下列叙述中，正确的是
A．原子的质量主要集中在原子核上
B．构成物质的微粒只有分子和原子
C．气体易被压缩，说明构成气体的分子在不断运动
D．物质在不同条件下的三态变化主要是由于分子的大小发生了变化
答案：A
题目分析：根据已有知识进行分析判断。
A、由于电子的质量约是质子和中子质量的1/1836，故可忽略不计，而原子核是由质子和中子构成的，所以原子的质量主要集中在原子核上，正确；
B、构成物质的微粒有很多，分子和原子只是其中的两种，错误；
C、由于气体分子间的间隔较大，加压时能使分子间隔变小，故气体易被压缩，错误；
D、物质在不同条件下的三态变化，主要是由于分子之间的间隔的变化而引起的，而不是分子大小发生了改变，错误。故选A
点评：理解和熟练掌握原子的构成，以及微观粒子性质方面的知识，是解答此题的关键。
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