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Nuclear Physics 1. An atom is the smallest unit of an element, which may or may not exist independently, but always takes part in a chemical reaction. 2. The central core of the atom is called nucleus. The nucleus contains protons and neutrons. 3. Neutrons and protons are collectively called nucleons. 4. Protons are positively charged particles, having a charge equal to 1.6 l0 19C and mass equal to 1.67 10 27 kg. 5. Neutrons are electrically neutral particles, having a mass slightly more than the protons. 6. Electrons are negatively charged particles, revolving around the nucleus. Their mass is equal to 9.1 10 31 kg and charge equal to 1.6 l0 19C. 7. An atom is electrically neutral, i.e., number of protons is equal to number of electrons. 8. A Z X represents the symbol for an atom of any element. X is the name of the element, A is the mass number of the element and Z is the atomic number of the element. 9. The number of nucleons in the atom of an element is called its mass number. 10. The number of protons in an atom is called atomic number. 11. The atoms of same element, having same atomic number, but different mass number are called isotopes. Class-X 1 Key points
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