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Chapter: 12th Physics : UNIT 9 : Atomic and Nuclear Physics

Composition of nucleus

The number of protons in the nucleus is called the atomic number and it is denoted by Z. The number of neutrons in the nucleus is called neutron number (N). The total number of neutrons and protons in the nucleus is called the mass number and it is denoted by A. Hence, A = Z+N.

Composition of nucleus

Atoms have a nucleus surrounded by electrons. The nucleus contains protons and neutrons. The neutrons are electrically neutral (q = 0) and the protons have positive charge (q =+ e) equal in magnitude of the charge of the electron (q =–e). The number of protons in the nucleus is called the atomic number and it is denoted by Z. The number of neutrons in the nucleus is called neutron number (N). The total number of neutrons and protons in the nucleus is called the mass number and it is denoted by A. Hence, A = Z+N.

The two constituents of nucleus namely neutrons and protons, are collectively called nucleons. The mass of a proton is 1.6726×10−27 kg which is roughly 1836 times the mass of the electron. The mass of a neutron is slightly greater than the mass of the proton and it is equal to 1.6749×10−27 kg.

To specify the nucleus of any element, we use the following general notation

AZX


where X is the chemical symbol of the element, A is the mass number and Z is the atomic number. For example, the nitrogen nucleus is represented by 157N . It implies that nitrogen contains 15 nucleons of which 7 are protons (Z = 7) and 8 are neutrons (N = A Z = 8).

Note that once the element is specified, the value of Z is known and subscript Z is sometimes omitted. For example, nitrogen nucleus is simply denoted as 15N and we call it as ‘nitrogen fifteen’.

Since the nucleus is made up of positively charged protons and electrically neutral neutrons, the overall charge of the nucleus is positive and it has the value of +Ze. But the atom is electrically neutral which implies that the number of electrons in the atom is equal to the number of protons in the nucleus.

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12th Physics : UNIT 9 : Atomic and Nuclear Physics


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