Lezione 1 - Atomo.pdf

The atom Teresa Gatti Department of Applied Science and Technology (DISAT) E-mail: [email protected]  There are about 91 elements found in nature. Over 20 have been made in laboratories. Each kind of atom is unique Carbon is not Hydrogen They have diff...

The atom Teresa Gatti Department of Applied Science and Technology (DISAT) E-mail: [email protected]  There are about 91 elements found in nature. Over 20 have been made in laboratories. Each kind of atom is unique Carbon is not Hydrogen They have different properties Structure, magnetic -meaning they can attract and repel other atoms-, melting, boiling, electrical, stability, reactivity (attract and repel), etc…  The Divisibility of Matter Ultimate particle Upon division, eventually a particle is reached which can no longer be divided. Atoms diameters are in the 10-10 m range (hundreds of pm or Å – angstroms-, 1 Å = 100 pm = 0.1 nm) We detect particles up to 10-15 m In theory particles of 10-35m exist, we don’t have instruments that sensitive “Nothing exists except atoms and empty space; everything else is opinion.” - Democritus 460–370 B.C. Modern Evidence for Atoms IBM's Almaden Research Center in San Jose, California, April 1990 Democritus 400 BC This is the Greek philosopher Democritus who began the search for a description of matter more than 2400 years ago. He started asking: Could matter be divided into smaller and smaller pieces forever, or is there a limit to the number of times a piece of matter can be divided? Atomos His theory: Matter cannot be divided into smaller and smaller pieces forever, eventually the smallest possible piece would be obtained. This piece would be indivisible. He named the smallest piece of matter “atomos,” meaning “not to be cut.” Atomos § For Democritus, atoms are small, hard particles all made of the same material but with different shapes and sizes. § Atoms are infinite in number, always moving and capable of joining together.  This theory was ignored and forgotten for more than 2000 years! Why? The eminent philosophers of the time, Aristotle and Plato, had a more respected, (and ultimately wrong) theory. Aristotle and Plato favored the earth, fire, air and water approach to the nature of matter. Their ideas were taken for granted because of their eminence as philosophers. The atomos idea was buried for approximately 2000 years.  Lavoisier: The atomic nature of matter (1/2) Mass conservation law (1785): “The mass of the products of a chemical reaction is exactly equal to that of reactants” CaCl2 + Na2SO4 à CaSO4 +2NaCl (In water)  Lavoisier: The atomic nature of matter (2/2) wood combustion ash + oxygen + carbon dioxide + water magnesium combustion magnesium + oxygen oxide Wood Magnesium After combustion wood turns into ash (accompanied by CO2 and H2O), whereas magnesium increases its mass becoming MgO. In both cases, though, the mass of reactants equals the mass of products.  Dalton: The atomic theory DALTON POSTULATES 1. The matter is made of ATOMS, very small particles of a given element that cannot be divided, created or destroyed. 2. The atoms of an element cannot turn into atoms of another element: in a chemical reaction the original substances get separated into atoms that do recombine to form different substances. 3. The atoms of an element show identical mass and properties and are different from any atom of other elements. 4. The compounds are formed by the chemical combination of a specific number of different atoms.  Dalton: The atomic theory Law of simple multiple proportions (Dalton) “If A and B react to form two or more compounds (e.g. AB and AB2), the two different masses of B that get combined with the same mass of A are multiples of a small prime number” This discovery was the first great success of Dalton’s atomic theory. This law was not induced from experimental results, but was derived from theory and then tested by experiments.  Dalton: Limits of the atomic theory Why do atoms combine in certain given ratios and not in others? For instance, why two H atoms and one atom of O form water and no compound is stable with 3 atoms of H and one of O? The atomic theory (atoms like billiard balls) could not explain the existence of sub-atomic particles, electrically charged, that some scientists (i.e. THOMSON, MILLIKAN & RUTHERFORD) discovered later, leading to the nuclear atomic model. … what did these scientists discover?  Inside the Atom Johnstone Stoney (1891): It was discovered that compounds can be decomposed by an electric current, for example 96490 C of electricity is required to liberate 1 g of hydrogen from water. à electricity exists in discrete units and that the units are associated with atoms. He suggested the name electron for such unit of electricity. Joseph John Thomson: if a glass tube is fitted with electrodes and a potential difference (10000 V) is applied (at very low pressure) the glass of the tube glows (fluoresces) with faint greenish light. The light is due to bombardment of the glass by rays liberated at the cathode (cathode rays).  Perrin: The charge of the electron Magnet Fluorescent screen Jean Perrin 1895: the french physicist Jean Perrin demonstrated that cathode rays consist of negatively charged particles. When a magnet is applied near the tube where cathode rays are emitted, the beam is deflected in a direction corresponding to a negative charge on the particle  Thomson: The cathodic tube Cathode rays are affected simultaneously by a magnetic field and an electric field Thomson was able to calculate the following: E Velocity of particles: Hev = Ee Þ v = H 2 Mass/charge ratio of such particles: v e E m = Hev = Ee Þ = 2 r m H r  Millikan: The charge and mass of an electron 1. The charge on the droplets was measured observing the different rate of fall observing that the different values had a common factor: 1,6 10-19 C (i.e. the smallest electric charge that was then associated to the charge of the electron). 2. the mass of the electron could thus be calculated based on the Thomson relationship (e/m). The atomic model by Thomson Cloud of positive particles Negative particles  Rutherford’s Experiment How can you prove something is empty? Put something through it. Use large target atoms. Use very thin sheets of target so they do not absorb “bullet”. Use very small particles as “bullet” with very high energy. But not so small that electrons will effect it. Bullet = alpha particles; target atoms = gold foil a particles have a mass of 4 amu & charge of +2 Gold has a mass of 197 amu and is very malleable. 20  The Rutherford experiment http://www.youtube.com/watch?v=5pZj0u_XMbc The results can be explained only under the assumption that most of the mass of the atom is concentrated into a small particleà the atomic nucleus  Rutherford’s Interpretation— The Nuclear Model 1. The atom contains a tiny dense center called the nucleus. The amount of space taken by the nucleus is only about 1/10 trillionth the volume of the atom. 2. The nucleus has essentially the entire mass of the atom. The electrons weigh so little they contribute practically no mass to the atom. 3. The nucleus is positively charged. The amount of positive charge balances the negative charge of the electrons. 4. The electrons are dispersed in the empty space of the atom surrounding the nucleus. Like water droplets in a cloud.  Some open issues How could beryllium have 4 protons stuck together in the nucleus? Shouldn’t they repel each other? If a beryllium atom has 4 protons, then it should weigh 4 amu, but it actually weighs 9.01 amu! Where is the extra mass coming from? Each proton weighs 1 amu. Remember: The electron’s mass is only about 0.00055 amu and Be has only 4 electrons—it can’t account for the extra 5 amu of mass.  There must be something else down there To answer these questions, Rutherford proposed that there was another particle in the nucleus—it is called a neutron. Neutrons have no charge and a mass of 1 amu. The masses of the proton and neutron are both approximately 1 amu.  Inside the atom (1/2) The atoms are made of sub-atomic particles: electrons, protons, neutrons characterised by the following parameters PARTICLE CHARGE MASS Neutron (n) None 1.67. 10-27 kg Proton (p) Positive +1 1.67. 10-27 kg Electron (e) Negative -1 9.07. 10-31 kg (1840 times lighter!) The nucleus is surrounded by electrons (e-) having a negative charge. The nucleus is characterised by “strong nuclear interactions”. Neutrons Today we know that protons and Protons neutrons are made of “quarks”. NUCLEUS Electrons 4He  Inside the atom (2/2) In a neutral atom, protons and electrons are equal in number A Symbol Z The number of protons Z, the atomic number, makes an element chemically different from another. The number of nucleons (protons + neutrons) is named A, the mass number. The atom dimensions are in the order of magnitude of one Ångstrom (1Å = 1.10-10 m). Protons and neutrons measure 1·10-15 m, whereas quarks 1·10-18 m. The nucleus is 5 orders of magnitude smaller than the atom.  The atomic mass unit (amu) The atom mass is a fundamental property which is measured by convention setting a reference to 1/12 of the mass of carbon 12 6C = 12 amu 12 1 amu = 1/12 mass of 6 C 1 amu = 1.66 10-27 kg = 1.66 10-24 g proton mass = 1.6726 10-27 kg = 1.007 amu neutron mass = 1.6749 10-27 kg = 1.008 amu electron mass = 0.00055 amu  Ions and molecules When protons and electrons are different in number, an ion is formed. If protons prevail a positively charged cation is formed (a cation will be attracted toward the cathode, the negatively charged terminal) es. Li+ 3 protons 2 electrons If electrons prevail a negatively charged anion is formed (an anion will be attracted toward the anode, the positively charged terminal) es. F- 9 protons 10 electrons The atoms may be linked together to form molecules. E.g. O2 molecules are made of 2 O atoms H2O is a molecule made of 2 H and 1 O atoms  Isotopes (1/3) Atoms of the same element may exhibit different A numbers; in these case they are called isotopes: e.g. 16 17 18 8O 8 O 8O 1 H protium, 2 H deuterium (D), 3 H tritium (T) 1 1 1 proton 1 proton 1 proton 1 neutron 2 neutrons  Isotopes (2/3) Because isotopes have the same number of protons and electrons, they have essentially the same chemical and physical properties…. … however for hydrogen the mass differences between isotopes are comparable to the masses of the atoms themselves.  Isotopes (3/3) Most elements in nature are characterised by different isotopes with different relative per cent abundance: 14Si 92.21%; 14Si 4.70%; 14Si 3.09% 28 29 30 1H 99.985%; 1H 0.015%; 1H traces 1 2 3 6C 98.89%; 6C 1.11%; 6C traces 12 13 14 Relative isotope abundance:  The measurement of the atomic mass THE MASS SPECTROMETER Mass spectrometry (MS) measures the mass-to-charge ratio of ions to identify and quantify molecules in simple and complex mixtures The mass spectrum for zirconium  Limitations of the Rutherford model It cannot explain some evidences, e.g.: Emission spectra Absorption spectra Photoelectric effect Moreover, why does the electron keep travelling around the nucleus and does not fall onto it? …this will be clearer in lecture 4, dedicated to atomic orbitals!