Nuclei
The nucleus is the central core of the atom. The radius of the nucleus is smaller than the radius of the atom. The volume of the nucleus is ten times the volume of the atom. The nucleus contains most of the mass of the atom.
The nucleus consists of protons and neutrons The protons are positively charged. The number of protons in a nucleus is called the atomic number (Z). The total number of protons and neutrons in a nucleus Is called mass number (A).
NUCLEAR PHYSICS GLOSSARY: DEFINITIONS, TERMS, MEANING, EXPLANATION
Atomic Mass Unit
The instrument used to determine accurately the atomic mass of an element is called a mass spectrometer. The unit in which atomic and nuclear masses are measured is called the atomic mass unit (u). One atomic mass unit is defined as 1/12th of the mass of the C-12 atom.
1 amu = 1.660539 x 10-27kg.
In terms of this unit, we have the mass of proton mp=1.00866u, the mass of neutron mn=1.00866u, and the mass of electron = 0.00055u.
Neutron
James Chadwick discovered the neutron in 1932. The neutrons are electrically neutral. The mass of the neutron is nearly the same as the mass of the protons. The protons and neutrons in a nucleus are collectively called nucleons.
The number of neutrons in the nucleus is given by N=A-Z. The nucleus of an atom is represented by AXZ. Neutrons have a high penetrating power. The free neutron is unstable, but the neutron is stable inside the nucleus. Neutrons have low ionizing power.
Models of Atom: Thomson, Rutherford, Bohr, Plum Pudding, Electromagnetic Rays, Experiment, Etc.
Isotopes
The atoms of an element having the same atomic number, but different atomic mass is called isotopes. For example, 6Li3 and 7Li3 are the two isotopes of lithium, 1H1, 2H1 (deuterium), 3H1 (tritium) are the three isotopes of hydrogen. The physical and chemical properties of isotopes remain the same.
The relative abundance is different for different isotopes. So, the average mass of an atom is obtained by the weighted average of the masses of the different isotopes.
Isobars
The atoms having the same atomic mass, but different atomic numbers are called isobars. For example: 37Cl17 and 37S16, 40Ca20 and 40Ar18.
Isotones
Nuclei with the same number of neutrons but different atom numbers are called isotones. For example, 37Cl17 and 39K19 have the same neutron as 18.
General characteristics of a nucleus
Size of the nucleus
The nuclear size can be determined from the scattering of alpha particles from the thin gold foil. Nuclei are assumed to be spherical of size 10-15m to 10-14m and the experiments show that the volume of a nucleus is directly proportional to its mass number present in it.
The radius of the nucleus R=R0A1/3, is approximately equal to 1.2 fermis.
Nuclear charge
The charge of the nucleus is because of the charge of the proton. Since proton carries a positive charge +e=1.6×10-19C, the total charge on the nucleus of atomic number Z is +Ze. (e is the electronic charge).
Nuclear mass
It is the total mass and neutrons present in a nucleus and is denoted by m. If mp and mn are masses of proton and neutron, the nuclear mass of a nucleus of atomic number Z and mass number A will have a mass ZMp+ (A-Z) mn. It is measured in terms of atomic mass unit (u).
Nuclear Density
It is the ratio of nuclear mass to nuclear size. The density of a nucleus is independent of its mass number. This density is very large compared to ordinary matter. For example water density = 103 kg, but for the nucleus, it is of the order of 1017/m3.
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