UNIT 4 ATOMIC STRUCTURE

Transcription

1 PHYSICS AND CHEMISTRY UNIT 4 ATOMIC STRUCTURE Arantza Rubio Mier 3º ESO

2 INDEX 1. DALTON S ATOMIC THEORY ATOMIC MODELS Thomson s atomic model Rutherford s atomic model Bohr s atomic model DISTRIBUTION OF ELECTRONS ATOMIC NUMBER. MASS NUMBER. ISOTOPES. IONS RADIOACTIVITY Radioisotopes Applications of radioisotopes NUCLEAR ENERGY Nuclear fission Nuclear fusion...12 ACTIVITIES...13 Arantza Rubio Mier Page 2

3 1. DALTON S ATOMIC THEORY In 1803, Dalton proposed a modern atomic model based on experimentation. This theory can be summarized in: a) All matter is made of atoms. Atoms are indivisible and indestructible b) All atoms of a given element are identical in mass and properties c) Each element has different atoms d) Compounds are formed by a combination of two or more different kinds of atoms e) A chemical reaction involves a rearrangement of atoms, not a change in the atoms themselves Dalton s atomic theory is not completely true but it was one of the greatest developments in Chemistry. This model could explain all the laws of chemistry and physics (with the exception of electricity, radioactivity and the emission of waves from atoms). The knowledge of atoms existence allows chemists to discover new elements, their atomic masses, the Periodic Table and the formula of a lot of compounds. It is very important to take into account that these assumptions of Dalton ś atomic theory must be reformulated. First of all, atoms are not indivisible, because they have smaller particles, such as protons or electrons. Secondly, we can find different atoms of the same element which have different masses (isotopes), but the average mass of these atoms is always the same because of the great number of atoms we can find in a sample. Arantza Rubio Mier Page 3

4 2. ATOMIC MODELS First evidence of the electric properties of matter was made by Thales of Miletus, a Greek scientist. He recorded that glass or amber attracts little pieces of paper when it is rubbed with wool or silk. The attractions can be explained by a motion of electric charges from glass to wool or vice versa. In 1831, Michael Faraday carried out electrolysis experiments, in which a chemical reaction was produced by an electric current. The amount of products collected was proportional to the amount of charge which had passed through the circuit. Consequently there must be a link between matter and electric charge. The atom has to be made of different particles, so it is not indivisible (as Dalton thought). In 1897, Thomson showed by an experiment that there were small charged particles inside the atoms. Later, Millikan calculated the charge of this particle, - 1, C. Today these particles are called electrons. In1886, Goldstein showed that there were some positive particles with the same charge than electrons. They were called protons Thomson s atomic model It was proposed en In this model, the atom can be considered as a uniform positively charged sphere in which electrons are embedded. The amount of positive and negative charge is the same, so the atom is electrically balanced. This model is also called the plum pudding model However, this model of the atom had to be abandoned, because new experiments showed that at least most of the positive charge was concentrated in the centre of the atom, not over the entire volume Rutherford s atomic model In 1911, the physicists Ernest Rutherford carried out an experiment in which he showed the inner structure of atoms. He decided to shoot alpha particles, taken from a radioactive sample, at a thin gold foil. The alpha particles are positively charged and they were expected to be scattered at very low angles but, surprisingly, a few particles were detected at large angles, even at 180º The results of the experiment were: Most of the alpha particles did pass through as expected Arantza Rubio Mier Page 4

5 Many others (between 1 or 2 %) were deflected at small angles A few (1:105) were reflected back to the alpha source through angles much larger than 90 degrees. Rutherford explained the results of the experiment proposing a new atom model according to these assumptions: Atoms are divided into a tiny core, called nucleus, and an electron shell. Most of the atom must be empty space Nucleus of an atom is positively charged and contains almost the mass of the atom. Now we know that the nucleus contains protons and neutrons Electrons orbit around the nucleus in circular orbits. Nucleus holds the electrons by means of an electric force between protons and electrons Some years later, in 1912, Chadwick discovered the neutrons. These particles do not have any electric charge and their mass is a little bit bigger that the mass of the proton. These particles are in the nucleus with the protons. Properties of electrons, protons and neutrons are summarized in the following table. These particles are tiny so a new unit is used (unidad de masa atómica, u, or atomic mass unit, amu) 1 u = 1, kg 1 e = 1, C Arantza Rubio Mier Page 5

6 2.3. Bohr s atomic model According to Rutherford s model, the electron moves continuously around the nucleus. This is a problem, because it is known that, if a charged particle moves around the nucleus, it should emit energy in the form of radiation. In this case, the electron would lose energy continuously and it would finally fall on the nucleus. But this does not happen, so the conclusion is that there must be something that Rutherford did not take into account. In 1913, Niels Bohr proposed his model. It was a modification of the earlier Rutherford model. It contains some errors, but it is important because it describes most of the accepted features of atomic theory without the mathematical difficulties of the modern model. Bohr supposed: a) Electrons move in circular orbits around the atomic nucleus b) Only certain orbits are permitted c) In these permitted orbits, the electrons would not radiate (would not create radio waves) d) Radiation is absorbed or emitted when an electron moves from one orbit to another. Light of certain colours would be created when the electron jumps form one orbit to the other one. 3. DISTRIBUTION OF ELECTRONS Electrons are in several energy levels (orbits). Each level contains 2n 2 electrons Level (n) Number of electrons (2n 2 ) Arantza Rubio Mier Page 6

7 There are some rules about the number of electrons in certain levels: if the third level (n=3) has 8 electrons, the fourth level starts filling even though the third level accepts 18 electrons. The distribution of electrons is known as electronic configuration. For example, the electronic configuration of an element with 12 electrons is ATOMIC NUMBER. MASS NUMBER. ISOTOPES. IONS As we have explained before, atoms consist of smaller particles, protons, neutrons and electrons. Protons are positive particles which have a unit of atomic mass. Neutrons are neutral and their mass is the same as that of the protons. Electrons are lighter, their mass is negligible compared with the mass of protons, but they are negatively charged and their charge is the same as protons. a) Atomic number (Z): it is the number of protons in an element. It serves to identify elements. It is represented as a subscript before the symbol of the element, 6 C. For example, atomic number of carbon is 6, so it means that every atom of carbon has 6 protons. If the atom in neutral, the number of electrons has to be the same, so it would have 6 electrons. b) Mass number (A): it is the sum of the number of protons and neutrons of a nucleus. It indicates the atomic mass of the element. It is represented as a superscript before the symbol of the element. A = Z + N Example: 37 17Cl has a mass number of 37. Its nucleus contains 17 protons, 17 electrons and = 20 neutrons Arantza Rubio Mier Page 7

8 c) Isotopes: atoms with the same number of protons but different number of neutrons (the same Z, but different A). They are different forms of a single element. Example: carbon-12 ( 12 6C) and carbon-14 ( 14 6C) are both isotopes of carbon. The first one has 6 neutrons and the other one has 8 neutrons The atom of hydrogen (Z =1) has three isotopes: 1 1H, 2 1H, 3 1H d) Ion: An atom or molecule which has gained or lost one or more of its electrons, giving it a net positive or negative electrical charge. There are two types of ions: - Cation: it has lost electrons, so the charge is positive (number of electrons < number of protons) - Anion: it has gained electrons, so the charge is negative (number of electrons > number of protons) e) Atomic mass of an element: It is the mass of one atom and it is almost the addition of the number of protons and neutrons (because the mass of electrons is so small that it can be ignored). The unit used to measure the atomic mass is the atomic mass unit (amu) and an isotope of 12-C is used as a pattern. It has been established that its mass is 12 amu (because it has 6 protons and 6 neutrons). So, the atomic mass unit is the twelfth part of the mass of the 12-C Example: when we say that the atomic mass of oxygen is 16, we are saying that the atom of oxygen has 16 times more mass than the twelfth part of the mass of the atom of 12-C Arantza Rubio Mier Page 8

9 Most of the elements have different isotopes and this has to be taken into account to calculate the atomic mass of the element. It is calculated by multiplying each isotope s mass by its abundance. If the abundance is a percent, you have to divide your answer by 100. For this reason, atomic masses are often decimal numbers 5. RADIOACTIVITY In 1895, Henry Becquerel discovered the emission of an unknown radiation by uranium salts in his laboratory. Radioactivity is a process whereby unstable atomic nuclei release energy or particles and become more stable. The energy and particles which are released during the decomposition process are called radiation. There are three types of radiation: a) Alpha radiation (α): It consists of 2 protons and 2 neutrons, so its mass number is 4 and it has positive charge. It is very dangerous because of its ionising properties, but it can be stopped by a thin piece of paper or human skin b) Beta radiation, β: It consists of electrons that can travel through the atmosphere across longer distances. It can be absorbed by a PVC barrier or a thick metal layer c) Gamma radiation, γ: It consists of electromagnetic waves of high frequency. It has no charge. Although it is not so ionising radiation, it Arantza Rubio Mier Page 9

10 can travel very long distances and it can be stopped just by a concrete barrier or a thick lead layer Radioisotopes Radioisotopes are radioactive isotopes of an element. For example, 14 C is a radioactive isotope of 12 C. They are not always harmful. They have different uses in different areas such as medicine, chemistry, biology, archaeology, agriculture, industry and engineering Applications of radioisotopes a) Determination of the age of minerals and rocks and archaeological objects: many minerals contain radioactive isotopes. The age of any of these minerals can be determined by counting the isotopes in the mineral and using the known decay rate to calculate the time required in that process. Example: radiocarbon dating (prueba del carbono 14) Arantza Rubio Mier Page 10

11 b) Medicine: Radioactive isotopes can be used in diagnosis and therapy of different illness, such as cancer. The radioisotope is dissolved into a substance, so that it can travel to the organ that we want to study. Example: 131- iodine is used for the treatment of thyroid cancer and 132- iodine is used for the diagnosis of the same cancer. c) Industry: X- rays to examine steel plates, welding and construction d) Others: Auxiliary generator for satellites, precision clocks, sterilization of agricultural pests, etc. 6. NUCLEAR ENERGY Nuclear energy comes from nuclear fission and fusion Nuclear fission A heavy nucleus (uranium, plutonium, etc ) splits into several smaller fragments. These fragments, or fission products, are about equal to half the original mass. Two or three neutrons are also emitted. These neutrons can break other nucleus. This is called chain reaction. In this process, a big amount of energy is released. Applications: the nuclear energy that is produced in a nuclear reactor is used to produce electricity Uranium is used in nuclear plants. The energy that is released is used to heat up the water, so it becomes a gas. The gas moves the turbines which are connected to a generator and it produces electricity. Advantages: Small amount of combustible produces a great amount of energy: 1 kg of uranium produces as much energy as 2000 tons of oil Disadvantages: There is risk of radioactive contamination and a great amount of wastes are produced. These wastes are very difficult to eliminate. Arantza Rubio Mier Page 11

12 6.2. Nuclear fusion It joins two light elements (usually hydrogen), creating a heavier element (helium) and releasing a tremendous amount of energy in the process. Applications: It has been used as a bomb. Nowadays, its peaceful use is being researched Advantages: The amount of energy obtained is bigger than in the nuclear fission. There are large reserves of energy (hydrogen from ocean water) and it almost does not produce waste or polluting substances Disadvantages: It is necessary an extremely high temperature (about million of degrees) to start the nuclear reaction. Magnetic fields are used to keep this matter in a determine space. Still no nuclear reactor has been designed Arantza Rubio Mier Page 12

13 ACTIVITIES ACTIVITY 1- Explain if the following sentence is true or false: A neutral atom is the one that has charges ACTIVITY 2- Choose the best option. The charge of the electron is: a) Bigger than proton s charge b) Smaller than proton s charge c) The same as the proton s charge ACTIVITY 3- If a fluoride atom wins an electron, which is its charge? ACTIVITY 4- Which are the differences between atomic number and mass number? ACTIVITY 5- When we write the symbol of an element, where do we have to write atomic and mass numbers? Write an example ACTIVITY 6- What is an isotope? Write an example ACTIVITY 7- Knowing the data of these atoms: A B C D 13 protons 14 protons 13 neutrons 15 neutrons 13 protons 14 neutrons a) Which are different isotopes of the same element? b) Which are the same isotopes? c) Are B and C atoms of the same element? ACTIVITY 8- Complete the following table: 14 protons 15 neutrons Isotope Element Z A Protons Electrons Neutrons 9 4 Be 16 8 O 18 8 O Mg Mg U U ACTIVITY 9- An imaginary element has two isotopes, A and B. Their atomic masses are 78 and 80. Which is the atomic mass of the element if they have the same abundance? ACTIVITY 10- Calculate the atomic mass of lithium if it has a 7,42% of an 6 Li and a 92,58% of 7 Li Arantza Rubio Mier Page 13

14 ACTIVITY 11- Write the electronic configuration of these elements: a) Phosphorus, P (Z =15) b) Chlorine, Cl (Z= 17) c) Magnesium, Mg (Z= 12) d) Calcium, Ca (Z= 20) e) Lithium, Li (Z=3) ACTIVITY 12- Complete the following table: Element Z A Protons Electrons Neutrons Configuration Na Si Ca Ne Mg Be 9 4 ACTIVITY 13- Complete the following table: Ion Z A Protons Electrons Neutrons Charge 16 8 O 2- Al Cl P Li ACTIVITY 14- Draw the following atoms: 9 4 Be, 31 15P, 20 10Ne, 4 2He, 1 1H, 16 8O, 16 8O 2-, 24 12Mg, 24 12Mg 2+ ACTIVITY 15- Indicate the number of protons, neutrons and electrons and the electric charge. Is there any isotope? 1 1 H, 17 8O, 60 28Ni, 26 13Al 3+, 16 8O, 29 14Si, 35 17Cl -, 2 1H ACTIVITY 16- Write the suitable type of radiation in each case: a) They have no electric charge b) It consists of 2 protons and 2 electrons c) It has electrons d) It can just be stopped by a concrete barrier e) It can be stopped by a the human skin f) It can be stopped by a thick metal layer Arantza Rubio Mier Page 14