C 06J General Chemistry Course Outline (7 lectures) The Structure of the Atom Atomic particles: protons, neutron and electrons Mass Relationships of Atoms Atomic number, mass number, isotopes, mass and atomic spectroscopy (line spectra- Rydberg equation and the Balmer series) The Electronic Structure of the Atom Bohr model The wave nature of the electron Quantum numbers, atomic orbitals, electronic configuration The periodic table Atomic Structure Matter Anything that has mass and occupies space It exists in three states: solid, liquid and gas Element A substance that cannot be chemically broken down further For example, hydrogen, sodium, chlorine, silicon Atomic Structure The Greek philosopher proposed that matter is made up tiny particles called atoms Atom: derived from the Greek word Atomos (indivisible) Dalton’s Atomic Theory John Dalton (1766-1844) Elements are made up of tiny particles called atoms Atoms of the same element have the same mass (definite average mass), while atoms of different elements have different masses Atoms of the same element are chemically alike and atoms different elements are chemically different. Elements combine chemically in whole number ratios to form different substances Atoms are not changed during chemical reactions This theory does not explain what the atom is made up of Structure of the Atom Evidence supporting the structure of the atom was gathered using radioactive elements Several scientists conducted experiments that led to what is known today as the structure of the atom The atom consists of a positively charged nucleus The nucleus is very small and dense and has a diameter of 10-12 cm 1 Angstrom (Å) = 10-8 cm Therefore diameter of the nucleus is 10-4 Å Diameter of the atom is 1-5 Å Structure of the atom Evidence supporting the presence of a nucleus Experiment conducted by English physicist Ernest Rutherford In an evacuate tube, a beam of alpha particles were directed at a thin gold sheet Most of the particles passed through the sheet A few were slightly deflected Very few were greatly deflected (1 in 20,000) It was assumed that the alpha particles bounced back if they approached a positively charged nucleus head on Those that were slighly deflected passed close to the nucleus Most particles passed straight through the gold foil, which implied that the atom consists of a lot of space Rutherford’s Experiment www.sol.sci.uop.edu/~jfalward/physics17/chapter13/chapter113.html The Nucleus Except for the hydrogen atom, the nucleus of every atom consists of two types of particles: protons and neutrons Protons Positively charged Mass: 1.673 x 10-24 g The positive charge is equal in magnitude to the negative charge on the electron In each atom # of protons = # of electrons As a result the atom is neutral The Nucleus Neutrons Discovered by English scientist James C. Chadwick in 1932 Neutral particles (no electric charge) Mass: 1.6375 x 10-24 g Thomson’s Experiment This experiment involved the use of a cathode ray tube This is a glass tube from which the air has been removed and with two pieces of metals called electrodes attached An electric current flows through the tube when a sufficient voltage is applied The current flow is from the cathode (negatively charged) to the anode (positively charged) If the tube is not fully evacuated and still contains small amounts of air or other gases, the current flow is visible as a glow called a cathode ray This beam is produced at the negative electrode and is deflected towards the positive electrode The beam can also be deflected by a magnet or an electrically charged plate Thomson proposed that the cathode ray must consist of tiny negatively charged particle called electrons Being emitted from different kinds of metal, elements must contain electrons Thomson’s Cathode Ray Tube Experiment http://online.cctt.org.physicslab/content/phy2HON/lessonnotes/modern/electronbeams.asp Electrons Discovered by English scientist J. J. Thompson These are negatively charged particles They move about the nucleus with different energies Mass: 9.110 x 10-28 g Radius: 2.818 x 10-13 cm Structure of the Atom Comparison of Subatomic Particles Particle Mass/g Mass/amu Charge/C Charge/e Electron 9.109 x 10-28 5.486 x 10-4 -1.602 x 10-19 -1 Proton 1.673 x 10-24 1.007 +1.602 x 10-19 +1 Neutron 1.675 x 10-24 1.009 0 0 Atomic and Mass Number Atomic Number (Z) The number of protons present in the nucleus The number of electrons around the nucleus Mass number (A) The sum of the number of protons (Z) and neutrons (N) present in the nucleus A=Z+N Isotopes The number of protons is not always equal to the number of neutrons Therefore atoms of the same element can have different mass numbers These are called isotopes For example, Hydrogen has three isotopes Isotopes behave almost identical in their chemical reactions, the number of neutrons present has very little effect on the atoms chemical property The chemical property is determined by the number of electrons present Isotopes of Hydrogen Protium 1 proton 0 neutron 1 electron http:/encarta.msn.com/media_461531710_-1_1/Hydrogen_Isotopes.html Exercise How many protons, neutrons and electrons are present in this isotope of uranium 23592U? Answer 92 protons 92 electrons 143 neutrons Atomic Weight An element’s atomic weight is a weighted average of the isotopic masses of the element’s naturally occurring isotopes The mass of an atom is extremely small in grams and so is measured in atomic mass units (amu) instead 1 amu = 1/12 mass of carbon-12 = 1.66054 x 10-24 g For example, carbon Exercise Copper metal has two naturally occurring isotopes copper-63 (69.17%, isotopic mass 62.94 amu) and copper-65 (30.83%, isotopic mass 64.93 amu). Calculate the atomic weight of copper. Electromagnetic Spectrum Made up of a range of electromagnetic radiation Electromagnetic radiation is characterised by frequency, wavelength and amplitude www.cem.msu.edu/~reusch/virtualtext/spectrpy/UV-Vis/spectrum.htm Visible Light/White Light www.cem.msu.edu/~reusch/virtualtext/spectrpy/UV-Vis/spectrum.htm www.cem.msu.edu/~reusch/virtualtext/spectrpy/UV-Vis/spectrum.htm Wave Characteristics Frequency (ν): number of wave peaks or cycles per unit time. Units: s-1 or Hertz Wavelength (λ): distance from one crest or wave peak to another. Units: nm Amplitude: the height of the wave from the center line between peaks and troughs Wave Characteristics www.cem.msu.edu/~reusch/virtualtext/spectrpy/UV-Vis/spectrum.htm Waves The rate at which electromagnetic radiation travels is referred to as the speed of light c Speed of light = 2.997 x 108 m/s Wavelength (m) x Frequency (s-1) = Speed (m/s) c=λxν λ and ν are inversely related The light blue glow given off by mercury lamps has a wavelength of 436 nm. What is its frequency? Exercise What is the wavelength (in meters) of an FM radio wave with frequency ν = 102.5 MHz and a medical X ray with ν = 9.55 x 1017 Hz? Last Lecture Structure of the atom Subatomic particles Rutherford’s experiment Thomson’s experiment Atomic number Mass number, atomic mass unit Isotopes Electromagnetic spectrum Waves characteristics (c = λ ν)……..λ and ν are inversely related Atomic Spectra Visible light consists of a continuous distribution of wavelengths, i.e.400 (violet)-700 (red) nm These different wavelengths travel at different rates through a prism resulting in the splitting of of white light into the various different colours When atoms are excited they give off visible light This visible light given off by an excited atom is not in the form of a continuous distribution as with light from the sun Instead it consists of only a few wavelengths This results in a series of lines and blank spaces called a line spectrum Line Spectra http://neon.chem.uidaho.edu/~honours/spectra http://csep10.phys.utk.edu/astra162/lect/light/absorption.html Atomic Spectra Each element has its own line spectrum Na: yellow light K: purple light For hydrogen, a spectrum of four lines is produced at 656.3 nm (red), 481.6 nm (blue-green), 434.0 nm (blue) and 410.1 nm (indigo). According to Johann Balmer these wavelengths could be expressed as: 1/λ = R {1/22 – 1/n2} (Balmer Equation) R = Rydberg constant (1.097 x 10-2 nm-1) n = integer greater then 2 Atomic Spectra Line spectra are also present in the non-visible region of the spectrum These give rise to spectral line called Lyman series (far UV) as well as Paschen, Brackett and Pfund series (IR region) Those resulting from the visible region are referred to as the Balmer series Jahannes Rydberg used the Balmer equation to show that every aspect of the spectrum can fit the equation This resulted in the Balmer-Rydberg equation Balmer-Rydberg Equation 1/λ = R {1/m2-1/n2} Lyman series, m = 1 Balmer series, m = 2 Paschen series, m = 3 m and n are integers, where n is greater than m Points to note Light can behave like waves as well as small particles (so far we have looked at the wave characteristics) Excited electrons do not give off/emit light of continuous wavelength As a result, this does not give a continuous spectrum Light of specific energy is emitted (quantum) giving rise to a line spectrum Questions What are the two longest wavelengths lines (in nm) in the Lyman series of the hydrogen spectrum? What is the shortest wavelength line (in nm) in the Lyman series of the hydrogen spectrum? Exercise The Balmer equation can be extended beyond the visible portion of the electromagnetic spectrum to include lines in the ultraviolet. What is the wavelength (in nanometers) of ultraviolet light in the Balmer series corresponding to a value of n = 7? Particle-like Properties of Electromagnetic Radiation Blackbody radiation: visible glow given off by objects when heated The intensity of black body radiation varies with the wavelength of the light emitted As an object is heated, the colour change is dull red bright orange white The intensity does not rise indefinitely Instead, it reaches a maximum (λ = 500 nm) and then falls rapidlyIntensity of Blackbody Radiation vs. Wavelength Explanation The intensity of black body radiation does not rise indefinitely Therefore the energy given off cannot be continuous It is quantised, i.e. emitted in discrete units called quanta The energy of this radiation depends on the frequency Planck Equation E = hν h = Planck’s constant (6.626 x 10-34 Js) We already know that ν = c/λ Therefore E = hc/λ Question Calculate the energy of one quantum of red light of frequency 4.62 x 1014 s-1 and wavelength 649 nm. Recall Shorter wavelengths imply high frequency Longer wavelengths imply low frequency The energy of radiation/light depends on the frequency NOT intensity. Exercise What is the energy of photons of FM radiowaves with ν = 102.5 MHz Particle-like Properties of Electromagnetic Radiation For an electron to be ejected from the surface of a metal the light or energy supplied must be above a certain threshold value This threshold is different for every metal Einstein assumed that a beam of light behaves like a stream of particles with specific amounts of energies/frequencies These particles are called photons If the frequency is not above the threshold value, then no electron is ejected Wave-like Properties of Matter de Broglie suggested that if light can behave like matter (having particle characteristics), then matter can also exhibit some properties similar to those of light (i.e. wave-like properties) Both have wave and particle characteristics Energy, E = mc2 (Einstein) m = E/c2 m = mass of photon, c = speed of light According to Planck, E = hc/λ hc/ λ = mc2 de Broglie Equation de Broglie suggested that this equation could be applied to an electron by replacing the speed of light (photon) c with that of the speed of the electron, v As a result, the wavelength of an electron can be calculated Hence: de Broglie equation λ = h/mv Question What is the de Broglie wavelength of an electron of mass 9.11 x 10-31 kg and velocity 2.2 x 106 ms-1? Exercise What is the de Broglie wavelength of a pitched baseball with a mass of 120 g and a speed of 100 mph (44.7 m/s)? Exercise What is the de Broglie wavelength (in meters) of a small car with mass 1150 kg travelling at a speed of 55.0 mph (24.6 m/s)? The Electronic Structure of the Atom Bohr Model Bohr described the atom as a nucleus with an electron orbiting it, similar to the planets orbiting the sun Certain orbits correspond to specific energy levels that are available to the electron Electrons have specific energy levels available to them Model failed for atoms with more than one electron The path is now considered to be less definite Rather than a strict path, the electron seems to occupy empty space around the nucleus (electron cloud) Quantum Mechanical Model This abandons the notion of an electron as a small particle that moves in a specific path around the nucleus It focuses on the wavelike properties of the electron Werner Heisenburg showed that it is impossible to know exactly where the electron is and what path it follows Wave Function/Orbital Has a specific energy associated with it It contains information about the electron’s position (3D) For H atom the lowest energy level available to an electron is associated with the wave function called a 1s orbital An electron in a 1s orbital will occupy a spherical region of space around the nucleus The electron is most likely to be found closer to the nucleus, and less likely to be found as you move further away from the nucleus The electron’s path or movement is uncertain It assumes a wavelike description, rather than a particle like description Wave Function and Quantum Numbers A wave function or orbital describes the behaviour of an electron (position & path) Each wave function contains three variables called quantum numbers These are represented as n, l and ml They describe the energy level of the orbital as well as the shape and orientation of the region of space in which an electron will be found The Principal Quantum Number (n) This is a positive integer (n = 1,2,3…) It determines the size and energy level of the orbital As n increases the number of allowed orbitals increase and the size of the orbitals become larger This allows for electrons to be located far from the nucleus As n increases the energy of the electron in the orbital also increases According to n, orbitals are grouped into layers called shells around the nucleus The Principal Quantum Number (n) n=1 n=2 n=3 n=4 K shell L shell M shell N shell The Angular-momentum/Orbital Quantum Number (l) This defines the three dimensional shape of the orbital in which the electron moves The number of possible shapes is equal to the principal quantum number When the principal quantum number is n, the angular-momentum quantum number has values from 0 to n-1 According to l, orbitals are grouped into subshells referred to as s, p, d, f The Magnetic Quantum Number (ml) This defines the spatial orientation of the orbital along a standard set of coordinate axes When the angular-momentum quantum number is l, the magnetic quantum number has values from –l to +l For a group of orbitals having the same principal quantum number n, and the same shape l, the different spatial orientations for the orbital is (2l+1) The Spin Quantum Number ms This indicates the property of the electron described by two conditions It can be thought of as being right-handed or lefthanded, clockwise or counter clockwise Electron spin It can have either of two values: +1/2 or –1/2 The spin quantum number is independent of the first three quantum numbers Quantum Numbers The position of the orbital described by the first three quantum numbers can be occupied by two electrons, but these must have opposite spins No two electrons can have the same set of four quantum numbers (Pauli Exclusion Principle) No two electrons can have the same energy Question Identify the shell and subshell of an electron with the quantum numbers: n = 3, l = 1 and ml = 1 Exercise Give the possible combinations of quantum numbers for a 4p orbital. Exercise Give the possible combinations of quantum numbers for the following orbitals: (a) 3s (b) 2p (c) 4d Exercise Give the orbital notations for electrons with the following quantum numbers: (a) n = 2, l = 1, ml = 1 (b) n = 4, l = 3, ml = -2 (c) n = 3, l = 2, ml = -1 Explanation of Line Spectrum using the quantum mechanical model Electrons occupy orbitals Orbitals have specific energy levels The energy available to electrons are quantized When an atom is heated, the energy causes an electron to jump from one energy level to another (i.e. from a lower energy orbital to a higher energy orbital) The excited atom is unstable and quickly returns to the lower energy level This is accompanied by emission of the amount of energy that was added (i.e. the difference between the higher and lower energy orbitals) The energy of the orbitals are quantized, and the energy emitted is also quantized As a result, we observe the emission of only specific frequenices of radiation The variables m and n in the Balmer – Rydberg equation for hydrogen corresponds to the principal quantum numbers of the two orbitals involved in the electronic transition Question What is the energy difference (kJ/mol) between the first and second shells of the hydrogen atom if the first emission in the Lyman series occurs at λ = 121.5 nm? Orbital Shapes This corresponds to the angular-momentum quantum number (ℓ) The designations are s, p, d, f s Orbital These are spherical The probability of finding an electron depends on the distance from the nucleus, not on direction There is only one possible orientation of a sphere: mℓ = 0 There is one s orbital per shell The size on the orbital increases as you go to a higher shell Beyond the first shell, there are several regions of maximum probability s orbital http://www.chemsoc.org/ExemplarChem/entries/2004/dublin_fowler/sorbitals p Orbital These are dumbbell-shaped The electrons are distributed in identical lobes on either side of the nucleus The lobes are separated by a nodal plane that cuts through the nucleus When ℓ = 1, ml has three possibilities, one along each axis As n increases, the size of the p orbital increases p orbitals px py pz http://www.chemsoc.org/ExemplarChem/entries/2004/dublin_fowler/porbitals d orbital These are found in the third and higher shells There are five d orbitals with two different shapes Four of them are clover-leaf shaped, having four lobes of maximum probability and two nodal planes The fifth d orbital is dumbbell shaped along the z axis with a donut region along the xy plane All five have the same energy d orbitals dz2 dx2-y2 http://www.chemsoc.org/ExemplarChem/entries/2004/dublin_fowler/dorbitals d orbital dxy dxz dyz http://www.chemsoc.org/ExemplarChem/entries/2004/dublin_fowler/dorbitals f orbitals There are seven f orbitals Each having eight lobes of maximum electron probability separated by three nodes For hydrogen, the energy of an orbital is determined by n For multi-electron atom, the energy level of the orbitals depend on the shell as well as the subshells Within a given shell, the orbitals have slightly different energies This is due to electron-electron repulsion As a result, outer shell electrons are shielded from the effects of the nucleus by the inner shell electrons The overall/net nuclear charge felt by an electron is called the effective nuclear charge Zeff and is lower than the actual nuclear charge Z A lower angular momentum quantum number corresponds to a higher Zeff and therefore a lower energy level Electronic Configuration Guided by a set of rules called the aufbau principles Rules of the aufbau principles Lower energy level orbitals are filled before higher energy level orbitals An orbital can hold only two electrons, these two electrons must have opposite spins (PAULI EXCLUSION PRINCIPLE) For degenerate orbitals, one electron is placed in each orbital (with the same spin quantum number), making them half full, then the second electron is added Hund’s Rule If two or more orbitals with the same energy are available, one electron goes into each until all are half-full. The electrons in the half-filled orbitals all have the same value for their spin quantum number. Electron Configuration For carbon and nitrogen, the electrons are in different p orbitals i.e. px, py, pz The singly occupied orbitals have the same value of the spin quantum number +1/2 or –1/2 Orbital filling diagrams are used to specify electron configuration In these diagrams electron are represented as arrows (pointing up or down) A single arrow indicates a half filled orbital while an up and down pair indicate a full orbital The Periodic Table The periodic table can be divided into four regions or blocks of elements based on the orbitals being filled Elements to the left: s block Elements in the middle: d block Elements to the right: p block The detached elements at the bottom are called lanthanides and actinides, and these form the f block elements Periodic Table Question Give the ground state electron configuration of arsenic (Z = 33). Draw orbital filling diagram, indicating the electron as up or down. Exercise Give the expected ground state electron configuration for the following and draw orbital filling diagrams for the first two Ti (Z = 22) Zn (Z = 30) Sn (Z = 50) Exercise What is the likely ground state electron configuration for the sodium ion Na+ formed by loss of an electron from a neutral sodium atom? Anomalies with Electronic Configuration There are a few anomalies concerning electronic configuration Most of these anomalies usually occur in the elements with atomic number greater than 40 At this point the energy differences between the subshells are small For example Cr and Cu An electron moves from a 4s to an energetically similar 3d orbital A transfer in this way allows for decreased electron-electron repulsion and therefore lowers the overall energy of the atom Electron Configuration of Ions Elements on the left side of the periodic table (metals) tend to give up electrons to form cations Halogens and some other non-metallic elements accept electrons to form anions Electrons given up by a metal come from the highest energy occupied orbital The electrons accepted by a non-metal go to the lowest energy occupied orbital Electron Configuration of Ions Na: 1s2 2s2 2p6 3s1 Na+: 1s2 2s2 2p6 Ne configuration Cl: 1s2 2s2 2p6 3s2 3p5 Cl–: 1s2 2s2 2p6 3s2 3p6 Ar configuration Electron Configuration of Ions For Main Group Elements All the elements in group 1A of the periodic table form singly charged positive ions (cations) by losing one electron Likewise, the elements in group 2A form doubly positive ions by losing two electrons The halogens in group 7A gain one electron to form singly charged negative ions Group 6A non metals gain two electrons to form doubly charged negative ions Electron Configuration of Ions For transition metals These form cations by losing their valence s electrons, then their d electrons Mass Spectrometry Used to determine a compound’s molecular formula by determining its molecular weight Mass spectrometer: an instrument used to determine both atomic weight and molecular weight Mass Spectrometer http://www.chemguide.co.uk/analysis/masspec/howitworks.html Mass Spectrometry The sample is vaporized It is then injected (as a gas) into an evacuated chamber In this chamber, it is bombarded with a beam of high energy electrons This beam of electrons is able to knock off other electrons from the sample molecules making them positively charged ions Ions of different masses are produced since some ionized molecules fragment into smaller ions These different ions are accelerated by an electric field and pass between the poles of a strong magnet As a result they are deflected through a curved pipe Mass Spectrometry The radius of deflection depends on the mass of the ion Lighter ions are deflected more strongly than the heavier ones Varying the strength of the magnetic field allows for focusing of different ions through a slit and into a detector This gives rise to a mass spectrum results, which is a plot of ion mass versus intensity (molecular weight of various ions versus the relative number of ions produced) The heaviest ion is generally due to the ionized molecule itself By determining the mass of this ion the molecular weight of the molecule/compound can be determined Mass Spectrum http://www.chemguide.co.uk/analysis/masspec/howitworks.html Exercise (a)Define the term atomic weight A certain element has only two naturally occurring isotopes. One isotope has an abundance of 57.3 % and isotoic mass of 120.904 amu. Given that the atomic weight of the element is 121.757 amu, determine the isotopic masss of the other isotope. What element is it most likely to be? (b) Briefly state the rules of the aufbau (building up) principle (c) Draw the box and arrow diagram for the 3d, 4s and 4p orbitals of the atom having the following electronic configuration: [Ar] 3d10 4s2 4p2 Give the ground state electron configuration of the elements with the following atomic numbers (i) Z = 13 (ii) Z = 17 (iii) Z = 21 Exercise (a) On the basis of modern atomic theory, explain what is meant by an atomic orbital. (b) What three quantum numbers are used to distinguish one atomic orbital from another and what range of values is allowed for each quantum number? (c) Write the set of four quantum numbers for each of the electrons in a ground state neon atom. Exercise Write down the Balmer-Rydberg equation in terms of whole numbers m and n. Hence, given that the frequency of the emitted light due to a transition from the n = 5 level to amother level m is 2.969 x 1014 Hz, calculate the value of m. To which series of the atomic spectrum does the transition belong? Determine the energy of the radiation emitted. Exercise (a)The quantum mechanical wave function, ψ, for electrons in atoms is a function of n, ℓ and mℓ Write down the meaning of n, ℓ and mℓ. List the possible values of n, ℓ and mℓ for all the electrons in Na+ and ClSketch the shape of the orbitals with l = 1 and l = 2. (b) According to quantum mechanics the electron does not move in a defined circular orbit around the nucleus. Briefly explain the quantum mechanical view of the electron. (c) Calculate the wavelength of the line with the longest wavelength in the Lyman series of the hydrogen spectrum. (The Lyman series is made up of electron transitions to the n = 1 level of the hydrogen atom)