Lecture 5 & 6


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Chemistry I for Eelo students

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Lecture 5 & 6

  1. 1. Eelo American University, Borama, Awdal, 11/6/2008 Somaliland EELO AMERICAN UNIVERSITY BORAMA, AWDAL Democritus SOMALILAND 400 BC • This is the Greek philosopher Democritus who began the search for a description of matter more than 2400 years ago. Md. Faysal Ahamed Khan – He asked: Could matter be Lecture 5-Structure of Atom divided into smaller and smaller pieces forever, or was there a & limit to the number of times a Lecture 6 - Quantum theory & Atomic piece of matter could be divided? structure Welcome to the class of Chemistry I Course No. CHEM 211 Credit hours 3 Atomos Atomos • His theory: Matter could § To Democritus, atoms not be divided into were small, hard particles smaller and smaller pieces that were all made of the same material but were forever, eventually the different shapes and smallest possible piece sizes. would be obtained. § Atoms were infinite in • This piece would be number, always moving indivisible. and capable of joining • He named the smallest together. piece of matter “atomos,” meaning “not divisible.” This theory was ignored and forgotten for more than 2000 years! Prepared byMd. Faysal Ahamed Khan
  2. 2. Eelo American University, Borama, Awdal, 11/6/2008 Somaliland • The eminent philosophers of the time, Aristotle and Plato, had more respect, and ultimately wrong theory. Aristotle and Plato favored the earth, fire, air and water approach to the nature of matter. Their ideas was believed because of their eminence as philosophers. The atomos idea was buried for approximately 2000 years. Laws of conservation Dalton’s Model of Mass In the early 1800s, the English Chemist John Dalton performed a number of experiments that eventually Mass can neither be created nor destroyed in Chemical led to the acceptance of the reaction idea of atoms. His theory of atom was based on the following laws: 1. Laws of conservation of 2H2 + O2 = 2 H2O mass 4x1.008 16x2 2x(2x1.008 + 16) =4.032 gm =32 gm = 2x18.016 2. Laws of constant = 36.032 gm composition 36.032 gm 3. Laws of multiple proportion Prepared byMd. Faysal Ahamed Khan
  3. 3. Eelo American University, Borama, Awdal, 11/6/2008 Somaliland Laws of multiple proportion Laws of Constant Composition Same elements can combine in different No matter what its source, a particular chemical ways to form different substances, whose compound is composed of the same elements in mass ratios are small whole number the same fraction by mass multiples of each other. That is if elements A and B reacts to form two compounds, the different masses of B By Mass water is: 88.8% oxygen that combine with a fixed mass of A can 11.2% hydrogen be express as a ratio of small whole numbers. How Dalton’s theory explains the Dalton’s Theory mass laws • He deduced that all elements are • Mass Conservation: Since each type of atoms have composed of atoms. Atoms are a fixed mass, a chemical reaction, in which atoms indivisible and indestructible are just combined differently with each other, cannot particles. possibly result a mass change. • Atoms of one element cannot be • Definite Composition: A compound is a This theory converted into atoms of another combination of specific ratio of different atoms, became one elements each of which has a particular mass. Thus, each of the foundations • Atoms of the same element are element in a compound constitutes a fixed fraction of modern identical in mass & properties. of total mass. chemistry. Atoms of different elements are • Multiple Proportions: Atoms of an element have different. the same mass and are indivisible. Because different • Compounds are formed by the numbers of B atoms combine with each A atom in joining of atoms of two or more different compounds, the mass of element B that elements by a specific ratio. combine with a fixed mass of element A give a small, whole-number ratio. Prepared byMd. Faysal Ahamed Khan
  4. 4. Eelo American University, Borama, Awdal, 11/6/2008 Somaliland Thomson’s Plum Pudding Limitation of Dalton’s model Model • In 1897, the English • Atoms are divisible scientist J.J. • In a nuclear reaction atoms of one Thomson provided elements often changes to atoms of the first hint that an other elements atom is made of even smaller • Isotopes & Isobars particles. He proposed a model of the atom that is sometimes called the “Plum Pudding” model or Raisin Bread model. Thomson Model Thomson Model Where did they • Atoms were made from a • This surprised come positively charged Thomson, because from? substance with the atoms of the gas negatively charged electrons scattered about, were uncharged. like raisins in a pudding. Where had the negative charges • Thomson studied the come from? passage of an electric current through a gas. Thomson concluded that the negative charges came from within the atom. • As the current passed through the gas, it A particle smaller than an atom had to exist. gave off rays of negatively charged The atom was divisible! particles. Prepared byMd. Faysal Ahamed Khan
  5. 5. Eelo American University, Borama, Awdal, 11/6/2008 Somaliland Rutherford’s Gold Foil Experiment Thomson called the negatively charged “corpuscles,” • In 1910, after the today known as electrons. discovery of radioactivity, emission of particles and/or radiation from Since the gas was known to be neutral, having no atoms of certain elements, charge, he reasoned that there must be positively the English physicist charged particles in the atom. Ernest Rutherford used one type of radioactive But he could never find them. particle in a series of experiment that that solved the mysteries of the atomic structure. • Rutherford’s experiment Involved firing a stream of tiny positively charged particles at a thin sheet of gold foil (2000 atoms thick) – Most of the positively charged “bullets” passed right through the gold atoms in the sheet of gold foil without changing course at all. – Some of the positively charged “bullets,” however, did bounce away from the gold sheet as if they had hit something solid. He knew that positive charges repel positive charges. Expect: 1. Mostly small angle scattering 2. No backward scattering events Results: 1. Some small scattering events 2. Several backward scatterings!!! Prepared byMd. Faysal Ahamed Khan
  6. 6. Eelo American University, Borama, Awdal, 11/6/2008 Somaliland Rutherford • This could only mean that the gold atoms in • Rutherford reasoned that all of an atom’s the sheet were mostly open space. Atoms positively charged particles were contained were not a pudding filled with a positively in the nucleus. The negatively charged charged material. particles were scattered outside the nucleus • Rutherford concluded that an atom had a around the atom’s edge. small, dense, positively charged center that repelled his positively charged “bullets.” • He called the center of the atom the A new question arises. If the electron is “nucleus” negatively charged, won't the attraction • The nucleus is tiny compared to the atom as for electrons by the nucleus cause the a whole. electron to fall into the nucleus and therefore atoms should collapse. Rutherford attempts to explain his Limitation of Rutherford’s experimental results. model He knew about the solar system - the attraction • A nucleus and an electron attract each other, of the planets by the sun - universal gravitation. so if they are to remain apart, the energy of Yet planets are not pulled into the sun. They are electron’s motion (kinetic energy) must in motion around the sun and this motion balance the energy of attraction (potential prevents them from being pulled into the sun. energy). According to the laws of classical physics says that a negative particle moving So Rutherford puts the electron in motion in a curved path around a positive one must around the nucleus. emit radiation and thus lose energy, so orbiting electron will lose energy continuously and spiral into nucleus. Electron crashes into the nucleus!? Prepared byMd. Faysal Ahamed Khan
  7. 7. Eelo American University, Borama, Awdal, 11/6/2008 Somaliland Limitation (Cont’d) • In solar system, the planets are electrically neutral but in atoms the electrons and protons are electrically Quantum theory & Atomic charged • The shape and size of electrons structure orbiting path is still undetermined and Rutherford did not give any idea about Lecture 6 that also • In case of more than one electrons atom Rutherford did not explain how they orbit around the nucleus From the failure of Rutherford's model Electromagnetic radiation • Neils Bohr, a Danish Physicist, points out that laws of physics do not apply for • Energy can be transmitted through the submicroscopic world of ATOM space by Electromagnetic radiation • In 1913 Bohr proposed a new model • Electro magnetic radiation consists of based on the modern quantum theory of energy waves which have both electrical & • With this model he was able to explain magnetic properties - why orbiting e- do not collapsed to nucleus Ex: stone thrown to the pond creates wave like - how atomic spectra occurs electromagnetic waves • To understand Bohr theory first we need • Waves convey the energy from one to know place to another - Nature of electromagnetic radiation • They travels through empty space at - Atomic Spectra the speed of light - Quantum theory of energy Prepared byMd. Faysal Ahamed Khan
  8. 8. Eelo American University, Borama, Awdal, 11/6/2008 Somaliland wavelength Visible light Wave nature of Light Amplitude • Light is electromagnetic radiation - crossed electric and magnetic waves: wavelength Node Ultaviolet radiation Properties : Wavelength - distance between consecutive peaks - Wavelength, (nm) crests - measured in m, nm, angstroms. Frequency, n (s-1, frequency - (nu) - number of times per second a crest passes a given point (cycles per second) Hz) Amplitude, A All waves have: Magnetic vector frequency and wavelength symbol: ν (Greek letter “nu”) λ (Greek “lambda”) units: “cycles per sec” = Hertz “distance” (nm) • All radiation: λ•ν= c where c = velocity of light = 3.00 x 108 m/sec Note: Long wavelength → small frequency Short wavelength → high frequency Reference: See Fig:7.3 of page 257 of Silberg’s Chemistry Book • Wave nature of light is shown by classical wave properties such as increasing increasing wavelength • interference frequency • diffraction Example:Red light has λ = 700 nm.Calculate the frequency, ν. • refraction 8 c = 3.00 x 10 m/s = 4.29 x 10 14 Hz ν= λ 7.00 x 10 -7 m Prepared byMd. Faysal Ahamed Khan
  9. 9. Eelo American University, Borama, Awdal, 11/6/2008 Somaliland Particle nature of light Quantum theory of electromagnetic radiation Blackbody radiation & quantization of energy § In 1900 Max Planck observed the fact § He proposed that light radiation produced When a solid body is heated it’s began to emit from a hot body discontinuously, in small visible light and with the increase in unit of wave, each of which have a temperature the light began to brighter specific frequency, which increase with Observed changes in intensity & wavelength temperature. of emitted light Continuous wave - This is blackbody radiation – Discontinuous waves or QUANTA Attempts to explain this observed change by classical wave theory failed § Unit of wave = Quantum (plural, quanta) - because in a continuous wave wavelength § The energy of Quanta is given by the /frequency is fixed and not changing in its path following relation E = nhν Photoelectric effect & photon theory of - So, Energy is QUANTIZED = that is you light get energy in certain no. quantity like particle/ packet, that is you can count, it is When a beam of light of sufficiently high not continuous frequency is allowed to strike a metal surface - Where did the radiation/quantum energy in vacuum, e- ejected from the surface- this is came from? Ans: Hot object’s radiation is Photoelectric effect emitted by the atoms contains within it. - It means atoms contains only certain quantity of energy - So the energy of atom is also QUANTIZED - So, change in atoms energy means gain or loss of one or more “packets” of energy - So, an atom changes its energy state by emitting one or more quanta of energy Prepared byMd. Faysal Ahamed Khan
  10. 10. Eelo American University, Borama, Awdal, 11/6/2008 Somaliland Einstein takes Planck's theory and extends it to the Observation 1 structure of light itself. He doesn't think of it as a Violet light will cause potassium to eject continuous wave, but as chunks/parts of electrons, no amount of red light, no matter 1wavelength. He proposed light itself like energy how bright (intense) it is will not have any is also particulate, occurring as quanta. He calls these chunks of light, photons. effect. Einstein was surprised that the Explanation of observation 1: threshold energy was related to color rather In order to release an e from metal surface, the than the intensity. incident photon has first to overcome the They are only ejected when the frequency of attractive force exerted by the positive ion of the the light exceeds a certain threshold value for metal each particular metal. The energy of photon proportional to its frequency of incident light Observation 2 More electrons are ejected with brighter light Explanation of observation 2: of a certain color, but the energy of each The more the intensity = the more no. of photon = each photon releasing one e but the energy of electron is the same. each e is fixed as frequency of each photon is fixed Spectrum • Visible light/white light is radiant energy coming from sun or from incandescent lamps. • It composed of light waves in the range of 400- Planck’s quantum theory & Einstein’s 800nm. Each wave has a characteristic color photon theory proved that • When a beam of white light from an incandescent light bulb pass through a prism, diff wavelengths are refracted at diff angles, when received on a screen these - Energy/electromagnetic radiation/light form series of color bands: VIBGYOR, Shorter λ's are has fixed quantity & it is discrete bent more than longer λ 's. particle • Thus a continuous spectrum results But those were only true for matter only previously. Prepared byMd. Faysal Ahamed Khan
  11. 11. Eelo American University, Borama, Awdal, 11/6/2008 Somaliland Pass a current through a hydrogen gas in a discharge tube at low pressure. The gas glows. Bohr Model When examined through a spectroscope, a line • In 1913, the spectrum is observed, not a continuous one as Danish scientist seen with the light bulb. Niels Bohr proposed a model for H atom and explain the line spectra of Hydrogen. • In his model, Bohr used Planck’s and Einstein’s idea about quantized energy and proposed three The wave theory of light could not explain postulates: the line spectra of excited gases Bohr Model Bohr model cont’d 1. Electrons travel around the nucleus in specific orbits at fixed distance from 2. While in these orbits, an e does not the nucleus and e- in each orbit have a radiate (or lose) energy. These orbit definite energy. is called energy levels. 3. The electron can moves to another orbit/energy level only by absorbing or emitting a photon/quantum, whose energy equals the difference in energy between the two orbits/levels. Ephoton = Estate A – Estate B = E = hν Prepared byMd. Faysal Ahamed Khan
  12. 12. Eelo American University, Borama, Awdal, 11/6/2008 Somaliland Bohr explanation for line LIMITATION of BOHR spectrum MODEL • Bohr ‘s model explain that an atomic But problems existed with Bohr theory — spectrum is not – theory only successful for the H atom, continuous because a that is it failed for more that one electron atom has only certain atom discrete energy levels – Electrons do not travels in fixed orbits or states • A spectrum line results when a photon of specific energy (and thus specific energy) is emitted as electron moves from a higher energy states to a lower energy states THE WAVE MODEL The Wave Model • Today’s atomic • In fact, it is impossible to determine the model is based exact location of an electron. The probable on the principles location of an electron is based on how of wave much energy the electron has. mechanics. • According to the modern atomic model, at • According to the atom has a small positively charged nucleus theory of wave surrounded by a large region in which there mechanics, are enough electrons to make an atom electrons do not neutral. move about an atom in a definite path/ORBIT. Prepared byMd. Faysal Ahamed Khan
  13. 13. Eelo American University, Borama, Awdal, 11/6/2008 Somaliland Electron Cloud: • ELECTRON CLOUD is a space in which electrons are likely to be found. • Electrons whirl about the nucleus billions of times in one second • They are not moving around in random patterns. • Location of electrons depends upon how much energy the electron has. • Depending on their energy they are locked into a certain area in the cloud. • Electrons with the lowest energy are found in the energy level closest to the nucleus • Electrons with the highest energy are found in the outermost energy levels, farther from the nucleus. Prepared byMd. Faysal Ahamed Khan