题目

氯元素有、两种同位素，其中的中子数和电子数之差为 A. 17    B. 20 C. 1    D. 3 答案：【答案】D 【解析】的中子数是37－17＝20，质子数等于核外电子数，所以中子数和电子数之差为20－17＝3，答案选D。 点睛：掌握原子的组成以及构成原子的几种微粒之间的关系是解答的关键，即。—Liu Fang, I don't know you can play the Pipa and Guzheng so well.—Well, actually I __________ it since I was six years old.A. plays B. have been playingC. played D. had been playing