electricity is the movement or flow of electrons from one atom to another.
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