The fragments of a stone broken in two, even if reduced to a powder, are still a stone, and every resulting fragment can

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1 NAME: LEVEL: ACTIVITY SHEETS PHYSICS AND CHEMISTRY 3º ESO LESSON 4 THE STRUCTURE OF THE ATOM The fragments a stone broken in two, even if reduced to a powder, are still a stone, and every resulting fragment can be divided again Can those divisions and subdivisions go on forever? Some ancient Greek philosophers asked themselves that question and started a chain speculations that, nowadays, are still concerning the scientists, twenty five centuries latter The Greek philosopher Leucipus (490 BC) is the first person whose name we know which tackled the problem dividing the matter, and came to the conclusion that the process could not go on forever A younger man, Democritus ( BC), Leucipus disciple, accepted the idea some fragments matter, so tiny that they were indivisible He called atoms to those fragments The meaning the word atom in Greek is indivisible For Democritus, any kind matter was made up atoms, and if there were space in among the atoms, this space did not have anything but vacuum ACTIVITY 1: The Dalton model the atom 1-How many types atoms are there in the diagram above? 2-According to Dalton What is an atom? 3-What is the difference between an atom and another atom? 4-What is the difference between molecule and atom? 5-Dalton thought that atoms were indivisible spheres What do you think about this? 6-What is the difference between the molecules hydrogen and oxygen and the molecules water and sulphuric acid? 7-Dalton explained correctly the difference between an chemical element and a chemical compound What is it? The conservation mass in a chemical reaction and the Dalton model the atom 1

2 READING 1: The Thomson model the atom (1904) The English physicist J J Thomson discovered the electron in 1879 when investigating the nature cathode rays produced inside the vacuum glass tube The cathode rays were jets very tiny particles negatively charged that were called electrons The negative electrified electrons were so tiny that Thomson guessed that they were enclosed in a much bigger positively charged sphere Thompson s model was compared to a British dessert called plum pudding, hence the name plum pudding model the atom (negatively charged raisings surrounded by positively charged pudding ) READING 2: The Rutherford model the atom (1911) The Thomson model the atom was accepted for a short time Four years after this model was published, a new experiment, carry out by Ernest Rutherford and other physicians, disproved it It is called the gold foil experiment Positively charged alpha particles where fired at a very thin sheet gold foil Most the particles passed through the foil without suffering any deflection and some the particles suffered little deflections Scientist new this because they had set up around the foil a detecting screen However, what surprised Rutherford was that a few alpha particles were reflected back to the alpha source, as if they bumped against something dense, hard, and bounced back On considering, this fact was the result a single collision, and when calculations were made, Rutherford take a model in which the greater part the mass the atom was concentrated in a minute nucleus, positively charged, and moving around the nucleus were the negatively charged electrons According to this model, the atom is essentially an empty space The unexpected results the experiment demonstrated for the first time the existence the atomic nucleus, leading the downfall the plum pudding model the atom, and the development the Rutherford (nuclear) model Structure the atom At the centre the atom is a tiny nucleus made up protons and neutrons Outside the nucleus and very far away from it, the electrons move around forming the electric shell Most the atom is empty space! Nearly all the mass the atom is in the nucleus, because the mass the electrons is negligible compared to the mass protons and neutrons The protons have a positive charge and the electrons have a negative charge Neutrons do not have a charge An atom has not an overall charge; it is electrically neutral This is because the number positive protons is equal to the number negative electrons ACTIVITY 2: Atomic models 1-Explain why the scientists considered insufficient the Dalton model the atom 2-Why the Thomson model is called the plum pudding model? What do the raisings represent? 3-Why do we know that the atom is essentially a hollow structure? 4-Why did Rutherford suggest that the atom should have a nucleus? 5-What type charge do the different subatomic particles have? 6-Explain why the atom is electrically neutral 7-Determine which the subatomic particles (electron, proton y neutron) is related to sentences below a) It has a positive electric charge d) It is in the nucleus the atom b) It has the tiniest mass e) It moves around the nucleus at full speed c) It does not have any electric charge f) It has negative electric charge 2

3 READING 3: The Bohr model the atom (1913) The Danish physicist Niels Bohr accepted the Rutherford nuclear model the atom and besides explained the structure the electron shell You already know that at the centre the atom is a tiny nucleus, made up protons and neutrons The general name given to protons and neutrons is nucleons because they are found in the nucleus The number protons plus neutrons is called the mass number (A, also called nucleon number) What Bohr said was that outside the nucleus, the electrons move around in circular trajectories called electron shells or energy levels The shells are at a certain distance from the nucleus There can be several electron shells further and further away from the nucleus Each these can hold a limited number electrons (as much as 2, 8, 18, 2n 2, where n is the level, 1, 2, 3 ) Isotopes Every element has its own proton number it has a particular number protons For instance, oxygen is oxygen because it has eight protons If it had nine protons, it would not be oxygen any more but another element called fluorine Therefore, knowing the number protons, that is the atomic number (Z), we can identify the name the element The atom represented above is sodium, which symbol is Na You can see that Z = 11 and A = 23, therefore, this atom has 11 protons and 23 nucleons The number neutrons in this sodium atom is = 12 neutrons You already know; atoms the same element always have the same atomic number However, in many elements some the atoms have different numbers neutrons from others Atoms the same element with different numbers neutrons are called isotopes Isotopes have different mass number For example, there are three isotopes hydrogen: Hydrogen we find in nature is always a mixture these three isotopes The most abundant by far is the first one Atomic models DALTON THOMSON RUTHERFORD BOHR 3

4 ACTIVITY 3: Atomic number, mass number Isotopes 1-What is the atomic number, Z? 2-What is the mass number, A? 3-If you are asked for the number electrons, what information do you need to know, Z or A? And, if you want to know the number protons? 4-Complete the table on the right 5-What are isotopes? Write some example Atom Z A Carbon C Carbon C Carbon C Sodium Na Aluminium Al Calcium Ca Zinc Zn Hydrogen H protons ACTIVITY 4: Relative atomic mass (A r ) and relative molecular masses (M r ) electrons neutrons A Z X C 13 C Al 13 Atom Aluminium Sulphur Calcium Carbon Chlorine Copper Hydrogen Iron Magnesium Mercury Nitrogen Oxygen Lead Potassium Sodium Zinc Symbol Al S Ca C Cl Cu H Fe Mg Hg N O Pb K Na Zn Mass (u) ,5 63,5 1 55,8 24,3 200, , ,4 However incredible it seems, scientists got to measure the mass the atoms You can imagine it was a long and difficult task, but nowadays we have accurate information On the left you a have a table which contains the atomic mass some atoms Measuring the mass the atoms in grams does not seem useful, as it is a too big unit for the atoms Therefore, a new unit had to be defined, the atomic mass unit, briefly written amu or simply u But, what is 1 amu? The mass the tiniest atom That is, the hydrogen atom 1 uma = mass hydrogen atom If atomic masses are known, it is very easy to calculate the relative molecular masses, M r EXAMPLE: In order to calculate the relative molecular mass water, H 2 O, we have to sum the masses 2 atoms hydrogen and 1 atom oxygen: M r (H 2 O ) = 2 1 u u = 18 u Questions 1-Work out the relative molecular masses : a) Sodium chloride: NaCl; b) Hydrogen: H 2 ; c) Glucose: C 6 H 12 O 6 2-Can there be a atomic mass smaller than 1 u? Argue 3-What does it mean that the atomic mass sulphur is 32 u? 4-Work out the relative molecular masses, M r, the following substances: a) Oxygen: O 2 ; b) Calcium carbonate: CaCO 3 ; c) Hydrochloric acid: HCl; d) Aluminium nitrate: Al (NO 3 ) 3 ; e) Butane: C 4 H 10 ; f) Sucrose (Sugar): C 12 H 22 O 11 4

5 ACTIVITY 5: Relationship between the relative atomic mass and the mass number Look the information in the table: Proton Electron Neutron Mass: 1 u Charge: + 1 Mass: 1/2000 u Charge: 1 Mass: 1 u Charge: 0 1-What will be the relative atomic mass one atom that has one proton, one neutron and one electron? Draw it and write its complete symbol /2000 = = u 2 u 2-Do you think that the mass electrons can be neglected to calculate the mass an atom? Why? 5 3-Indicate what is the mass number, A, and work out the relative atomic mass, Ar, 2 He Draw it** 7 4-Indicate what is the mass number, A, and calculate the relative atomic mass, Ar, 3 Li Draw it 5-State the number protons, neutrons and electrons in this lead atom: 6-Draw a magnesium atom, which has 12 protons and 13 neutrons? Pb 7 **Drawing, according to the Bohr model, 4 Be (Beryllium): 4p +, 3n o and 4e READING 4: Isotopes and relative atomic masses As you know, some elements contain atoms that have different masses, although all them have the same atomic number These atoms are given the name isotopes, since they occupy the same (iso) place (topos) in the periodic table For example, if we could see the atoms that made up the gaseous substance called chlorine, Cl 2, we would distinguish two types chlorine atoms: 17 Cl and 17 Cl That is, the first type has a relative atomic mass 35 u and the second one 37 u So, why the relative atomic mass the chlorine element is 35 5 u? Because we have to calculate the average relative atomic mass taking into account the proportion or abundance on every isotope Look a t the diagram on the right How to do it: We calculate the average relative atomic mass taking into account the relative abundances the isotopes Exercise = 35'5 u % these atoms are 17 Cl and 25% are Neon consists two isotopes, with mass numbers 20 and 22, and with relative abundances 90 % and 10 % respectively Calculate the average relative atomic mass Neon [Sol = 20 2 u] Cl 5

6 ACTIVITY 6: Electronic distribution 1-Knowing the value Z, distribute the electrons in their appropriate shell or energy level Group IA 1H Group VIIIA 2He Group IIA Group IIIA Group IVA Group VA Group VIA Group VIIA 3Li 4Be 5B 6C 7N 8O 9F 10Ne 11Na 12Mg 13Al 14Si 15P 16S 17Cl 18Ar 2-Look at the elements the first column (group I A) They are called alkali metals group What are their names? What do they all have in common? 3-Elements group II A are called alkaline earth metals group What are their names? What do they have in common? 4-Now pay attention to the names below: Group III A: Group IV A: Group V A: Group VI A: Boron group Carbon group Nitrogen group Oxygen group Group VII A: Halogen group Group VIII A: Noble gases group Which elements constitute each group? What do the elements that belong to the same group in common? 5-Do you find any relationship between electronic distribution and similarity in properties? 6-Could you predict anything about the electronic configuration potassium? And about calcium? And about bromine? (Consult the periodic table) 6

7 ACTIVITY 7: Atoms and ions 1-Define the concepts ion, anion and cation 2-Complete the table on the right, which contains the A groups The B groups or transition series have, for the moment, little interest and they are more complicated 3-All atoms have the tendency to be like the nearest noble gas and get the same electronic configuration in the outermost shell (Rule the octet) For this purpose, atoms have to gain or to lose electrons, and by doing this they form ions Complete the table below Group I A II A Name Alkali metals e in the outermost shell Atom Group It tends to Formed ion IIIA Li I A Lose 1 electron Li + IV A O VI A Gain 2 electrons O 2- V A Ca VI A Cl VII A N VIII A * *Except Helium, which has 2 4-Why element number 7 is similar to elements 15 or 33 and, however, it is very different from elements 6 or 8, which are by it side? 5-Explain the following statements: a) METALS form cations, b) NON METALS form anions, c) the noble gases do not form ions 6-Explain the charge acquired by the atoms: Li, Be, B, C, N, O, F and Ne, which respectively belong to the groups IA, IIA, IIIA, etc (For instance, Li (Lithium) belongs to group IA, so it lose one electron: we will write Li +1 ) 7- Explain the charge acquired by the atoms Na, Mg, Al, Si, P, S, Cl y Ar, which respectively belong to the groups IA, IIA, IIIA, etc Ionisation the lithium atom Li 1e Li + 7

8 ACTIVITY 8: Electronic configurations atoms 1-Complete the tables below Atom protons neutrons electrons Electronic distribution 1 st shell 2 nd shell 3 rd shell a) O b) S c) Cl d) N 7 8 Atom protons neutrons electrons Electronic distribution 1 st shell 2 nd shell 3 rd shell e) Na f) Mg Write the element symbol both tables: Ex a) 17 8 O ACTIVIDAD 9: Configuraciones electrónicas de los iones 1-Complete the tables below: Anion protons neutrons electrons Electronic distribution 1 st shell 2 nd shell 3 rd shell a) O 2 b) S 2 c) Cl d) N 3 Catión protons neutrons electrons Electronic distribution 1 st shell 2 nd shell 3 rd shell e) Na + f) Mg 2+ 2-State the number protons, neutrons and electrons in the following atoms or ions 5 23 a) 3 Li c) Mg e) 16 S 55 g) b) 3 Li d) 12 Mg + 41 f) Ca Ex, a) p + = 3, n o = 2, e = 3; b) p + = 3, n o = 2, e = h) 23 Fe + 23 Fe

9 READING 5: Radioactivity Unstable nucleus The atoms some elements have unstable nuclei because they have a number neutrons much bigger than the number protons This fact happens, most all, from the element number 83 (bismuth) This is why the nuclei these atoms spontaneously emit radiation until they reach stability Consequently, the nuclei the atoms are transformed, changing their proton number and decaying into a different atom This phenomenon, which happens in the nucleus the atoms is called radioactivity Three types radiation are produced in this phenomenon They are α- particles, β-particles and γ- rays Alpha particles, α They are made up two protons and two neutrons Therefore, their mass number is 4 and they have positive charge They are emitted at high speed, but they soon slow down and stop in the air They have poor penetration power: cm metal or a sheet paper stop this radiation Beta particles, β They are electrons (negative charge) moving at high speed Their penetrating power is medium: They are stopped by 2 cm aluminium Gamma rays, γ They are high-energy electromagnetic radiation, like X-rays, and move at the speed light They do not have electric charge Their penetrating power is good This radiation passes through 10 cm aluminium, but it is stopped by 10 cm lead or concrete Radioisotopes Radioisotopes are radioactive isotopes an element For instance, 6 and 92 U are radioactive isotopes, but 6 C and 92 U are not From the physical and chemical point view, a radioisotope is identical to the inactive isotope However, a radioisotope is a marked atom that can be traced when it is injected and moves inside the human body due to the radiation it constantly emit (α, β and γ), and because this, it has a many applications On the other hand, the radiation given out by radioisotopes can be harmful because it can kill or damage cells our body producing cancer 14 C 235 Radiation hazard sign Use radioisotopes One the most important medical uses radioisotopes is in cancer treatment The radiation given out by the radioisotope is used to kill cancer cells The radiation has to be controlled so that it only hits the cancer cells and not the healthy cells around the tumour An isotope cobalt is ten used to this Other medical uses include radioisotopes as tracers A tracer is injected into a patient and its progress around the body is followed using an external detector A well-known example is the use iodine-131, which is absorbed by the thyroid gland just like iodine- 127, but gives out radiation which can be detected to indicate whether the thyroid gland is taking iodine as it should Gamma rays are used to sterilise medical instruments, by killing all the microbes The radiation foodstuffs with gamma rays kills all microorganisms in the food without using heat or adding harmful or taste-affecting chemicals Therefore, it is a very effective method sterilization Carbon-14 is a radioisotope carbon-12 A small but constant amount C-14 is constantly produced in the atmosphere All the living beings absorb CO 2, and this is why we find C-14 in all the living matter When a living being dies, no new carbon atoms are incorporated from the atmosphere, but those C-14 atoms continue to decay If we compare the amount C-14 to the amount C-12 in a fossil, as the ratio slowly reduces by the time, the age a fossil or an archaeological remain can be estimated This method is called radiocarbon dating QUESTIONS 1-Why are not the nuclei some atoms stable? 2-How do they show their instability (what do the emit)? 3-When do they get stability? 4-What is the name this phenomenon that happens in the nucleus the atom? 5-How are called the radiations emitted? 6-What are the most important characteristics these particles and rays? 7-What are the radioisotopes? 8-What are the differences and similarities a radioactive isotope and its non-radioactive isotope? 9-What are useful for the radioactive isotopes? 9

10 READING 6: Nuclear energy Nuclear energy comes from the energy stored or locked up in the nuclei atoms This energy is only released from nuclear reactions (changes in the atomic nucleus), which can be either nuclear fission or nuclear fusion Nuclear fission The scientists discovered that if a heavy nucleus, like uranium or plutonium, absorbs a slowmoving neutron, the nucleus split up, releasing loads energy in form heat and spits up two or three neutrons, as you can see in the diagram on the right The released neutrons might go on to hit other nuclei, causing them to split and release even more neutrons, which hit even more nuclei and so on and so on This process is known as a chain reaction This is represented in the diagram below When a large atom, like uranium, splits in two it will form two new lighter elements (eg krypton and barium) These nuclei are usually radioactive because have the wrong number neutrons in them This is the big problem with nuclear power; it produces huge amounts radioactive material which is very difficult and expensive to dispose safely Each nucleus splitting (called a fission) gives out a lot energy, a lot more energy than you get from fossil fuels Uses The process described above is made inside a nuclear reactor Here, a controlled chain reaction takes place and the nuclear energy released is used to get electricity A nuclear power station is an electrical station that instead using fossil fuels uses radioactive substances, like uranium-235, releasing enormous amount heat, which is used to heat water to produce steam and drive the generators electricity Advantages Small amount nuclear fuel releases a lot energy It is a non-renewable resource It will run out one day However there is still plenty uranium left in the ground It do not produce any the greenhouse gases (eg CO2) which contribute to global warming Disadvantages The main problem is with the disposal waste, which are dangerous The products left over after nuclear fission are generally radioactive, so they cannot just be thrown away Nuclear fusion Nuclear fusion is just the opposite fission; in this case, two light nuclei (eg hydrogen) can combine to create a larger nucleus (helium) Fusion releases a lot energy, more than fission for a given mass fuel So people is trying to develop fusion reactors to make electricity Fusion does not leave behind a lot radioactive waste and there is plenty hydrogen about to fuel Fusion hydrogen nuclei is the energy source that the Sun constantly irradiates 10

11 Uses There are a few experimental fusion reactors around, but none them are generating electricity yet At the moment it takes more power to get up to the right temperature than the reactor can produce Advantages Fusion does not leave behind a lot radioactive waste and there is plenty hydrogen about to fuel Disadvantages The big problem is that fusion only happens at really high temperatures; over ºC No material can stand that kind temperature without being vaporised, so fusion reactors are really difficult to build You have to conatin the hot hydrogen in a magnetic field instead in a physical container Questions 1-What is nuclear fission? 2-What is it used for? 3-Pros and cons about nuclear fission 4-What is nuclear fusion? 5-What is it used for? 6-Pros and cons about nuclear fusion? 11