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Chapter Two The Atomic Theory of Matter

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Aristotle 400 B.C. - Democritus thought matter could not be divided indefinitely. 350 B.C - Aristotle modified an earlier theory that matter was made of four elements: earth, fire, water, air. Democritus Aristotle was wrong. However, his theory persisted for 2000 years. fire air water earth This led to the idea of atoms in a void.

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Relied on experimentation not the development of scientific theories. Developed some useful techniques in distillation, evaporation, crystallisation and filtration Aristotle rejected the idea of atoms, but accepted and refined the notion of four elements water, air, fire and earth One of the major goals of an alchemist was to discover a substance that would turn base metals such as iron, copper, and lead into gold.

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Robert Boyle Attempted to isolate Aristotles four elements Defined an element as being a substance that could not be broken down into simpler substances Was able to distinguish clearly between elements, compounds and mixtures

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Antoine Lavoisier Changed chemistry from a qualitative to a quantitative science through his tin experiment Showed that the mass of the products in a reaction is equal to the mass of the reactants proving the law of conservation of mass

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1800 -Dalton proposed a modern atomic model based on experimentation not on pure reason. All matter is made of atoms. Atoms of an element are identical. Each element has different atoms. Atoms of different elements combine in constant ratios to form compounds. Atoms are rearranged in reactions. His ideas account for the law of conservation of mass (atoms are neither created nor destroyed) and the law of constant composition (elements combine in fixed ratios).

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Discovery of subatomic particle i) The Discovery of Electrons *1897 J.J. Thomson- cathode ray experiment Thomsons experiment involved the use of cathode-ray tube. When a sufficiently high voltage is applied across the electrode, an electric current flows through the tube from negatively charged electrode ( the cathode) to the positively charged electrode (the anode).

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Later experiments had shown that cathode-ray can be deflected by electric or magnetic field. Because the beam is produced at a negative electrode and is deflected toward a positive plate, Thomson proposed that cathode rays are negatively charged fundamental particles found in all atoms which, we now called electrons. Furthermore, because electrons are emitted from electrodes made of many different metals, all these substances must contain electrons. By careful measuring the amount of deflection caused by electric and magnetic fields of known strength, He established the ratio of mass to electric charge for cathode ray that is, m/e = -5.6857x10 -9 g/coulomb.

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Complete revision questions page 32 (1 2) Summarise how subatomic particles were originally discovered through investigations on electricity What is a cathode ray?

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1) Daltons Billiard ball model (1800-1900) Atoms are solid and indivisible. 2)Thompson Plum pudding model (1900) Negative electrons in a positive framework. 3)The Rutherford model (around 1910) Atoms are mostly empty space. Negative electrons orbit a positive nucleus. Materials, when rubbed, can develop a charge difference. This electricity is called cathode rays when passed through an evacuated tube (demos). These rays have a small mass and are negative. Thompson noted that these negative subatomic particles were a fundamental part of all atoms.

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1896 Antoine Becquerel discovered uranium ores emit invisible rays through working with some photographic plates Marie and Pierre Curie experimented with radioactivity discovering that uranium, radium and polonium were disintegrating over time and emitting radiation

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Rutherford shot alpha ( ) particles at gold foil. Most particles passed through. So, atoms are mostly empty. Some positive -particles deflected or bounced back! Thus, a nucleus is positive & holds most of an atoms mass. Radioactiv e substance path of invisible -particles Lead block Zinc sulfide screen Thin gold foil

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1909 Ernest Rutherford ~ use particle to study the inner structure of atoms. When he directed a beam of -particles at a thin gold foil, he found that The majority of -particles penetrated the foil undeflected. Some particles experienced slightly deflections. A few (about one in every 20,000) suffered rather serious deflections as they penetrated the foil. A similar number did not pass through the foil at all, but bounced back in the direction from which they had come.

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James Chadwick Proposed the existence of uncharged particles through his experiments with Beryllium foil Showed that neutrons had almost the same mass as protons

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Therefore Modern picture of an atom, then, consist of three types of particles- electrons, protons and neutron. Electric Charge Mass ParticleSI (C ) Atomic SI (g)amu Located Electron -1.602x10 -19 -1 9.109x10 -28 5.49x10 -4 outside nucleus Proton+1.602x10 -19 +1 1.673x10 -24 1.0073 in nucleus Neutron 0 0 1.675x10 -24 1.0087 in nucleus

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Modern physics has revealed successively deeper layers of structure in ordinary matter. Matter is composed, on a tiny scale, of particles called atoms. Atoms are in turn made up of minuscule nuclei surrounded by a cloud of particles called electrons. Nuclei are composed of particles called protons and neutrons, which are themselves made up of even smaller particles called quarks. Quarks are believed to be fundamental, meaning that they cannot be broken up into smaller particles.

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Complete the revision questions page 36 (3 9). Check and review your answers.

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Frederick Soddy Found two kinds of thorium and isolated the difference in the number of neutrons. The Mass Spectrometer (provides information about;) The number of isotopes in a given sample of element The relative isotopic mass of each isotope The percentage abundance of the isotopes

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Most elements consist of a mixture of isotopes The relative atomic mass (A r )of an element represents the average mass of one atom, taking into consideration the number of isotopes of the element, their relative isotopic mass (RIM) and their relative abundance.

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(RIM first isotope x abundance)+(RIM second isotope x abundance) A r = ______________________________________________________ 100

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Using the data in the table calculate the RIM of Cl 37 IsotopeRIM% Abundance Cl 35 (Z = 17)34.9775.80 Cl 37 (Z = 17)unknown24.20 (RIM first isotope x abundance)+(RIM second isotope x abundance) A r (Cl) = ______________________________________________________ 100 Check your answer and workings on page 38

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Complete the revision questions page 39 (10 18). Check and review your answers.

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Modern atomic theory is concerned primarily with electron arrangement around the nucleus and has helped the understanding of the chemical properties and behaviour of atoms. In chemical reactions, when atoms react bonds are broken and atoms rearrange themselves to form new bonds. The formation of new bonds involves the redistribution of electrons.

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There are 2 types of spectra: continuous spectra & line spectra. Its when electrons fall back down that they release a photon. These jumps down from shell to shell account for the line spectra seen in gas discharge tubes (through spectroscopes). Electrons orbit the nucleus in shells Electrons can be bumped up to a higher shell if hit by an electron or a photon of light.

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If atoms emit only discrete wavelengths, they must have discrete energies. Each orbit corresponds to a different energy level An electron can move between energy levels but cannot remain between levels Quantum jump moving from one energy level to another A specific quantity of energy (a photon) is associated with each quantum jump made by an electron

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An electron can move from a lower energy level to a higher level by absorbing energy (eg flame) When the electron falls back to a lower level, energy is released as a consequence of the quantum jump. The energy given out is the difference in energy between the two energy levels and will be associated with a specific wavelength. Each wavelength corresponds to a coloured line in the emission spectrum.

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In the Quantum Mechanical Model an electron around a nucleus may be visualised as a cloud of negative charge.

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1. The energy levels of electrons are designated by principal quantum numbers, n, and are assigned specific values : n = 1, 2, 3, 4, 5 etc. These principal quantum numbers may be referred to as shells and are also called the K, L, M, and N shells. 2. Within each shell, several different subshells exist. The number of subshells equals the shell number Eg Shell number 2, has 2 subshells Each subshell corresponds to a different electron cloud shape Subshells are represented by the letters: s, p, d, f

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The way in which electrons are arranged around the nucleus generally lowest energy first Note that the 4s subshell is filled before the 3d subshell (3d is of a higher energy)

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Electron Configuration Exited States When an atom moves to a higher energy level than the ground state by absorbing energy, its electron configuration changes The outermost electron moves to a higher energy level subshell Eg Ne (ground state) 1s 2 2s 2 2p 6 Eg Ne (exited state) 1s 2 2s 2 2p 5 3s 1

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Electron Configuration The group and period in which an element is found is easily read from the electron configuration (is the number of valence electrons in the last shell being filled) Eg Be (Z = 4) 1s 2 2s 2 (the highest shell number being filled) Group ? _______ Ar (Z = 18) 1s 2 2s 2 2p 6 3s 2 3p 6 Period ? _______

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Electron Configuration Group number is found by using the total number of electrons in the last shell to count across the periodic table The period is the number of the last shell occupied by electrons Work through Sample Problems 2.2 and 2.3

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Consider an atom of Potassium: Potassium has 19 electrons. These are arranged in shells Nucleus The inner shell has __ electrons The next shell has __ electrons The next shell has the remaining __ electron Electron structure = 2,8,8,1

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ElementShell 1Shell 2Shell 3Shell 4 Hydrogen H 1 electron0 electron Helium He 2 electron0 electron

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ElementShell 1Shell 2Shell 3Shell 4 Lithium Li 2 electron1 electron0 electron Beryllium Be 2 electron 0 electron

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ElementShell 1Shell 2Shell 3Shell 4 Boron B 2 electron3 electron0 electron Carbon C 2 electron4 electron0 electron

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ElementShell 1Shell 2Shell 3Shell 4 Nitrogen N 2 electron5 electron0 electron Oxygen O 2 electron6 electron0 electron

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ElementShell 1Shell 2Shell 3Shell 4 Fluorine F 2 electron7 electron0 electron Neon Ne 2 electron8 electron0 electron

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ElementShell 1Shell 2Shell 3Shell 4 Sodium Na 2 electron8 electron1 electron0 electron Magnesium Mg 2 electron8 electron2 electron0 electron

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ElementShell 1Shell 2Shell 3Shell 4 Aluminium Al 2 electron8 electron3 electron0 electron Silicon Si 2 electron8 electron4 electron0 electron

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ElementShell 1Shell 2Shell 3Shell 4 Phosphorus P 2 electron8 electron5 electron0 electron Sulphur S 2 electron8 electron6 electron0 electron

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ElementShell 1Shell 2Shell 3Shell 4 Chlorine Cl 2 electron8 electron7 electron0 electron Argon Ar 2 electron8 electron 0 electron

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ElementShell 1Shell 2Shell 3Shell 4 Potassium2 electron8 electron 1 electron Calcium Ca 2 electron8 electron 2 electron

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Hydrogen 1,0,0,0 Helium 2,0,0,0 Lithium 2,1,0,0 Beryllium 2,2,0,0 Boron 2,3,0,0 Carbon 2,4,0,0 Nitrogen 2,5,0,0

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Oxygen 2,6,0,0 Fluorine 2,7,0,0 Neon 2,8,0,0 Sodium 2,8,1,0 Magnesium 2,8,2,0 Aluminium 2,8,3,0 Silicon 2,8,4,0

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Phosphorus 2,8,5,0 Sulphur 2,8,6,0 Chlorine 2,8,7,0 Argon 2,8,8,0 Potassium 2,8,8,1 Calcium 2,8,8,2

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Complete the revision questions pages 44, 45 (21 26) Work through the Sample Problems 2.4, 2.5 Complete the revision questions page 45 (27 31)

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Atomic numbers, Mass numbers There are 3 types of subatomic particles. We already know about electrons (e ) & protons (p + ). Neutrons (n 0 ) were also shown to exist (1930s). They have: no charge, a mass similar to protons Elements are often symbolized with their mass number and atomic number E.g. Oxygen: O 16 8 These values are given on the periodic table. For now, round the mass # to a whole number. These numbers tell you a lot about atoms. # of protons = # of electrons = atomic number # of neutrons = mass number atomic number Calculate # of e , n 0, p + for Ca, Ar, and Br.

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3545358035Br 1822184018Ar 20 4020Ca ee n0n0 p+p+ MassAtomic

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3 p + 4 n 0 2e 1e Li shorthand Bohr - Rutherford diagrams Putting all this together, we get B-R diagrams To draw them you must know the # of protons, neutrons, and electrons (2,8,8,2 filling order) Draw protons (p + ), (n 0 ) in circle (i.e. nucleus) Draw electrons around in shells 2 p + 2 n 0 He 3 p + 4 n 0 Li Draw Be, B, Al and shorthand diagrams for O, Na

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11 p+ 12 n 2e 8e 1e Na 8 p+ 8 n 2e 6e O 4 p+ 5 n Be 5 p+ 6 n B 13 p+ 14 n Al

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Isotopes and Radioisotopes Atoms of the same element that have different numbers of neutrons are called isotopes. Due to isotopes, mass #s are not round #s. Li (6.9) is made up of both 6 Li and 7 Li. Often, at least one isotope is unstable. It breaks down, releasing radioactivity. These types of isotopes are called radioisotopes Q- Sometimes an isotope is written without its atomic number - e.g. 35 S (or S-35). Why? Q- Draw B-R diagrams for the two Li isotopes. A- The atomic # of an element doesnt change Although the number of neutrons can vary, atoms have definite numbers of protons.

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3 p + 3 n 0 2e 1e 6 Li 7 Li 3 p + 4 n 0 2e 1e For more lessons, visit www.chalkbored.com www.chalkbored.com

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Chapter 2 Review Practice your electron dot diagrams here Complete the multiple choice questions pages 48, 49 Consider each of the review questions 1 - 11