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Topic 2: Atoms and The Atomic Theory Contents • Early chemical discoveries • Electrons and the nuclear atom • Chemical elements • Atomic masses • The Avogadro constant & the Mole concept Early Discoveries Democritus (B.C 470-400) Democritus is a philosoph lived in the ancient times of Greece. He brought up the modernest theory concerned with the nature of the physical world in that period of time. According to Democritus; • Every thing existing in nature is composed of atoms. • Atoms are very tiny particles which can not be perceived by our senses. • All of them are made of the same matter, but they all have different sizes,shapes and weights. • Atoms are those particles,which can not be split into parts,can not be annihilated and created Early Discoveries Lavoisier 1774 Law of Conservation of Mass Proust 1799 Law of Constant Composition Dalton 1803-1808 Dalton’s Atomic Theory Law of Conservation of Mass Lavoisier, known as the establisher of Chemistry The mass of substances formed by a chemical reaction is the same as the mass of substances entering into the reaction. Reactants Silvernitrate and potasium chromate Products Silverchromate and potasiumnitrate 2 AgNO3 (aq) + K2CrO4 (aq) Ag2CrO4 (s) + 2 KNO3 (aq) Law of Constant Composition All samples of a compound have the same composition- the same proportions by mass of the constituent elements. Proust 1799 This is called Law of Constant Composition Sample A 10.0000 g 1.1190 g H 8.8810 g O Composition % H=11.19 % O=88.81 Sample B 27.0000 g 3.0213 g H 23.9787 g O Water consists of %11,19 of hydrogen and %88,81 of oxygen Dalton’s Atom Theory • John Dalton (1766-1844) • • • John Dalton’s Theory involves three assumptions: All atoms of an element are alike in mass and other properties, but the atoms of one element are different from those of all other elements. Each chemical element is composed of minute, indestructible particles called atoms. Atoms can neither be created nor destroyed during a chemical change. In each of their compounds different elements combine in a simple numerical ratio.For example one atom of A to one of B(AB), or one atom A to two of B(AB 2 EFFECT OF ELECTRICITY and MAGNETISM ON THE ATOMIC STRUCTURE . Behaviour of electrically charged particles All objects of matter are made up of electrically charged particles. Q Q Neutral Q Q Q Q Positively charged Negatively charged • Electrically charged particles deflect in a magnetic field. Q1 Q2 F k 2 r k:Coulomb constant EFFECT OF ELECTRICITY and MAGNETISM ON THE ATOMIC STRUCTURE • Objects with like charges repel one another. •Objects with unlike charges attract one another. The force, (F),of attraction or repulsion is directly proportional to the magnitude or quantity of charges (Q1 ve Q2) and inversely proportional to the square of the distance between them (r2). This force is called Coulomb force. Behaviour of electrically charged particles Electrically charged particles are deflected from the straight line path into the curved path perpendicular to the field Negatively charged particles are deflected in one direction, whereas positively charged particles are deflected in the opposite direction, when they travel at right angles through a magnetic field. Assumption is made that the influence of the magnet as represented lines of force run from the north pole to the south pole of the magnet. The Discovery of Electrons CRT,the abbreviation for cathode ray tube has become an everyday expression. The CRT is the heart of the familiar computer monitor or TV set. The first cathode ray tube was made by Michael Faraday (1791-1867) . Faraday discovered cathode rays, a type of radiation emitted by the negative terminal cathode that crossed the evacuated tube to the positive terminal or anode. Later scientists found that cathode rays travel in straight lines and have properties that are independent of the cathode material. The Discovery of Electrons Cathode rays are invisible and they can only be detected by the light emitted by materials they strike. Fluorescence is the term used to describe the emission of light by a material when it is struck by energetic radiation. In 1897, Thomson established the ratio of mass to electric charge (m/e). He concluded that cathode rays are negatively charged fundamental particles of matter found in all atoms. Cathode rays subsequently became electrons, the term used firstly by George Stoney in 1874 Figure 2.1 Determining the mass/charge ratio for cathode rays The properties of Cathode rays In 1897, Thomson established the ratio of mass to electric charge (m/e). He concluded that cathode rays are negatively charged fundamental particles of matter found in all atoms. Cathode rays subsequently became electrons, the term used firstly by George Stoney in 1874 Cathode beam rays are deflected by the electrical and magnetic field. The ratio m/e = -5.6857 x 10-9 g/coulomb Millikan’s Oil Drop Experiment •Ions are produced by energetic X rays.Some of these ions become attached to oil droplets,giving them a charge. •The fall of a droplet between the condenser plates is speeded up or slowed down, depending on the magnitude and sign of the charge on the droplet Millikan’s Oil Drop Experiment Millikan concluded that the magnitude of the charge, q on a droplet is an integral multiple of the electronic charge,e ; q= ne (where n=1,2,3…..) q= ne Robert Millikan (1868-1953) • Millikan determined the electronic charge e through a series of oil drop experiments • By combining this value with an accurate value of the mass to charge ratio for an electron we get the mass of an electron . e = 1,6022 x 10-19 C me = 9,1094 x 10-28 g Figure 2.3 Millikan’s oil drop experiment Thomson’s Atom Model He atoms and ions according to Thomson’s atom model Thomson’s Atom Model After the discovery of electrons Thomson brought up an atom model. The positive charge needed to counterbalance the negative charges of electrons in a neutral atom is in the form of a nebulous cloud. Electrons float in this diffused cloud of positive charge (Plum pudding model). X-Rays and Radioactivity • 1895 Wilhelm Roentgen noticed that when cathode ray tubes were operating certain materials outside the tubes would glow or fluoresce. Because of the unknown nature of this radiation, he coined the term X-rays. • Antoine Becquerel discovered radioactivity as a result of series experiments with the uranium containing material in which he found out that it had emitted radiation continously without fluorescing. X-Rays and Radioactivity •Ernest Rutherford, 4 2 identified two types of 2 radiation from radioactive materials, alpha and beta 0 rays, Paul Villard 1 discovered gamma rays. He e • Marie and Pierre Curie discovered plenty of radioactive elements . The most important ones are Ra and Po. X-Rays and Radioactivity Alpha particles: carry two fundamental units of positive charge and have the same mass as helium atoms. They have +2 charges. Beta particles: are negatively charged particles produced by changes occuring within the nuclei of radioactive atoms and have the same properties as electrons. Gama rays are defined as the electromagnetic radiation of extremely high penetrating power. When the radiation is passed through the electric field, gamma rays are undeflected. Alpha particles and beta particles are deflected in the opposite directions of each other. Three types of radiation The Nuclear Atom The experiment of thin foils of gold In 1909 Geiger and Ernest Marsden bombarded very thin foils of gold with alpha particles, here is what they observed: The Nuclear Atom The scattering of α particles by metal foil The deflection of alpha particles according to the Thomson’s model The Nuclear Atom 1.The majority of α particles penetrated the foil undeflected 2.Some of α particles are slightly scattered as they encounter electrons 3.A few(about one in 20000) suffer rather serious deflections as they penetrate the foil 4.A similar number can not pass through the foil at all but bounce back in the direction from which they have come. These results can not be explained by Thomson’s model. The Nuclear Atom Rutherford’s Atom Model 1. Most of the mass and all of the positive charge of an atom are centered in a very small region called the nucleus. 2. The magnitude of positive charge is different for different other atoms and is approximately one half the atomic weight of the element. 3. There exist as many electrons outside the nucleus as there are units of positive charge on the nucleus. The atom as a whole is electrically neutral. The Nuclear Atom Rutherford discovered in 1919 the particles called protons, in studies involving the scattering of alpha particles by nitrogen atoms in air. In 1932, James Chadwick discovered the neutrons originated from the nuclei of atoms . They are uncharged particles. The sum of the protons and neutrons located in the nucleus of an atom is defined as nucleon. Rutherford’s atom model Fundamental Particles of Atom Electron (e) unique atom unit has negative charge. Proton (p) unique atom unit has positive charge . Neutron (n) is uncharged. ~ m p mn me Atomic mass unit: 1/12 of the mass of the atom known as carbon-12, its abbreviation is amu . Fundamental Particles of Atom The number of protons in the nucleus of an atom is called atomic number (Z), the total number of protons and neutrons in its nucleus is called mass number (A) . In a neutral atom Z p pe A pn n A Z Atomic Scale The mass of the heaviest atom is only 4.32 x 10-22 g and the diameter of this atom is 5 Å’. The units frequently used in the atomic scale: 1 amu (atomic mass unit) = 1.66054 x 10-24 kg 1 pm (pikometer) = 1 x 10-12 m 1 Å (Angstrom) = 1 x 10-10 m = 100 pm = 1 x 10-8 cm Tipical C-C bond length= 154 pm (1.54 Å). Chemical Elements All the atoms of an element have the same atomic number (Z). At present the elements with the atomic numbers ranging from Z=1 to Z=109 are known. Each element has a name and a distinctive symbol. For most elements the chemical symbol is an abbreviation of the English name and consists of one or two letters. Some elements known since ancient times have symbols based on their Latein names, such as Fe for iron(ferrum ) Chemical Elements Element Carbon Oxygen Nitrogen Sulphur Neon Silicon Symbol C O N S Ne Si Element Ferrium Plumbum Natrium Kalium Symbol Fe Pb Na K Elements beyond uranium (Z=92) do not occur naturally and must be synthesized in particle accelerators . Chemical Elements Number of protons + number of neutrons Number of protons A Z X Charge (p-e) Symbol of Element Dalton put up the argument that all of the atoms of each element had the same atomic mass but we know at present not every atom of one element may have the same atomic mass. Chemical Elements Example: 20 10 Ne 21 10 Ne 22 10 All of the Ne atoms have 10 protons in their nuclei.However the number of neutrons in their structures varies from 11 to 12. Ne Isotopes: Two or more atoms having the same atomic number (Z) but different mass numbers (A) are called isotopes 20 10 Percent Natural Abundance Ne %90,9 21 10 Ne %0,3 22 10 Ne %8,8 Chemical Elements I. 20 10 22 II. 10 Ne Ne 2 ion An element that has either lost or gained electrons is called an ion and carries a net electric charge. The number of protons never changes when an atom becomes an ion. The first neon ion has 10p, 10n, 9e . (+1 charged) The second one has 10p, 10n, 8e . (+2 charged) Chemical Elements Example: Indicate the number of protons,electrons and neutrons in Z p 17 A p n 35 17 n n 18 p e 17 Example: Write an appropriate symbol for the species consisting of 29 protons, 34 neutrons and 27 electrons Isotopic Masses We can not determine the mass of an individual atom just by adding up the masses of its fundamental particles. When an atomic nucleus is created from protons and neutrons, a small quantity of the original mass is converted to energy. This is the nuclear binding energy that holds the protons and neutrons together and we can not predict exactly how much this will be. Isotopic Masses How can we determine atomic masses? We arbitrarily choose one atom and assign it a certain mass. By international agreement this standard is an atom of the isotope C-12 which is assigned a mass of 12 amu. The masses of other atoms relative to this value are determined. To do this we use a mass spectrometer . Mass spectrometry A gaseous sample is ionized by bombardment with electrons in the lower part of the apparatus. The positive ions thus formed are subjected to an electrical force by the electrically charged velocity selector plates and a magnetic force by a magnetic field perpendicular to the page. Only ions with a particular velocity pass through and are deflected into circular paths by the magnetic field . Ions with different masses strike the detector(here the phtographic plate) in different regions. Mass spectrometry Mass spectrometry With mass spectral data,the ratio of the mass Example: of 16O to 12C is found to be 1,33291. What is the mass of an 16O atom? The ratio of the masses is 16O / 12C =1,33291 ,the mass of an 16O is 1,33921 times the mass 12C ; 16O (mass of oxygen) = 1,33291 x 12,00000 amu =15,9949 amu 16O is found By mass spectrometry an atom of Example: to have 1,06632 times the mass of 15N. What is the mass of an 15N, expressed in atomic mass units? 16O mass = 1,06632 x 15N mass 15N mass = 15,9949 amu / 1,06632 =15,00009 amu Atomic masses In a table of atomic masses the value listed for carbon is 12,011, yet the atomic standard mass is exactly 12. What is the reason for this difference? The atomic mass standard is based on a sample of carbon containing only atoms of 12C isotope, whereas naturally occuring carbon contains some 13C and 14C isotopes as well. The existence of these three isotopes causes the observed atomic mass to be greater than 12 Atomic masses Atomic mass (weight) of an element is the average of the isotopic masses, weighted according to the naturally occuring abundances of the isotopes. We can calculate it by using the formula given below: Atomic mass of an element Fractional abundance Mass of = of isotope x isotope(1) (1) Fractional abundance + of isotope x Mass of isotope (2) (2) +… Atomic masses 12C isotope: 12,00000 amu; percent natural abundance: 98,892 % 13C isotope: 13,00335 amu; percent natural abundance: 1,108 % Atomic mass of naturally occuring C is calculated as: At. mass Fractional Fractional of x Mass of abundance abundance naturally = x Mass of + 13C 12 13 of C 12C of C occuring carbon At. mass of = naturally occuring carbon 0,98892 12,00000 0,01108 13,00335 Atomic mass of C = 12,011 amu The Avogadro Constant and the concept of the mole • Since atoms are very tiny particles and there are as many as uncountable atoms used in Chemistry, it is impossible to measure their masses individually. For this reason a certain kind of system has been improved. The SI quantity describes it as mole. • A Mole is an amount of substance that contains the same number of elementary entities(atoms,molecules,other particles as there are 12C in in exactly 12 g of carbon. An italian scientist namely Avogadro provided that the number of elementary entities are defined as the Avogadro constant, 1 mol = 6,02214 x 1023 atoms,molecules The Avogadro Constant and the Concept of the Mole 12,00g 12C = 1mol 12C atom = 6,022x1023 12,00g sample of 12C 6,022 x 1023 12C 12C atoms 1,993 x 10-23g = the mass of a unique atoms 1 12C atom = 1,993 x 10-23g 1 12C atom = 12amu 1amu = 1,661 x 10-24g 1,661 x 10-24g 12C atom 1g 6,022 x 1023 6,022 x 1023 amu 1 mol Oxygen: 16 amu Oxygen= 16g/mol The atomic mass ( the mass of one atom) and the molecular weight (the mass of 6,022 x 1023 atoms) are quantitatively the same but the units of them are different : 1 oxygen atom = 16 amu 1 mol oxygen = 16 g F-19 Mg-24,26,25 Cl-35,37 Pb-207,206,208,204 The Avogadro Constant and the Concept of the Mole Example: Potasyum-40 (40K) is one of the few naturally occuring isotopes of elements of low atomic number. Its percent natural abundance is 0.012% . How many 40K atoms do you ingest by drinking one cup of whole milk containing 371 mg K ? K= 39,1 mK(mg) x (1g/1000mg) mK (g) x 1/MK (mol/g) nK(mol) nK = (371 mg K) x (10-3 g/mg) x (1 mol K) / (39,10 g K) = 9,49 x 10-3 mol K nK(mol) x NA atoms K x 0.012% atoms 40K atoms 40K = (9.49 x 10-3 mol K) x (6.022 x 1023 atoms K/mol K) x (1,2 x 10-4 40K/K) = 6,9 x 1017 40K