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# CBSE Class 12th Physics Notes for Chapter 13 Nuclei

CHAPTER 13 NUCLEI

• An atom has a nucleus. The nucleus is positively charged. The radius of the nucleus is smaller than the radius of an atom by a factor of 104. More than 99.9% mass of the atom is concentrated in the nucleus.

• On the atomic scale, mass is measured in atomic mass units (u). by definition, 1 atomic mass unit (1 u) is 1/12th mass of one atom of 12C i.e. 1 u = 1.660563 x 10-27

• A nucleus contains a neutral particle called neutron. Its mass is almost the same as that of a proton.

• The atomic number Z is the number of protons in the atomic nucleus of an element. The mass number A is the total number of protons and neutrons in the atomic nucleus. A=Z+N where N denotes the number of neutrons in the nucleus.

• A nuclear species is also known as nuclide.

• Isotopes: Nuclides with the same atomic number Z but different neutron number N.

• Isotones: Nuclides with the same number of neutrons N.

• Isobars: Nuclides with the same mass number A.

• Neutrons and protons are bound in a nucleus by the short-range strong nuclear force.

• Radioactivity is the phenomenon in which nuclei of a given species transform by giving out α or β or γ
• α- rays are helium nuclei
• β- rays are electrons
• γ- rays are electromagnetic radiation of wavelengths shorter than X-rays.

• Law of radioactive decay: N (t)= N(0) eλt where λ is the decay constant or disintegration constant.

• Nuclear fission occurs when a heavy nucleus breaks into two smaller fragments. Nuclear fusion occurs when lighter nuclei combine to form a larger nucleus.