19世纪中后期,人们已经认识到世界上的一切物质都是由元素组成的,元素的原子是由原子核和核外电子构成的。物质的放射性被发现后,人们对微观世界的认识又深入到原子核内部。科学家经研究发现,一切元素的原子核都是由质子和中子构成的(只有一种氢元素的原子除外),而同一元素所有原子的原子核里,质子数都相同,但中 In the mid-19th century, people realized that everything in the world is made up of elements whose atoms are made up of nuclei and extranuclear electrons. After the discovery of the radioactivity of matter, people’s understanding of the micro-world went deep into the nucleus. Scientists have found that the nuclei of all elements are made of protons and neutrons (except for one atom of hydrogen), while the number of protons in all the atoms of the same element is the same, but the number of