3 6 The atomic nucleus is made of protons and neutrons Proton(质子) is a positively charged particle in a nucleus a proton is clearly 2000 times as heavy as an electron but has the same (reverse) electric charge as an electron The number of protons each atom of a given element contains is called atomic number Neutron(fis another subatomic particle in nucleus, having the similar mass as the proton but electrically neutral. It has very important role in holding the atomic nucleus together. The atom for a given element should have a set number of proton but the number of neutron can be changed atoms of the same element that contain different numbers of neutrons are isotopes(同位素)3.6 The atomic nucleus is made of protons and neutrons Proton (质子) is a positively charged particle in a nucleus. A proton is clearly 2000 times as heavy as an electron but has the same (reverse) electric charge as an electron. The number of protons each atom of a given element contains is called atomic number. Neutron (中子) is another subatomic particle in nucleus, having the similar mass as the proton but electrically neutral. It has very important role in holding the atomic nucleus together. The atom for a given element should have a set number of proton, but the number of neutron can be changed. Atoms of the same element that contain different numbers of neutrons are isotopes (同位素)