RSMI 2130 Structure of Matter Lecture Notes PDF

RSMI 2130 Structure of Matter Dr. Nahla Atallah  Definition of physics Physics is a science of dealing with the most general properties and forms of motion of matter. The term matter describes all of the physical substances around us that has mass and takes up space...

RSMI 2130 Structure of Matter Dr. Nahla Atallah  Definition of physics Physics is a science of dealing with the most general properties and forms of motion of matter. The term matter describes all of the physical substances around us that has mass and takes up space The Universe is made up of matter and energy  History of the atom Greek Atom. Scientists at that time thought that all matter was composed of four substances: earth, water, air, and fire. According to them, all matter could be described as combinations of these four basic substances in various proportions, modified by four basic essences: wet, dry, hot, and cold. Figure 2-2 shows how this theory of matter was represented at that time The Greeks used the term atom, meaning “indivisible” to describe the smallest part of the four substances of matter. Each type of atom was represented by a symbol (Figure 2-3A). Radiation Oncology Physics: A Handbook for Teachers and Students - 14. Radiation Oncology Physics: A Handbook for Teachers and Students - 14.  Dalton Atom In 1808 John Dalton, an English schoolteacher, published a book summarizing his experiments, which showed that the elements could be classified according to integral values of atomic mass. According to Dalton, an element was composed of identical atoms that reacted the same way chemically. For example, all oxygen atoms were alike. (see Figure 2-3B)  Thomson Atom In the late 1890s while investigating the physical properties of cathode rays (electrons), J.J. Thomson concluded that electrons were an integral part of all. atoms. He described the atom as looking something like a plum pudding, in which the plums represented negative electric charges (electrons) and the pudding was a shapeless mass of uniform positive electrification (see Figure 2-3C). The number of electrons was thought to equal the quantity of positive electrification because the atom was known to be electrically neutral.  Rutherford’s model of the atom Rutherford's model shows that an atom is mostly empty space, with electrons orbiting a fixed, positively charged nucleus in set.  Bohr Atom In 1913 Niels Bohr improved Rutherford’s description of the atom. Bohr’s model was a miniature solar system in which the electrons revolved about the nucleus in prescribed orbits or energy levels. For our purposes, the Bohr atom (see Figure 2-3D) represents the best way to picture the atom, although the details of atomic structure are more accurately described by a newer model, called quantum chromodynamics (QCD).  Bohr Atom Simply put, the Bohr atom contains a small, dense, positively charged nucleus surrounded by negatively charged electrons that revolve in fixed, well-defined. Bohr : Electrons move in definite orbits around the nucleus The Atom The atom consists of two parts: 1. The nucleus which contains: protons neutrons 2. Orbiting electrons.  Atoms in nature are electrically neutral so: The number of electrons orbiting the nucleus equals the number of protons in the nucleus.  The nucleus of an atom is made up of tightly bound protons and neutrons, which are called nucleons.  The nucleus contains most of the atomic mass. Protons  Much larger and heavier than electrons  Protons have a positive charge (+)  Located in the nucleus of the atom Neutrons  Large and heavy like protons  Neutrons have no electrical charge  Located in the nucleus of the atom Electrons  Tiny, very light particles  Have a negative electrical charge (-)  Move around the outside of the nucleus in shells Fig1.2.  The electrons are maintained in their orbits around the nucleus by two opposing forces. The first of these, known as electrostatic force, is the attraction between the negative electrons and the positive protons. This attraction causes the electrons to be pulled toward the protons in the nucleus. In order to keep the electrons from dropping into the nucleus, the other force, known as centrifugal force, pulls the electrons away. The balance between these two forces keeps the electrons in orbit. Centrifugal force pulls the electrons away from the nucleus. electrostatic force, is the attraction between the negative electrons and the positive protons.  Electrostatic force is the attraction between the positive protons and negative electrons. Electrons in the orbit closest to the nucleus (the K-shell) will have a greater electrostatic force than will electrons in orbits further from the nucleus. The higher the atomic number of an atom (more protons), the higher the electrostatic force will be for all electrons in that atom.  A Z X A = number of protons + number of neutrons Z = number of protons N = A – Z = number of neutrons Number of neutrons = Mass Number – Atomic Number  Each shell is assigned a principal quantum number (n), beginning with one for the K-shell, two for the L-shell, and so on. The number of electrons each shell can contain is 2n2. Electrons are arranged in Energy Levels or Shells around the nucleus of an atom. first shell K a maximum of 2 electrons second shell L a maximum of 8 electrons third shell M a maximum of 18 electrons  Electron binding energy  The work that is required to remove an electron from an atom is called the electron binding energy.  The binding energy of outer-shell electrons is smaller than the binding energy of the inner-shell. Energetic particles can knock out inner-shell electrons only if their energy is equal to or greater than the electron binding energy.  A vacancy in the K-shell will be filled by an electron from a higher shell.  Electrons moving from an outer shell to an inner shell may emit excess energy as electromagnetic radiation. Electromagnetic radiation represents a transverse wave, in which the electric and magnetic fields oscillate perpendicular to the direction of the wave motion.  Wavelength (λ) is the distance between successive crests of waves.  Amplitude is the intensity defined by the height of the wave.  Frequency (f) is the number of wave oscillations per unit of time expressed in cycles per second, or in hertz (Hz).  The period is the time required for one wavelength to pass (1/f). X-rays are an example of electromagnetic radiation.  The product of the wavelength (λ) and frequency (f) of electromagnetic radiation is equal to the speed of light c (c = f.λ ).  Electromagnetic radiation is quantized, meaning that it exists in discrete quantities of energy called photons.  Photons may behave as waves or particles but have no mass.  Summary Because the atom contains all the neutrons and protons, the nucleus of the atom contains most of its mass. For example, the nucleus of a uranium atom contains 99.998% of the entire mass of the atom. The atom is essentially empty space. Possible electron orbits are grouped into different “ shells.” The arrangement of these shells helps reveal how an atom reacts chemically, that is, how it combines with other atoms to form molecules.  A neutral atom has the same number of electrons in orbit as protons in the nucleus. The number of protons determines the chemical element. Atoms that have the same number of protons but differ in the number of neutrons are isotopes. Electrons can exist only in certain shells, which represent different electron binding energies. For identification purposes, electron orbital shells are given the codes K, L, M, N, and so forth, to represent the relative binding energies of electrons from closest to the nucleus to farthest from the nucleus.  The closer an electron is to the nucleus, the greater is its binding energy. The electron distribution is as follows: 2 in the K shell, 8 in the L shell, 18 in the M shell, 32 in the N shell, 21 in the O shell, 9 in the P shell, and 2 in the Q shell. In their normal state atoms are electrically neutral; the electric charge on the atom is zero. The total number of electrons in the orbital shells is exactly equal to the number of protons in the nucleus.  References 1447_Radiologic_Science_for_Technologists. Diagnostic Radiology Physics: A Handbook for Teachers and Students.