STRUCTURE AND PROPERTIES OF THE NUCLEUS

In the early 1930s a model of the nucleus was developed that is still useful today. According to this model, a nucleus is considered as an aggregate of two types of particles: protons and neutrons. In the simplest atom (hydrogen atom), the nucleus is a proton. A proton is an elementary particle, which has a positive charge of

+ e  = 1.60 × 10 ⁻¹⁹C and a mass of m_p = −1.6726 × 10⁻²⁷ kg. A neutron is a neutral particle which has a mass almost identical to that of a proton, m_n = 1.6749-×-10−27   kg. These two particles are referred to collectively as nucleons. The different types of nuclei are referred to as nuclides. The number of protons in a nucleus (or nuclide) is called the atomic number and is designated by the symbol Z. The total number of nucleons, neutrons and protons, is called the mass number and is designated by the symbol A.

 The neutron number N is N = A − Z.

To specify a nuclide, we use the special symbol ^AX_Z, e.g. a normal nitrogen nuclide ¹⁵N₇ means that a nitrogen nucleus contains 7 protons and 15 nucleons and therefore it has 8 neutrons.

Nuclei of atoms of the same element that contain the same number of protons but different numbers of neutrons are called isotopes.

For example, ¹¹C₆, ¹²C₆, ¹³C₆, ¹⁴C₆, ¹⁵C₆ and ¹⁶C₆ are all isotopes of carbon. 98.9% of naturally occurring carbon is the isotope ¹²C₆ and about 1.1% is ¹³C_6.