3 Atomic Structure 15

3 Atomic Structure 15

3.1 Atoms You need to be familiar with the terms in italics The diameter of the nucleus is approximately 10-15 m and an atom 10-10 m. All matter consists of atoms. An atom can be thought of as having a central nucleus surrounded by a cloud of particles called electrons. The nucleus contains a number of particles called protons and neutrons, together termed nucleons. Each nucleon is nearly 2000 times the mass of an electron. This means the mass of an atom is concentrated in its nucleus, around which the much lighter electrons orbit. If a nucleus was scaled up to the size of an orange, the electrons would be orbiting ¾ of a mile away! 3.2 Atomic number (symbol: Z) see GRAHAM DT (1996) Principles of Radiological Physics (3 rd ed) Section 26.5 The number of protons in the nucleus of an atom, its atomic number, determines the type of element eg a nucleus with just one proton is hydrogen, which has an atomic number of one. A nucleus with 8 protons is oxygen, atomic number 8. A nucleus with 29 protons is copper, atomic number 29. The names of all the different types of elements and their atomic numbers can be found in the Periodic Table. 3.3 Mass number (symbol: A) The mass number of a nucleus gives the total number of nucleons in the nucleus ie the number of protons plus the number of neutrons. If you subtract the atomic number from the mass number you will end up with the number of neutrons in the nucleus. For example, a nucleus of carbon may have an atomic number of 6 - so 6 protons - and a mass number of 12. Subtracting 6 from 12 means there must be 6 neutrons as well as 6 protons in the nucleus. A nucleus of sodium has an atomic number of 11 and a mass number of 23. So it has 11 protons and 12 neutrons. 16

naturally or artificially. For example, a nucleus of carbon (atomic number 6) may have a mass number of 11, 12, 13 or 14, with 5, 6, 7 or 8 neutrons. Each is called an isotope of carbon. Some isotopes are stable and some are radioactive, depending on the ratio of protons to neutrons. 3.5 Chemical symbols An element can be represented by its chemical symbol with its atomic number as a subscript and its mass number as a superscript. An element identified in this way is known as a nuclide. For example: mass number 63 atomic number 29 Cu chemical symbol Check your answers in section 3.11 Look up the symbols in Graham (1996) if you are not sure of the names of the elements 3.4 es Isotop 3.6 Check you are familiar with the terminology: i) The nucleons in a nucleus of an atom are called? and? ii) iii) iv) The number of protons in the nucleus of an atom is known as? the total number of nucleons in the nucleus of an atom is known as? To calculate the number of neutrons in a nucleus, you subtract? from? v) Nuclei with the same atomic number but different mass numbers are called? vi) How many protons and neutrons are in the following nuclei? 14 138 208 N Ba Pb 7 56 82 The number of neutrons in a nucleus can vary, either occurring 17

The maximum number of electrons in a shell is 2n 2, where n = number of shell and K = 1 Electrons are the small, light particles orbiting the nucleus of an atom. They are arranged in shells, called K, L, M, N etc. Normally, the number of electrons orbiting the nucleus equals the number of protons in the nucleus. The number of electrons in each shell follows certain rules: for example, the maximum number in the K shell is 2, the L shell 8 and the M shell 18. However, the outermost shell (valence shell) may only hold 8 electrons. This shell determines the chemical, thermal, optical and electrical properties of the atom. Copper for example has an atomic number of 29. There are 2 electrons in the K shell, 8 in the L shell, 18 in the M shell and 1 in the N shell. This single valence electron easily leaves the atom and acts as a free electron. Hence copper is a good conductor of heat and electricity. 3.8 Electric charge We can use the shorthand +ve and -ve Protons and electrons carry an electric charge, meaning they exert a force on each other similar to the North and South poles of magnets. Protons are said to carry a positive electric charge and electrons a negative electric charge. The number of positivelycharged protons in the nucleus of an atom normally balances the number of negatively-charged electrons orbiting the nucleus. WHAT HAPPENS WHEN YOU BRING THE NORTH POLES OF TWO MAGNETS NEAR TO EACH OTHER? WHAT HAPPENS WHEN YOU BRING A NORTH AND A SOUTH POLE NEAR TO EACH OTHER? 3.7 ns Electro In the same way as like magnetic poles repel each other due to the magnetic force of repulsion, like charges repel each other due to the electrical force of repulsion. Unlike magnetic poles attract each other in the same way as unlike charges attract each other. 18

positive charges in an atom cancel each other out, so the atom is electrically stable. If an electron is removed from the atom, the positive protons will outnumber the remaining negative electrons, leaving the atom with a net positive charge. The ionised atom is known as a positive ion. When an electron is removed from a shell, a certain amount of energy is used to overcome the attractive force of the protons in the nucleus: this is called the binding energy. The more protons in the nucleus, the higher the binding energy required to remove an electron. The closer the shell to the nucleus, the higher the binding energy. Z is the symbol for atomic number. See section 3.2 Table 3: Binding energies for tungsten (Z = 74) kev are kiloelectronvolts: units of energy. See section 1.3 Add up the total number of electrons to check they balance the number of protons in the tungsten nucleus kinetic energy is energy of a moving object. See section 1.1 Ionising Radiation warning symbol shell binding energy (kev) no. of electrons K 70 2 L 11 8 M 2 18 N 0.6 32 O 0.07 12 P 0.02 2 Atoms may be ionised by charged particles such as free electrons: kinetic energy carried by the free electron is transferred to overcome a bound electron s binding energy. This is part of the process of x-ray production in the x-ray tube. Atoms may also be ionised by x-ray photons: the energy of the x-ray is used to overcome the binding energy of one of the electrons. If atoms making up important structures in the body, such as DNA, are ionised, chemical changes occur which may cause permanent damage. This is why x- rays can be dangerous. 3.9 ion Ionisat Normally the negative and 19

atomic particles recognise chemical symbols and identify atomic and mass numbers define the term isotope describe the basic properties of electric charges explain what is meant by ionisation 3.11 Answers to Section 3.6 i) protons and neutrons ii) atomic number iii) mass number iv) subtract atomic number from mass number v) isotopes vi) nitrogen (N) has 7 protons and 7 neutrons barium (Ba) has 56 protons and 82 neutrons lead (Pb) has 82 protons and 126 neutrons 3.10 Check that you can: name the principle 20