Subatomic particles The atom is made from three particles: protons, neutrons and electrons.1
Unified atomic mass unit (u)
The unified atomic mass unit (u) is a unit of mass used to express atomic and molecular masses. It is the approximate mass of a proton or a neutron. It is also referred to as ‘universal mass unit’;the old name for it is the ‘atomic mass unit’(amu).
The unified atomic mass unit is defined as one-twelfth of the mass of an unbound atom of carbon-12: 1 u 1.66 1027 kg
Fig 21.1: Illustration of an atom The Electron
ELECTRON An electron (e) is an indivisible quantity of charge that orbits the nucleus of an atom.
Charge: An electron has a negative charge of 1.6 1019 C. It is thus deflected in electric and magnetic fields.We often refer to its charge as ‘1’ where we do not give it a unit.This is because all charged bodies have a total electric charge that is an integer multiple of the charge on an electron.
Mass: An electron has a mass of 1 840 u 1 9.09 1031 kg.
PROPERTIES OF SUBATOMIC PARTICLES Proton
Neutron Mass Charge Location 1 u 1 Nucleus 1 u 0 Nucleus Table 21.1: Mass, charge and location of subatomic particles In a neutral atom the number of electrons equals the number of protons.
1In Chapter 23 we will see that protons and neutrons can be further subdivided into quarks.The fact that protons and neutrons are not fundamental has no bearing on the behaviour in the observable world. It is beyond the scope of Leaving Cert. Physics; however, it has now been shown that fundamental ‘particles’ such as the electrons behave like waves, just as photons of light do. The world-renowned physicist Richard Feynman once said: ‘The electron is a theory we use; it is so useful in understanding the way nature works that we can almost call it real.’ The study of fundamental particles is Theoretical Physics. At Leaving Cert. we merely scratch the surface of the subject, but if you take the time to delve deeper a whole new bizarre and fascinating world is waiting to be revealed.
