RMI 213 Principles of Medical Imaging Lecture 3 PDF

RMI 213 Principles of medical imaging Lecture 3 The Structure of Matter Slide 1 fchs.ac.ae Learning Outcomes At the conclusion of this lecture, associated tutorial and practical session (if relevant), you will be able to: 1. Relate the history – t...

RMI 213 Principles of medical imaging Lecture 3 The Structure of Matter Slide 1 fchs.ac.ae Learning Outcomes At the conclusion of this lecture, associated tutorial and practical session (if relevant), you will be able to: 1. Relate the history – the evolution – of model of the atom 2. Describe the currently accepted structure of the atom 3. Describe electron shells, stability and instability within atomic structure 4. Discuss radioactivity and describe the characteristics of alpha and beta particles 5. Explain the differences between the particulate form and the electromagnetic form of ionizing radiation Slide 2 fchs.ac.ae Prescribed Text Bushong, S.C., Radiologic Science for Technologists, 10th edition, Mosby/Elsevier; St Louis, 2012, pages 26-43. Notes: 1. Each lecture in this course will relate very closely to a specific set of pages in the above text. It is strongly recommended that students read the pages indicated prior to coming to the lecture. 2. The students outcomes listed at the commencement of each lecture are essentially those found in the prescribed text for the relevant chapter. Slide 3 fchs.ac.ae Atomic Species The atom is the basic ‘building block’ of matter An atom is the smallest particle that has all the properties of an element The Greeks believed there were 4 elements; represented in the diagram opposite We now know there are about 118 elements 26 of these are artificially produced Bushong, Figure 2-2, page 28 92 are naturally occurring Mendeleev arranged these elements in what, today, we know as the Periodic Table (dates from the about 1860s) The Bohr atom is a reasonable approximation of a real atom Slide 4 fchs.ac.ae The Periodic Table http://chemicool.com/ Slide 5 fchs.ac.ae Slide 6 fchs.ac.ae Bohr Model of the Atom The Bohr model of the atom is a quantum model (1913) Electrons exist in stable orbits with fixed energies The electron extent defines the volume of the atom Electrons travel in defined circular orbits around the nucleus. Electrons are negatively - charged At the center is the positively-charged nucleus The nucleus provides the mass of the atom The nucleus comprises protons and neutrons Protons are positively + charged Neutrons are uncharged Slide 7 fchs.ac.ae Atomic Particles Electron Charge: qe = –1.6  10-19 C Mass: me = 9.1  10-31 kg Proton Charge: qp = +1.6  10-19 C Mass: mp = 1.673  10-27 kg Neutron Charge: qn = 0 Mass: mn = 1.675  10-27 kg Bushong Figure 2-5, page 30 Slide 8 fchs.ac.ae z Slide 9 fchs.ac.ae Atomic Mass Protons and neutrons together are referred to as nucleons The atomic mass unit (amu) 1 amu = 1/12 mass of C-12 atom me = 0.000549 amu mp = 1.00728 amu mn = 1.00867 amu Atomic mass number A (an approximate mass) This is essentially the number of nucleons For the neutron and the proton, A = 1 For the electron, A = 0 Slide 10 fchs.ac.ae Atomic Structure The atom is virtually empty space The atomic number Z is the number of protons; Np = Z for a neutral atom, Z is the number of electrons Z determines the chemistry of the atom The number of neutrons is Nn = N The number of nucleons is N + P The atomic mass is normally given in amu Isotopes are atomic species with same Z but different N Slide 11 fchs.ac.ae Atomic Structure Electrons exist in stable orbits, grouped in shells Shells are labeled K, L, M, etc., from that closest to the nucleus Electron binding energy, BEe , is the energy required to ionize the atom – to remove an electron K shell can contain a maximum of 2 electrons L shell can contain a maximum of 8 electrons M shell can contain a maximum of 18 electrons N shell can contain a maximum of 32 electrons Closed (filled) shells represent very stable (un-reactive) atoms Slide 12 fchs.ac.ae Electron Shells Ionization is the removal of an electron or the addition of an electron to the neutral atom X-rays are able to ionize atoms, hence the term ionizing radiation BEes are around 5 eV and upwards N is generally slightly greater than Z, but there are exceptions Bushong Figure 2-6, page 31 Slide 13 fchs.ac.ae Ionization Ionization of a carbon atom by an x-ray leaves the atom with a net electric charge of +1 The ionized atom and the released electron are called an ion pair Bushong Figure 2-7, page 32 Slide 14 fchs.ac.ae Electron Orbit Electrons go around the nucleus in fixed orbits Electrons are attracted to the positively charged nucleus Although accelerating, electrons exists in stable orbits +Z –1 speed Monash University Department of Medical Imaging and Radiation Sciences Slide 15 fchs.ac.ae Atomic Nomenclature An alphabetic abbreviation is used for the chemical symbol This also defines Z Two systems are used – take the example of barium, Ba: 135Ba56 The subscript, Z = 56, is really redundant (why?) The superscript is the atomic mass number, A = N + Z From this we see that N = 135 – 56 = 79 The actual atomic mass of this element is 134.91 amu There are other isotopes of barium An alternate notation is Ba-135 (easier to write) Slide 16 fchs.ac.ae Atomic Nomenclature A more complete notation can also be used: Atomic mass number Valence state A +/– Chemical symbol Z X # Atomic number Number of neutrons In the alternate notation X–n (e.g. Ba-135), n is the atomic mass number, X leads you to the atomic number, Z. Slide 17 fchs.ac.ae Some Elements Some elements are important in a study of radiologic science; K-shell binding energies are particularly important in determining x-ray energies. Bushong Table 2-3, page 35 Slide 18 fchs.ac.ae Nomenclature with A, Z and N Isotopes are atomic nuclei with the same Z but different N Example: I-130 and I-131 (I = iodine) Isobars are atomic nuclei with the same A but different Z Example: I-131 and Xe-131 (Xe = xenon) Isotones are atomic nuclei with the same N but different A Example: I-130 and Xe-131 Isomers are atomic nuclei with the same Z and same A They have different nuclear energy states Example: Tc-99 and Tc-99m (Tc = technetium, the ‘m’ denotes an ‘excited’ nuclear state) Slide 19 fchs.ac.ae Combinations of Atoms A molecule is a collection of atoms with a fixed ratio of elemental species in a fixed arrangement Na + Cl → NaCl Here NaCl is a molecule (common salt in this example) The structure depends on the type of bonding Types of bonds include Covalent (think of some examples) Ionic (think of some examples) Compounds Are a quantity of matter of the one molecule Slide 20 fchs.ac.ae Summary Check that you can satisfy the learning outcomes for this lecture Go over calculations/exercises undertaken during the lecture Make sure you can define the following terms: electron, proton, neutron atomic number, mass number, atomic mass unit electron arrangement in the atom arrangement of the Periodic Table radioactivity and half-life alpha-particle and beta-particles (two types) Slide 21 fchs.ac.ae