All nuclei are made up of two types of particles, namely: protons and neutrons. The only exception is the ordinary hydrogen nucleus, which is a single proton.
Atomic Number, Neutron Number, Mass Number and Isotopes
The following quantities are used to describe the atomic nucleus:
- The atomic number, Z sometimes referred to as the charge number, which equals the number of protons in the nucleus.
- The neutron number, N, which equals the number of neutrons in the nucleus.
- The mass number, A, which equals the number of nucleons (neutrons plus protons) in the nucleus.
The symbol used to show how many protons and neutrons are present in nuclei is:
Where X represents the chemical symbol for the element for instance:
The above represents Radium that has a mass number of 226 and atomic number of 88, it thus contains 88 protons and 138 neutrons.
The nuclei of all atoms of a specific element contain the same number of protons but often consist of different numbers of neutrons. Nuclei that are related in this way are called isotopes. To put it in other words, the isotopes of an element have the same Z value but different N and A values.
The natural abundances of isotopes can differ substantially. For instance, the following are the four isotopes of carbon:
The natural abundance of the carbon isotope below is 98.9%:
Whereas that of the carbon isotope below is only 1.1%:
Some isotopes do not occur naturally but can be produced in the laboratory through nuclear reactions.
Related: Hadrons vs. Leptons
Charge and Mass
The proton carries a single proton charge, equal in magnitude to the electron charge where e = 1.602 X 10-19 C. The neutron is electrically neutral. Since the neutron has no charge, it is not easy to detect it.
The atomic mass (the mass of an atom containing a nucleus and Z electrons) can be measured with the mass spectrometer. The proton is approximately 1836 times as massive as the electron, and the masses of the proton and the neutron are almost equal.
For atomic masses, the atomic mass unit, u, can be defined in such a way that the mass of the isotope 12C is exactly 12 u. That is, the mass of an atom is measured relative to the mass of an atom of the neutral carbon-12 isotope (the nucleus plus six electrons). Therefore, the mass of 12C is exactly 12 u, where 1 u = 1.660540 x 10-27 kg. The proton and neutron each have a mass of approximately 1 u, and the electron has a mass that is only a small fraction of an atomic mass unit:
- mass of proton = 1.0072765 u
- mass of neutron = 1.0086659 u
- mass of electron = 0.00054857990 u
Since the rest energy of a particle is given by E = mc2, it is often suitable to express the atomic mass unit in terms of its rest-energy equivalent. For one atomic mass unit, we have:
= 931.4943 MeV
The mass is typically express in terms of unit MeV/c2, therefore the mass of 1 u is:
1 u = 931.4943 MeV/c2
Also Read: Principal Quantum Number
