The elementary structure of the atom

Chapter 18 The elementary structure of the atom

18.1 Aim

The aim of this chapter is to introduce the reader to the key components of an atom. The principal particles which form the nucleus will be identified, as will the factors which determine whether or not the nucleus is stable. The differing electron orbitals and the influence of the electron orbitals on the chemical properties of the material will be discussed. The consequences of transitions of electrons between orbitals will also be identified.

18.2 Introduction

Any attempt to understand the universe around us must start with the fundamental question: what is matter made of? The atom as the fundamental building block of matter has been the subject of a great deal of both theoretical debate and experimental study by physicists. Many of the modern theories concerning atomic and subatomic structures are extremely complex and are the subject of a number of textbooks in their own right. Most of the phenomena which we encounter in radiography can be explained using a relatively simple planetary model of the atom. In this model, there are solid electrons orbiting a solid nucleus – some phenomena can also be explained using the quantum physics model and where this is appropriate this model will be referred to in Insights.

The planetary model of the atom was first described by Rutherford in 1911. It describes an atom consisting of a small, positively charged central nucleus (containing protons and neutrons) around which negatively charged electrons move in defined orbitals. This model can be used to illustrate the carbon atom, as shown in Figure 18.1. As we can see from the diagram, the nucleus of this atom consists of 12 elementary particles – six protons and six neutrons. These particles are bound together in a small volume of extremely high density – about three thousand million, million times greater than the density of water. The protons in the nucleus carry a positive charge and the electrons carry an equal negative charge, so this atom is electrically neutral – the neutrons carry no charge. The electrons are arranged in orbitals or shells called K, L, M … starting from the orbital closest to the nucleus. The K-shell can only contain two electrons and, in the case of the carbon atom, the L-shell contains the remaining four. Different atoms contain different numbers of protons and neutrons in the nucleus and different numbers of electron configurations; this will be discussed later in this chapter. The particles which make up the atom are very tiny and the atom consists largely of empty space. For example, if an atom were to be enlarged until it was the size of a house, the size of the nucleus would be about the size of a pin-head, although it contains 99.95% of the total mass of the atom.

Figure 18.1 The basic structure of a carbon atom. At the centre of the atom is a nucleus which contains six protons (positively charged) and six neutrons (zero charge) – i.e. the nucleus contains 12 nucleons. Six electrons (negatively charged) orbit the nucleus in defined orbitals. As the atom contains equal numbers of positive and negative charges, the whole atom is electrically neutral.

The masses and charges of the subatomic particles which will be considered in this and later chapters are summarized in Table 18.1.

Table 18.1 Masses and charges of the main subatomic particles

		REST MASS		REST ENERGY
PARTICLE	SYMBOL	kg	AMU^a	MeV	CHARGE^b	COMMENTS
Proton	p	1.672×10⁻²⁷	1.007	938	+1	Nucleon, i.e. present in the atomic nucleus
Neutron	n	1.675×10⁻²⁷	1.009	939	0	Nucleon, i.e. present in the atomic nucleus
Alpha-particle	α	6.645×10⁻²⁷	4.003	3718	+2	Two protons and two neutrons
						Ejected in α decay
Electron	e⁻ or β⁻	9.109×10⁻³¹	0.00055 or 1/1820	0.511	−1	Form stable discrete orbits around nuclei
						Ejected from nucleus in β decay
Positron	e⁺ or β⁺	9.109×10⁻³¹	0.00055 or 1/1820	0.511	+1	Antiparticle of the electron – produces annihilation radiation when both meet
Pi meson	π⁺	2.480×10⁻²⁸	0.150	139	+1	Keep the nucleus together (π⁰ and π⁻ also exist)
Neutrino	ν	0	0	0	0	Emitted during β decay and electron capture
						Very weak attenuation by matter
Photon or quantum	h_v	–	–	–	0	Travels at 3×10⁸ m.s⁻¹ Forms part of the electromagnetic spectrum

a 1 amu is 1 atomic mass unit which is one-twelfth of the mass of a neutral C¹²⁶ atom.

b A charge of +1 is +1.602×10⁻¹⁹ coulomb.

Rutherford carried out some elegant experimental work connected with the scattering of alpha-particles by atoms. From this, he concluded that the only explanation for the wide scattering angles which he found experimentally was given by assuming that the atom consists of a very heavy positively charged nucleus with orbiting electrons. The alternative model, whereby all the subatomic particles were contained in a very small volume, was unacceptable because it would produce much smaller scattering angles for the alpha-particles.

Similar experiments have recently been carried out by physicists to try to establish whether protons and neutrons can be broken into smaller pieces. Unfortunately, if the energy of the projectile is large enough to break up the proton or neutron, then the energy is great enough to create new particles – remember E=mc² – and so it is difficult to tell which are fragments of the proton and which have been created as a result of energy being converted into matter. More subtle experiments use high-energy electrons to bombard the protons and neutrons and measure the angle of deflection of these particles. These angles are again over a wide range, suggesting that there are small solid structures within the protons and neutrons. These may be the basic building blocks of the universe; they are known as quarks and 18 different types of quark have been identified. Further discussion regarding this search to identify whether there is a smaller structure within the quark is well outside the scope of this text.

18.3 The atomic nucleus

The number of protons and neutrons in the atomic nucleus determines both the mass and the charge of the nucleus and the configuration of electron orbitals of the atom. There are several important terms, which we will use in this and in following chapters of this text, that require definition at this stage. These terms, which will help us to understand atomic structure, are defined in Table 18.2.

Table 18.2 Terms used to describe a nucleus

TERM	SYMBOL	DEFINITION
Nucleon		A proton or neutron within a nucleus
Atomic number	Z	The number of protons in the nucleus
Atomic mass number	A	The total number of nucleons in the nucleus
Neutron number	N	The number of neutrons within the nucleus
Nuclide		A nucleus with a specific value of Z and A
Element	E	A nucleus with a given value of Z
Isotope (of an element)		Any nucleus which contains the same number of protons as the given nucleus but has a different mass number
Isobar		Any nucleus which has the same atomic mass number as another nucleus (i.e. has the same value of A)
Radionuclide or radioisotope		Any nuclide or isotope which is radioactive

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Tags: Principles and Applications of Radiological Physics

Mar 6, 2016 | Posted by admin in GENERAL RADIOLOGY | Comments Off

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