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Chapter 5 Early Atomic Theory and Structure Empedocle (440 BC): all matter consists of 4 “elements” earth, fire, water and air. Democritus (470-370 BC): matter is composed of indivisible particles (Greek atomos - not cuttable). Aristotle (384-322 BC): endorsed Empedoclean theory, so that it dominated until 17th century. John Dalton (1766-1844), 2000 years after Democritus, revived concept of atoms: Dalton’s rules: 1. elements are composed of indivisible particles-atoms. 2. atoms of the same element are alike in mass and size. 3. atoms of different elements are not alike – they have different masses and sizes. 4. compounds are union of two or more atoms of different elements. 5. compounds contain simple numerical ratios of atoms. 6. more than one compound can be formed of atoms of two elements. Dalton rules (cont.) Consequence of simple numerical ratio of atoms in compound: Law of definite composition – a compound contains two or more elements in a definite proportion by mass. For instance, 2g of hydrogen and 16 g of oxygen combine to form 18g of water. Law of multiple proportions – atoms of two or more elements may combine in different ratios to produce more than one compound. H2O and H2O2 are composed of two types of atoms, but different relative number. compound water hydrogen peroxide Copper (I) chloride Copper(II) chloride Methyl alcohol Ethyl alcohol formula H2O H2O2 CuCl CuCl2 CH4O C2H6O percent composition 11.2 % H, 88.8% O 5.9% H, 94.1% O 64.2% Cu, 35.8% Cl 47.3% Cu, 52.7% Cl 37.5% C, 12.6% H, 49.9% O 52.1%C, 13.1% H, 34.7% O All atoms of one type have the same mass, so they can be measured either by counting or by weighing. Water Hydrogen peroxide While most of the Dalton’s theory still holds, some modifications were necessary: 1. An atom is composed of subatomic particles, 2. Most elements contain atoms have isotopes - that have different masses. Electric Charge Matter may have electric charge 1. charge may be of two types, positive and negative 2. unlike charges attract, like repel 3. can be transferred from one object to the other by contact or induction 4. the smaller the distance, the stronger the force between two charges x q q x 1 2 (F – force, k - constant, q1, q2 - charges, r - distance) F=k 2 r G.J. Stoney (1826-1911) named the unit of electricity electron. Joseph Thompson (1856-1940) experimentally showed the existence of electron, which is: negative in charge, deflected by magnetic and electric fields, and capable of moving small paddle wheel. Michael Faraday (1791-1867) 1. Some compounds dissolved in water conduct electricity, others decompose 2. Particles of some elements are attracted to the positive or negative electrode. 3. Concluded that they are charged and called them ions (Greek-wanderer). Svante Arhenius (1859-1927) 1. Melted NaCl also conduct electricity. NaCl breaks up to Na+ and Cl- ions. 2. Ions move to oppositely charged electrode. Ion that goes towards positively charged cathode is called cation. The other goes to positive anode is called anion. Charge and Periodic Table IA H+ Li+ Na+ K+ Rb+ Cs+ IIA Invented by Dmitri Mendeleev, the Periodic Table shows recurring trends in properties of elements and their charges. VIIIA IIIA Be2+ Mg2+ Ca2+ IIIB IVB VB Cr2+ Cr3+ Sr2+ VIB VIIB Fe2+ Fe3+ ------ VIIIB ------- Al3+ IB IIB Cu2+ Cu+ Zn2+ Ag+ Cd2+ IVA VA VIA N3- O2- P3- S2- VIIA F- ClBr- I- Ba2+ Group IA: Group IIA: always 1+ always 2+ Charge = group number Elements of B groups (transition metals) form ions of different charges Group VA: Group VIA: Group VIIA: often 3usually 2- always 1Charge = 8 - group number Metals form positive charges, nonmetals negative charges. Group VIIIA: no charge: noble gases Subatomic particles Atom is extremely small, cannot be observed by optical microscope. Atomic diameters range from 0.1 to 0.5 nm. Consider this dot on the right. If its diameter is 1 mm, 10 million H atoms could be arranged in it. Yet, there are particles smaller than H atom. They are called subatomic particles. Three subatomic particles are sufficiently long-lived to be used in experiments: electron, proton and neutron. Proton and electron have charges opposite in sign, identical in absolute value. Proton and neutron have almost equal masses. Number of other short-lived subatomic particles are detected in recent years. quarks, neutrinos, antineutrinos, positrons and antiprotons ... Their existence is still debated. The Evolution of Atomic Theory Becquerel in 1896 discovered radioactivity. Radioactive elements spontaneously emit alpha particles, beta particles and gamma rays from their nuclei. By 1907 Rutherford found that alpha particles emitted by certain radioactive elements were helium nuclei. J.J. Thomson discovered electron in 1897. Shortly after, he proposed the first atomic ‘Plum-Pudding’ model where negatively-charged electrons (‘plums’) are embedded into a positively-charged sphere (‘pudding’). + Thomson and Goldstein discovered proton in 1907, and Chadwick discovered neutron in 1932. α particles should pass through the Rutherford’s experiment showed that the plumpudding model is incorrect. sphere because it is effectively neutral. α particle velocity ~ 1.4 x 107 m/s (~5% speed of light) The Evolution of Atomic Theory (cont.) Indeed, most of them did, but a few (1 in 20,000) were deflected, some in great If Thomson’s model was correct, αangles. It was “as if you were firing 15” particles would go straight through the foil. shell at a tissue paper, and it came back and hit you”. Rutherford’s Conclusions: 1. Atom consists of a nucleus and electrons. 2. Size of an atom is about 100,000 times bigger than that of its nucleus. Size of an atom: If the nucleus is as big as a marble, the atom is as big as football field. Thus, atoms are mostly empty space! 3. Atoms’ positive charge is concentrated in the nucleus. 4. Electron has negative charge, and nucleus has positive charge. Nucleus has both protons and neutrons. Mass of p is 1840 x mass of e- (1.67 x 10-24 g). In order to avoid falling onto the nucleus, electron must be constantly rotating with great speed, as if it was everywhere at once. Problem: classical physics says that the moving charge (electron) radiates energy. Creation of Ions An neutral atom has equal number of protons and electrons. Recall that the proton has equal charge to that of an electron, but opposite in sign. When atom loses one or more electrons, a positive cation is formed. When one or more electrons are added to a neutral atom, an anion is formed. The word ‘cation’ comes from the fact that positive ion (cation) moves towards negativelycharged (cathode) in an electrolyte solution. The negative ion (anion) moves towards positively-charged electrode (anode). Atomic Numbers of the Elements The atomic number of an element is equal to the number of protons in the nucleus of that element. It gives the identity of the element. Atomic number (Z) = number of protons in nucleus Mass number (A) = number of protons + number of neutrons = atomic number (Z) + number of neutrons Isotopes: atoms of the same element with different numbers of neutrons in their nuclei. Neutrons can be envisioned as glue that holds nuclei together. Charge: the difference in protons and electrons. The mass of 12C is 12.000 g. Mass Number Atomic Number A ZX There exists 1.11 % 13C and Charge traces of 14C. The mass of Element Symbol C atom (12.01 g) is weighted average of all atoms. Two isotopes of uranium 235 92 238 92 U U Protons: 92 Neutrons: 235-92=143 Protons: 92 Neutrons: 238-92=146 Three isotopes of hydrogen Hydrogen (Protium) Deuterium Tritium (radioactive) Masses of Elements and Isotopes To overcome the problem in measuring extremely small masses, a system of relative atomic masses using “atomic mass units” was devised to express the masses of elements using simple numbers. A mass of exactly 12 atomic mass units (amu) was assigned to 126 C. Element masses are average numbers, because of isotopes. Mass of H atom is 1.008 amu. The mass of a single atom is too small to measure on a balance. Using a mass spectrometer, the mass of the hydrogen atom was determined = 1.673 x 10-24 g. The standard to which the masses of all other atoms are compared to was chosen to be the most abundant isotope of carbon. 1 amu is defined as exactly equal to 1/12 the mass of a carbon - 12 atom: 1 amu = 1.6606 x 10-24 g 12 6 C Masses of elements and Isotopes (cont.) Isotopes of the same element have different masses. The listed atomic mass of an element is the average relative mass of the isotopes of that element compared to the mass of carbon-12 (exactly 12.0000…amu). To calculate the atomic mass, multiply the isotopic mass of each isotope by its abundance and add the results. Percent abundances must If the abundance is given as percent, divide it by 100. be divided by 100! Example: calculate average atomic mass of copper. Isotope Average Isotopic Abundance atomic mass (amu) (%) mass (amu) 63 29 Cu 62.9298 69.09 65 29 Cu 64.9278 30.91 (62.9298 amu) x 0.6909 = (64.9278 amu) x 0.3091 = 63.55 + 43.48 amu 20.07 amu 63.55 amu Finding the Number of Protons, Neutrons and Electrons Element Chlorine Gold Bromine Symbol 36Cl Au Z Mass# #p 17 79 56 36 197 Br - 35 Fill in the table below. 1. From Periodic table, find: Element, symbol, atomic number and #protons. At least one of those must be given, the other three are read from Periodic Table. Symbol Element Z = #p. 2. Find mass number and #n Mass# = #p + #n 3. Find charge or #electrons (charge) = #p – #e #e = #p – (charge) 80 17 79 35 #n #e charge 19 118 79 16 17 79 18 0 0 +2 -2 45 36 -1 Charge = 0 Locate Cl in periodic table Look up Atomic number (Z) for Cl Mass # is 36 = #p + #n #n = 36 – #p = 19 4. You can get #e and charge. Use step 3 to find #p, and from that identify element. See if you can solve the other two examples. The solutions are: 135Ba2+, and 32S2-. 56 16 Chapter 10 The Modern Atomic Theory and Periodic Table Rutherford model, based on classical (Newtonian) physics could not explain periodicity of the elements, nor how electrons stay in orbit around nucleus. A new model was needed. Light moves with speed of 300,000 km/s. It is characterized with wavelength λ. When passed through a prism, a familiar set of colors are seen. Spectrum is continuous as colors blend. Visible spectrum is just a tiny fraction of the electromagnetic radiation. Bohr’s Theory Niels Bohr in 1913 postulated that an electron inside an atom can possess only certain values of energy. Those “packs” of energies are called quanta (pl. of quantum). Atom is in the lowest energy when its electrons are closest to the nucleus. When atom absorbs quantum of energy, electrons are moved to higher orbits. Electrons can only have allowed energies, or the distances from the nucleus, not those between the allowed ones. It is like a person on a ladder who can stand on a 1st, 2nd, 3rd rung, but not between the rungs. Allowed orbits are called shells. They are characterized by the principal quantum number, n that goes from n = 1 (shell closest to the nucleus), to infinity. The maximum number of electrons a shell can hold increases with n as the radius of the shell increases. Number of electrons in a shell equals to 2n2. Li and Na have a single electron in the valence shell. Periodicity Elements have similar chemical properties because their valence shell configurations F and Cl have 7 electrons are similar. in the valence shell. The number of electrons in the valence shell is equal to the Roman-numeral group number for the representative elements. The dependence of chemical on valence shell occupancy holds for each group of elements. Helium is an exception. It behaves as a noble gas although it has only 2 ein the valence shell. Group IA has one electron, Group IIA has two… The lowest reactivity is found in the group VIIIA (noble gases). Elements of other groups can attain the valence shell of group VIIIA by producing compounds. Line Spectra Ground state s: 2 e- p: 6 ed: 10 e- f: 14 e- Excited state New, refined model includes subshells that are very close in energy and size. Every subshell is designated with a letter (s, p, d, f). s subshell can hold two electrons. Each subsequent subshell holds 4 e- more than the previous. Transition n=3 n=2 produces 13.1 eV – 11.2 eV = 1.9 eV, i.e. red color. Similarly, n=4 n=2 produces 13.8 – 11.3 = 2.6, green color; other colors in the visible spectrum are also found. Bohr’s model correctly predicted all transitions of H atom; transition into n=1 produces a line in UV, into n=2 (visible) and into n≥3 (infrared). But the model failed to predict spectrum for any atom with more than one e-. n s p d f 1 2 2 2 6 3 2 6 10 4 2 6 10 14 Each ‘box’ (orbital) contains 2 e- with paired spins. Filling of Subshells The number of subshells equals to n. They increase in energy in a shell: s < p < d < f. Subshells may cross each other; the crossing is more pronounced when spacing between shells reduces as the principle quantum number increases. 35Br This crossing is the reason for the socalled ‘potassium problem’. Present the electron configuration of 19K. 1s2 2s2 2p6 3s2 3p6 4s1 Valence electron Orbital diagram is slightly more complicated. 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5 Rearrange: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p5 Valence electrons Move He next to H to get four blocks, each with different width (s:2, p:6, d:10, f:14). Subshell fill up 16S S Try 26Fe: + 16 e Rule of filling p, d, f subshells: Each new electron goes in its own orbital, assuming the same spin as the others in that subshell. When all orbitals in a subshell have at least one electron, the next coming electron pairs with another in a half-filled orbital. 1s2 2s2 2p6 3s2 3p4 1s2 2s2 2p6 3s2 3p6 4s2 3d6 long e config. …first to leave are its or: [Ar] 4s2 3d6 electrons in the s orbital short e config. When a transition of the valence shell ! metal forms cation… Valence electrons: e- in unfilled shells + e- in the outermost shell. Valence electron jumps from Na to Cl The best way for Mg to achieve octet is to lose two electrons from its 3rd shell. Theoretically, Mg could also gain 6 e-, but the 6 e- excess is too much for Mg nucleus to hold on to. The best way for F to achieve octet is to gain one e-. Each e- from Mg 3rd shell jumps into 2nd shell of an F atom. Mg atom becomes Mg2+ cation. Each F atom becomes F- anion. All three ions have 8 valence e-. Compound Formation NaCl MgF2 Neutral atoms: Val.eNa: 1s2 2s2 2p6 3s1 Cl: 1s2 2s2 2p6 3s2 3p5 Note the size of atoms! Ions: no electrons in n = 3 shell of Na. Na+ has 8e- in n=2 shell. Cl- has 8e- in n=3 shell. Both ions satisfy octet rule. Note the size of ions! Atomic Size Understanding of atomic size with Bohr’s model. One consequence: shells get larger as n increases. A combination of same valence shell and increased nuclear charge shrinks the atom in a group. Quantum Mechanical Model Electron has wavelike characteristics, it can tunnel to appear in ‘forbidden’ places. Uncertainty principle states that it is impossible to know at any given time where the electron is, or where it is going. Instead, etravels around nucleus in an electron cloud or orbital. The QM model also explains why e- stays in orbit around the nucleus: An e- orbital is allowed only if the electron wave closes in on itself. Allowed orbit must close in on itself. Nonallowed orbit does not. Electron cloud exhibits wavelike motion, so that different orbitals have different shapes. It has a particular shape and energy determined by the principle quantum number n. HW Chapter 5 (p.95). 12. List similarities and differences in the three isotopes of hydrogen. 15. Write isotopic notation symbols for each of the following: Z=29, A=65; Z=20, A=45; Z=36, A=84 23. An element exists in two stable isotopic forms. One isotope has a mass of 62.9296 amu (69.17 % abundance). The other isotope has a mass of 64.9278 amu. Calculate the average mass of the element and determine its identity. 27. What experimental evidence supports these statements? (a) The nucleus of an atom is small; (b) The atom consists of both positive and negative charges; (c) The nucleus of the atom is positive. 39. Complete the following table with the appropriate data for each isotope given Assume all are neutral atoms. Element Symbol At.# Mass # #p #n #e 134Xe silver 107 9 92 143 92 41 19 HW, Chapter 10 (p.208) 3. Write the complete (long) and abbreviated (short) electronic configurations for the elements: Sc, Rb, Br, S. 17. Which elements have these configurations? 1s2 2s2 2p5. 1s2 2s2 2p6 3s1 [Ne] 3s2 3p4 [Ar]4s2 3d8. 21 (extra credit) For each of the element, write the corresponding orbital diagrams: F, S, Co, Kr, Ru. 35. Pick the electron structures that represent elements in the same chemical family: 1s2 2s1 1s2 2s2 2p6 3s2 3p6 1s2 2s2 2p4 1s2 2s2 2p6 3s2 3p6 4s2 1s2 2s2 2p2 1s2 2s2 2p6 3s2 3p6 4s1 1s2 2s2 2p6 3s2 3p4 1s2 2s2 2p6 3s2 3p6 4s2 3d1. 41. In which period and group does an electron first appear in an f orbital?