Materi 1 dan Materi 2 . Struktur materi : Struktur atom dan tinjauan perkembangan Teori Atom Sejaran Struktur Atom dan PerkembanganTeori Atom: Teori Atom Dalton. Model Atom JJ Thomson, Model Atom Ruterford, Model atom N Borh, Model atom mekanika gelombang 9/21/2020 1 Structure of matter Definition of Chemistry: The study of the properties, composition, and Structure of matter, the physical and chemical changes it undergoes, and the energy liberated or absorbed during those changes. Material Properties Bulk Structure Molecular Structure Atomic Structure 9/21/2020 2 ATOMIC STRUCTURE HISTORY Modern Concepts of Matter John Dalton (1803) - An atomist who formalized the idea of the atom into a viable scientific theory in order to explain a large amount of empirical data that could not be explained otherwise. Matter is composed of small “indivisible” particles called “atoms”. The atoms of each element are identical to each other in mass but different from the atoms of other elements. A compound contains atoms of two or more elements bound together in fixed proportions by mass. 9/21/2020 3 Definisi atom 2500 tahun yang lalu : atom = “not cut” INTI TEORI ATOM DALTON : 1. Materi terdiri atas atom atom 2. Atom merupakan bagian terkecil dari materi dan tidak dapat terbelah lagi 3. Atom dari suatu element memiliki massa dan sifat sama 4. Atom dari element berbeda memiliki massa dan sifat berbeda 5. Atom atom dapat bergabung membentuk senyawa dengan rasio massa tertentu Asumsi Dasar Teori Dalton Tiap unsur kimia tersusun oleh partikelpartikel kecil yang tidak bisa dihancurkan dan dibagi, yang disebut atom. Selama perubahan kimia, atom tidak bisa diciptakan dan juga tidak bisa dimusnahkan Semua atom dari suatu unsur mempunyai massa (berat) dan sifat yang sama, tetapi atom-atom dari suatu unsur berbeda dengan atom dari unsur yang lain, baik massa (berat) maupun sifatsifatnya berlainan. Dalam senyawa kimiawi, atom-atom dari unsur yang berlainan melakukan ikatan dengan perbandingan numerik yang sederhana : Misalnya satu atom A dan satu atom B (AB) satu atom A dan dua atom B (AB2). Perkembangan dan kemajuan pemahaman hubungan materi dan listrik. Fakta : Atom bukan unsur terkecil, ada sub atomic ATOMIC STRUCTURE Nucleus Model of a Helium-4 (4He) atom e- p+no no p+ eElectron Cloud NEXT ….. Teori Atom Dalton, Democritus → Thompson, Planck, Einstein, Millikan, Rutherford, Bohr, de Broglie, Heisenberg, Schrödinger, Chadwick, and many others. 9/21/2020 7 ATOMIC STRUCTURE Democritus - First atomic ideas Dalton - 1803 - First Atomic Theory J. J. Thompson - 1890s - Measured the charge/mass ratio of the electron (Cathode Rays) Fluorescent Material _ Cathode + Anode 9/21/2020 Electric Field Source (Off) 8 ATOMIC STRUCTURE Fluorescent Material _ Cathode + Anode Electric Field Source (Off) With the electric field off, the cathode ray is not deflected. 9/21/2020 9 ATOMIC STRUCTURE - Cathode + Anode Fluorescent Material + Electric Field Source (On) With the electric field on, the cathode ray is deflected away from the negative plate. The stronger the electric field, the greater the amount of deflection. Cathode + Anode 9/21/2020 Magnet 10 Pengamatan J.J. Thomson (1856-1940) Kode C = Katoda; A = Anoda; E = lempeng kondensor bermuatan listrik; M = magnet; F = layar berfluoresens. Berkas 1 : Hanya dengan adanya medan listrik, berkas sinar katoda dibelokkan keatas menyentuh layar pada titik 1. Berkas 2 : Hanya dengan adanya medan magnit, berkas sinar katoda dibelokkan kebawah menyentuh layar pada titik 2. Berkas 3 : Berkas sinar katoda akan lurus dan menyentuh layar dititik 3, bila medan listrik dan medan magnit sama besarnya With the magnetic field present, the cathode ray is deflected out of the magnetic field. The stronger the magnetic field, the greater the amount of deflection. e/m = E/H2r e = the charge on the electron m = the mass of the electron E = the electric field strength H = the magnetic field strength r = the radius of curvature of the electron beam Thompson, thus, measured the charge/mass ratio of the electron - 1.759 x 108 C/g 9/21/2020 12 Thomson’s Atomic Model J. J. Thomson Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model. Summary of Thompson’s Findings Cathode rays had the same properties no matter what metal was being used. Cathode rays appeared to be a constituent of all matter and, thus, appeared to be a “sub-atomic” particle. Cathode rays had a negative charge. Cathode rays have a charge-to-mass ratio of 1.7588 x 108 C/g. 9/21/2020 14 FAKTA YANG ADA …………….. •If a hydrogen atom absorbs radiation whose energy corresponds to the difference between that of n=1 and some higher value of n, the atom is said to be in an excited state. Excited states are unstable and quickly decay to the ground state, but not always in a single step. For example, if the electron is initially promoted to the n=3 state, it can decay either to the ground state or to the n=2 state, which then decays to n=1. Thus this single n=1→3 excitation can result in the three emission lines depicted in the diagram above, corresponding to n=3→1, n=3→2, and n=2→1. If, instead, enough energy is supplied to the atom to completely remove the electron, we end up with a hydrogen ion and an electron. When these two particles recombine (H+ + e– → H), the electron can initially find itself in a state corresponding to any value of n, leading to the emission of many lines. R. A. Millikan - Measured the charge of the electron. In his famous “oil-drop” experiment, Millikan was able to determine the charge on the electron independently of its mass. Then using Thompson’s charge-to-mass ratio, he was able to calculate the mass of the electron. e = 1.602 10 x 10-19 coulomb e/m = 1.7588 x 108 coulomb/gram (Thomson) m = 9.1091 x 10-28 gram 9/21/2020 16 Mass of the Electron Mass of the electron is 9.11 x 10-28 g The oil drop apparatus 1916 – Robert Millikan determines the mass of the electron: 1/1840 the mass of a hydrogen atom; has one unit of negative charge Menghitung massa elektron. Anda dapat memperoleh penyelesaian dengan mensubstitusikan nilai yang didapat Millikan pada hubungan: muatan/massa = 1,76 x 108 (C g–1). Maka, m = e/(1,76 x 108 C g–1) = 1,6 x 10–19 C/(1,76 x 108C g–1) = 9,1 x 10–28 g. Muatan elektron =1,6 x 10–19 C. Ratio muatan/massa (Thomson ) = 1,76 x108 C/g), Goldstein - measure the mass of proton Goldstein - Conducted “positive” ray experiments that lead to the identification of the proton. The charge was found to be identical to that of the electron and the mass was found to be 1.6726 x 10-24 g. 9/21/2020 19 Menghitung rasio massa elektron dan atom hidrogen. Massa atom Hidrogen (H) adalah: mH = 1 g/6 x 1023 = 1,67 x 10–24g. Jadi, me : mH = 9,1 x 10–28g : 1,67 x10–24g = 1 : 1,83 x 103. Sangat menakjubkan bahwa massa elektron sangat kecil. Bahkan atom yang paling ringanpun, hidrogen, sekitar 2000 kali lebih berat dari massa elektron. Menghitung ukuran atom Dengan menganggap molekul air berbentuk kubus, perkiraan ukuran satu atom (nyatakan dengan notasi saintifik 10x). Volume 1 mol air sekitra 18 cm3. Jadi volume 1 molekul air: v = 18 cm3/6 x 1023 = 3x10–23 cm3 = 30 x 10–24 cm3. Panjang sisi kubus adalah (30 x 10–24)1/3 cm = 3,1 x 10–8 cm. Nilai ini mengindikasikan bahwa ukuran atom sekitar 10–8 cm. Thomson mengasumsikan bahwa atom dengan dimensi sebesar itu adalah bola seragam bermuatan positif dan elektron-elektron kecil yang bermuatan negatif tersebar di bola tersebut. Dalam kaitan ini model Thomson sering disebut dengan “model bolu kismis”, kismisnya seolah elektron dan bolunya adalah atom. ….Nilai ini mengindikasikan bahwa ukuran atom sekitar 10–8 cm. Thomson mengasumsikan bahwa atom dengan dimensi sebesar itu adalah bola seragam bermuatan positif dan elektron-elektron kecil yang bermuatan negatif tersebar di bola tersebut. . Dalam kaitan ini model Thomson sering disebut dengan “model bolu kismis”, kismisnya seolah elektron dan bolunya adalah atom 9/21/2020 22 Apa kelemahan model atom Thomson? - tidak dapat menerangkan susunan muatan positif / negatif dalam sebuah atom. ATOMIC STRUCTURE Lanjutan …. 9/21/2020 24 ATOMIC STRUCTURE Ernest Rutherford - Developed the “nuclear” model of the atom. The Plum Pudding Model of the atom: + + + + - - - -- + + - - - - - + + - - - - ++ A smeared out “pudding” of positive charge with negative electron “plums” imbedded in it. The Metal Foil Experiments: Radioactive Material in Pb box. 9/21/2020 a-particles Fluorescent Screen Metal Foil 25 Ernest Rutherford’s Gold Foil Experiment - 1911 Alpha particles are helium nuclei - The alpha particles were fired at a thin sheet of gold foil ▪ Particles that hit on the detecting screen (film) are recorded ▪ Rutherford’s problem: In the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target? Target #1 Target #2 The Answers: Target #1 Target #2 Rutherford’s Findings Most of the particles passed right through ▪ A few particles were deflected ▪ VERY FEW were greatly deflected ▪ “Like howitzer shells bouncing off of tissue paper!” Conclusions: a) The nucleus is small b) The nucleus is dense c) The nucleus is positively charged The Rutherford Atomic Model Based on his experimental evidence: The atom is mostly empty space All the positive charge, and almost all the mass is concentrated in a small area in the center. He called this a “nucleus” The nucleus is composed of protons and neutrons (they make the nucleus!) The electrons distributed around the nucleus, and occupy most of the volume His model was called a “nuclear model” If the plum pudding model is correct, then all of the massive a-particles should pass right through without being deflected. In fact, most of the a - particles DID pass right through. However, a few of them were deflected at high angles, disproving the “plum pudding” model. Rutherford concluded from this that the atom consisted of a very dense nucleus containing all of the positive charge and most of the mass surrounded by electrons that orbited around the nucleus much as the planets orbit around the sun. 9/21/2020 31 Nucleus (INTI) Quiz 1. Assignment: Assume the diameter of the nucleus of a hydrogen atom is 1 x 10 -13 cm and the diameter of the atom is 1 x 10 -8 cm. 1. Calculate the volume of the nucleus and the volume of the atom in cm3 . 2. Calculate the volume of empty space in the atom. 3. Calculate the ratio of the volume of the nucleus to volume of the whole atom. 4. Calculate the density of the nucleus if the proton’s mass is 1.6726 x 10-24 g 9/21/2020 33 Problems with the Rutherford Model: It was known from experiment and electromagnetic theory that when charges are accelerated, they continuously emit radiation, i.e., they loose energy continuously. The “orbiting” electrons in the atom were, obviously, not doing this. The Fact : Atomic spectra and blackbody radiation were known to be DIScontinuous. The atoms were NOT collapsing. 9/21/2020 34 Atomic Spectra - Since the 19th century, it had been known that when elements and compounds are heated until they emit light (glow) they emit that light only at discrete frequencies, giving a line spectrum. - + Hydrogen Gas 9/21/2020 Line Spectrum 35 When white light is passed through a sample of the vapor of a substance, only discrete frequencies are absorbed, giving an absorption ban spectrum. These frequencies are identical to those of the line spectrum of the same element or compound. For hydrogen, the spectroscopists of the 19th Century found that the lines were related by the Rydberg equation: n/c = R[(1/m2) - (1/n2)] n = frequency c = speed of light 9/21/2020 R = Rydberg Constant m = 1, 2, 3, …. n = (m+1), (m+2), (m+3), …. 36 ATOMIC STRUCTURE Max Planck - In 1900 he was investigating the nature of black body radiation and tried to interpret his findings using accepted theories of electromagnetic radiation (light). He was NOT successful since these theories were based on the assumption that light had WAVE characteristics. To solve the problem he postulated that light was emitted from black bodies in discrete packets he called “quanta”. Einstein later called them “photons”. By assuming that the atoms of the black body emitted energy only at discrete frequencies, he was able to explain black body radiation. 9/21/2020 Einstein Equation E = hn 37 ATOMIC STRUCTURE Both spectroscopy and black body radiation indicated that atoms emitted energy only at discrete frequencies or energies rather than continuously. Is light a particle or a wave?? Why do atoms emit only discrete energies? What actually happens when light interacts with matter? What was wrong with Rutherford’s Model? 9/21/2020 38 ATOMIC STRUCTURE-BOHR Niels Bohr - Bohr corrected Rutherford’s model of the atom by formulating the following postulates: Electrons in atoms move only in discrete orbits around the nucleus. When in an orbit, the electron does NOT emit energy. They may move from one orbit to another but are NEVER residing in between orbits. 9/21/2020 When an electron moves from one orbit to another, it absorbs or emits a photon of light with a specific energy that depends on the difference in energy between the two orbits. 39 How the Bohr model explains the hydrogen line spectrum? Each spectral line represents an energy difference between two possible states of the atom. Each of these states corresponds to the electron in the hydrogen atom being in an "orbit" whose radius increases with the quantum number n. The lowest allowed value of n is 1; because the electron is as close to the nucleus as it can get, the energy of the system has its minimum (most negative) value. This is the "normal" (most stable) state of the hydrogen atom, and is called the ground state. The Bohr Model of the Atom Balmer Series (Visible) Lyman Series Paschen Series + (UV) (IR) 9/21/2020 41 The lowest possible energy state for an electron is called the GROUND STATE. All other states are called EXCITED STATES. En = (- 2.179 x 10-18 J)/n2 Ephoton = Ehigh - Elow Ephoton = [(- 2.179 x 10-18 J)/n2high] -[(- 2.179 x 10-18 J)/n2low] = - 2.179 x 10-18 J[(1/n2high) - (1/n2low)] Does this equation look familiar? Rydberg equation 9/21/2020 n/c = R[(1/m2) - (1/n2)] 42 Problem : Jari-jari r dapat diungkapan dalam persamaan r = n2aB, n = 1, 2, 3,... aB adalah jari-jari minimum (jari-jari Bohr= 5,2918 x 10– 11 m) bila n = 1. Tentukan jari-jari lainnya untuk n=2,3,4 Shell and Energy level RANGKUMAN TEORI ATOM BOHR 9/21/2020 45 Keterbatasan teori Bohr Keberhasilan teori Bohr begitu menakjubkan. Teori Bohr dengan sangat baik menggambarkan struktur atom hidrogen, dengan elektron berotasi mengelilingi inti dalam orbit melingkar. Setelah berbagai penyempurnaan, teori Bohr mampu menerangkankan spektrum atom mirip hidrogen dengan satu elektron seperti ion helium He+. Namun, spektra atom atom poli-elektronik tidak dapat dijelaskan. Kemudian menjadi jelas bahwa ada keterbatasan dalam teori ini yaitu tidak ada penjelasan persuasif tentang ikatan kimia dapat diperoleh. Dengan kata lain, teori Bohr adalah satu langkah ke arah teori struktur atom yang dapat berlaku bagi semua atom dan ikatan kimia. Pentingnya teori Bohr tidak dapat diremehkan karena teori ini dengan jelas menunjukkan pentingnya teori kuantum untuk memahami struktur atom, dan secara lebih umum struktur materi. Niels Bohr won the Nobel Prize for his work. However, the model only worked perfectly for hydrogen. What about all of those other elements?? Louis de Broglie - Thought that if light, which was thought to have wave characteristics, could also have particle characteristics, then perhaps electrons, which were thought to be particles, could have characteristics of waves. l = h/mv An electron in an atom was a “standing wave”! 9/21/2020 47 Karakter partikel dan karakter gelombang l = h/mv Particle character : massa, wave character : wavelength 9/21/2020 48 ATOMIC STRUCTURE Werner Heisenberg - Developed the “uncertainty” principle: It is impossible to make simultaneous and exact measurements of both the position (location) and the momentum of a sub-atomic particle such as an electron. (Dx)(Dp) h/2p Our knowledge of the inner workings of atoms and molecules must be based on probabilities rather than on absolute certainties. 9/21/2020 49 Quntum mechanism for atomic structure Erwin Schrödinger - Developed a form of quantum mechanics known as “wave mechanics”. Hy = Ey H = Hamiltonian operator E = Total energy of the system y = Wave function [(-h2)/(8p2m)]2y - [e2/r] y = Ey Kinetic Energy Term Potential Energy Term This is simply a quantum mechanical statement of the Law of Conservation of Energy 9/21/2020 50 Of the numerous solutions to the Schrödinger equation for hydrogen, only certain ones are allowed due to the following boundary conditions: Y, the wave function, must be continuous and finite. It must be single-valued at all points (There can’t be two different probabilities of finding an electron at one point in space). The probability of finding the electron, Y2, somewhere in space must = 1. + - Y2dxdydz = 1 Y has many values that meet these conditions. They are called “orbitals”. 9/21/2020 51 Ketidakpastian posisi electron 9/21/2020 52 Wave Function - A mathematical function associated with each possible state of an electron in an atom or molecule. It can be used to calculate the energy of an electron in the state the average and most probable distance from the nucleus the probability of finding the electron in any specified region of space. 9/21/2020 53 Quantum Numbers: Principle Quantum Number, n - An integer greater than zero that represents the principle energy level or “shell” that an electron occupies. n 1 2 3 4 etc. 9/21/2020 Energy Level 1st 2nd 3rd 4th etc. Shell K L M N etc. # of orbitals n2 1 2 9 16 etc. 54 Quantum number (n) 9/21/2020 55 ATOMIC STRUCTURE Azimuthal Quantum Number, l - The quantum number that designates the “subshell” an electron occupies. It is an indicator of the shape of an orbital in the subshell. It has integer values from 0 to n-1. l = 0, 1, 2, 3, …, n - 1 s p d f Magnetic Quantum Number, ml - The quantum number that determines the behavior of an electron in a magnetic field. It designates the orbital and has integer values from -l to +l including 0. ml = -l, …, -3, -2, -1, 0, +1, +2, +3, …, +l 9/21/2020 56 n 1 2 3 etc. l 0 0 1 0 1 2 etc. Orbital Name 1s 2s 2p 3s 3p 3d etc. # of ml Orbitals 0 1 0 1 -1, 0, +1 3 0 1 -1, 0, +1 3 -2, -1, 0, +1, +2 5 etc. etc. Spin Quantum Number, ms - The quantum number that designates the orientation of an electron in a magnetic field. It has half-integer values, +½ or -½. 9/21/2020 57 Orbital 1s vs orbital 2s 9/21/2020 58 9/21/2020 59 So what do atoms look like? A. Interpretation of Y: The probability of finding an electron in a small volume of space centered around some point is proportional to the value of Y2 at that point. B. Electron Probability Density vs. r C. Dot Density Representation: Imagine superimposing millions of photographs taken of an electron in rapid succession. D. Radial Densities 9/21/2020 60 Electron Configuration 9/21/2020 61 ATOMIC STRUCTURE Electron Configuration A. Many-electron atom: An atom that contains two or more electrons. B. Problems with the Bohr model: 1. It “assumed” quantization of the energy levels in hydrogen. 2. It failed to describe or predict the spectra of more complicated atoms. 9/21/2020 62 ATOMIC STRUCTURE C. What are the differences in electron energy levels in hydrogen vs. more complicated atoms? 3s 3p 2s 2p 3d Ground State Hydrogen Atom 1s 9/21/2020 63 ATOMIC STRUCTURE Splitting of the Degeneracy 2s 2p 2s 2p 1s 1s H 9/21/2020 Li 64 ATOMIC STRUCTURE Splitting of the Degeneracy 1. In hydrogen, all subshells and orbitals in a given principal energy level have the same energy. They are said to be Degenerate. 2. In many-electron atoms, s-orbitals have lower energy than p-orbitals which have lower energy than d-orbitals which have lower energy than f-orbitals, etc., etc. 3. Reason: Complex electrostatic interactions. 9/21/2020 65 + - - - - - Hydrogen ++ Helium +++ Lithium A. Shielding Effect - A decrease in the nuclear force of attraction for an electron caused by the presence of other electrons in underlying orbitals. B. Effective Nuclear Charge - A positive charge that may be less than the atomic number. It is the charge “felt” by outer electrons due to shielding by electrons in underlying orbitals. 9/21/2020 66 ATOMIC STRUCTURE The Pauli Exclusion Principle - No two electron in the same atom can have the same four quantum numbers. H + e- → H - Quantum Number n l ml ms 9/21/2020 Electron 1 1 0 0 +1/2 Electron 2 1 0 0 -1/2 67 The Aufbau Principle - A procedure for “building up” the electronic configuration of many-electron atoms wherein each electron is added consecutively to the lowest energy orbital available, taking into account the Pauli exclusion principle. Order of Filling 1s 2s 2p 3s 1s 2s 3s 4s 5s 9/21/2020 3p 4s 3d 4p 5s Increasing Energy 2p 3p 3d 4p 4d 4f 5p 5d 5f 5g 68 Designating Electron Configurations Standard Designation H 1s1 He 1s2 Li 1s2 2s1 B 1s2 2s2 2p1 Be 1s2 C 1s2 2s2 2p2 2s2 Orbital Diagram Designation H Li 1s He 2s Be 1s 9/21/2020 1s B 1s 2s 2p 1s 2s 2p C 1s 2s 69 Core Designation - A designation of electronic configuration wherein the outer shell electrons are shown along with the “core” configuration of the closest previous noble gas. Li Be [He] 2s2 Na [Ne] 3s1 Mg [Ne] 3s2 [Ar] 4s1 Ca [Ar] 4s2 Rb [Kr] 5s1 Sr [Kr] 5s2 K 9/21/2020 [He] 2s1 70 Hund’s Rule of Maximum Multiplicity - Electrons occupy a given subshell singly and with parallel spins until each orbital in the subshell has one electron. “Electrons try to stay as far apart as possible” Elevator Analogy 2 1 B [He] 2s 2p [He] C [He] 2s2 2p2 [He] N [He] 2s2 2p3 [He] Bus Seat Analogy 2s 9/21/2020 2p 71 Assignment: Write the electron configuration using all three types of designation for lead (Pb). Pb 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p2 Pb [Xe] 6s2 4f14 5d10 6p2 Electronic Configuration for postive ions (cations) Cations are formed by removing electrons in order of decreasing n value. Electrons with the same n value are removed in order of decreasing l value. 9/21/2020 72 Assignment: What are the electron configurations for Fe2+ Fe3+ Cr Cr3+ Se2- ? Questions? 9/21/2020 73 Sinar Kanal (Sinar Positif) Dalam tahun 1886 Eugen Goldstein melakukan serangkaian percobaan dan ia menemukan partikel jenis baru yang disebut sinar kanal (canal rays) atau sinar positif. Sinar katoda mengalir kearah anoda. Tumbukannya dengan sisa atom gas melepaskan elektron dari atom gas, menghasilkan ion yang bermuatan listrik positif. Ion-ion ini menuju ke katoda (-) tetapi sebagian dari ion ini lolos melewati lubang pada katoda dan merupakan arus partikel mengarah ke sisi lain. Berkas sinar positif ini disebut sinar positif atau sinar kanal. Sifat-sifat sinar kanal Partikel-partikelnya dibelokkan oleh medan listrik dan magnit dan arahnya menunjukkan bahwa muatannya positif. Perbandingan muatan dan massa (e/m) sinar positif lebih kecil daripada elektron. Perbandingan e/m sinar positif tergantung pada sifat gas dalam tabung. Perbandingan terbesar dimiliki oleh gas hidrogen. Untuk gas lain e/m merupakan pecahan integral (mis. ¼, 1/20 dari hidrogen). Perbandingan e/m dari sinar positif yang dihasilkan bila gas hidrogen ada dalam tabung adalah identik dengan e/m untuk gas hidrogen yang dihasilkan melalui air. Pengamatan ini dapat diterangkan dengan model atom yang dibuat J.J. Thomson yaitu model plum pudding. Kesimpulan dari sifat sinar kanal ini ialah semua atom terdiri dari satuan dasar yang bermuatan positif, pada atom H terdapat satu dan atom-atom lainnya mengandung jumlah lebih banyak. Satuan dasar ini sekarang disebut dengan proton. Unsur Radioaktif dan Radiasinya Ernest Rutherford membuktikan adanya dua jenis radiasi, sinar alfa dan sinar beta. Sinar a mempunyai kekuatan ionisasi besar tetapi daya tembusnya terhadap materi rendah. Sinar ini dapat ditahan oleh kertas biasa. Sinar ini adalah partikel yang membawa 2 satuan dasar muatan + dan mempunyai massa identik dengan He (Sinar a = ion He2+). Sinar sebaliknya memiliki kekuatan ionisasi rendah dan daya tembus besar. Sinar ini dapat melewati lempeng alumunium setebal 3 mm. Sinar ini memiliki partikel bermuatan negatif dengan e/m sama seperti elektron. Bentuk radiasi ketiga mempunyai daya tembus sangat besar dan tidak dibelokkan oleh medan listrik dan magnit. REM ini dikenal dengan sinar gamma (). Inti Atom Tahun 1909 Hans Geiger dan Ernest Marsden membuat serangkaian percobaan yang menggunakan lempeng emas yang sangat tipis dan logam lain (tebal 10-4 s.d. 10-5 cm) sebagai sasaran partikel a yang berasal dari radioaktif. Geiger dan Marsden mengamati bahwa” 1. Sebagian besar dari partikel a menembus lempeng logam tanpa pembelokkan. 2. Sebagian (~1 dari tiap 20.000) mengalami pembelokkan setelah menembus lempeng logam. 3. Dalam jumlah yang sama (poin 2) tidak menembus lempeng logam sama sekali tetapi berbalik sesuai arah datangnya sinar. Proton dan Neutron Pada tahun 1913 Moseley menemukan bahwa panjang gelombang sinar x bervariasi tergantung dari bahan sasarannya. Dengan menghubungkan hal ini ke persamaan matematis disimpulkan bahwa setiap unsur dapat ditetapkan dengan suatu bilangan bulat yang disebut nomor atom. Tahun 1919 Rutherford mengembangkan satuan dasar muatan positif yang disebut proton hasil risetnya dari jalur lintasan partikel a diudara. Konsep yang dipopulerkan oleh Rutherford adalah inti mengandung sejumlah proton yang sama dengan nomor atomnya dan sejumlah partikel netral yang disebut neutron agar sesuai dengan massa atom. Pada tahun 1930-an Chadwick membuktikan keberadaan neutron melalui percobaan pemboman berilium dan boron dengan partikel a, sehingga model atom yang terdiri dari elektron, proton dan neutron lengkap ditemukan. Subatomic Particles Particle Charge Mass (g) Location Electron (e-) -1 9.11 x 10-28 Electron cloud Proton (p+) +1 1.67 x 10-24 Nucleus Neutron (no) 0 1.67 x 10-24 Nucleus
