Atoms consist of a positively charged nucleus surrounded by a cloud of negatively charged electrons. Nuclei are very dense and extremely small, they contain more than 99.9% of an atom's mass and are 10,000 times smaller than an atom! The nucleus is a collection of electrically neutral particles called protons, which are positively charged. In turn, protons and neutrons consist of particles known as quarks. Get the best ICSE Class 12 Physics Study Material only on the Extramarks website.

An atom's chemical element is determined by the nucleus ' number of protons, or the atomic number, Z. The oxygen element has an atomic number Z=8, compared to Z=6. The nucleus ' atomic mass is given by, A= Z+N, where, N, is the nucleus ' number of neutrons. ICSE Class 12 Physics Study Material will be
Different element isotopes have different neutron numbers in their nucleus. For example, the most common type of carbon in the human body, the stable isotope carbon-12, has Z=6 and N=6, while the radioactive isotope used in carbon dating, carbon-14, has Z=6 and N=8. There are fewer than 300 stable nuclei and more than 3000 radioactive unstable nuclei. Get the ICSE Class 12 Physics Study Material can get students good marks.

The nucleus consists of protons (charge= + 1; mass= 1.007 units([ μ]) of atomic mass and neutrons. The number of protons in the nucleus is called the atomic number Z and defines the chemical element represented by the nucleus. The number of neutrons in the nucleus is known as the number of neutrons N, whereas the total number of neutrons and protons in the nucleus is known as the number of mass A, where A= N + Z. Collectively, the neutrons and protons are called nucleons. A nucleus containing a given N and Z is called a nuclide. Isotopes are nuclides with the same atomic number, such as 12 C and 14 C, whereas isotones are called nuclides with the same N, such as 14 C and 16 O. Nuclei like 14 N and 14 C are isobars with the same mass number. Nuclides are designated by a shorthand notation in which one writes, i.e. for a nucleus with six protons and eight neutrons, one writes, or, or only 14 C. The size of a nucleus is about 1 to 10 − 10 m, with a more accurate representation of the nuclear radius as 1.2 − A 1/3 − 10− 15 m. We can approximate the nucleus approximately like a sphere, so we can calculate its density where 1.66 − 10 −27 kg is the nucleon's mass. The nuclear density is therefore approximately 200,000 tons/mm 3 and is independent of A. Get the Physics ICSE Class 12 Sample Paper on the Extramarks websites.
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