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TOPIC 2 ATOMIC STRUCTURE 2.1 THE NUCLEAR ATOM ESSENTIAL IDEA The mass of an atom is concentrated in its minute, positively charged nucleus. NATURE OF SCIENCE (1.8) Evidence and improvements in instrumentation - alpha particles were used in the development of the nuclear model of the atom that was first proposed by Rutherford. NATURE OF SCIENCE (2.3) Paradigm shifts – the subatomic particle theory of matter represents a paradigm shift in science that occurred in the late 1800s. INTERNATIONAL-MINDEDNESS Isotope enrichment uses physical properties to separate isotopes of uranium and is employed in many countries as part of nuclear energy and weaponry programs. THEORY OF KNOWLEDGE Richard Feynman: “If all of scientific knowledge were to be destroyed and only one sentence passed on to the next generation, I believe it is that all things are made of atoms.” Are the models and theories which scientists create accurate descriptions of the natural world, or are they primarily useful interpretations for prediction, explanation and control of the natural world? No subatomic particle can be or will be directly observed. Which ways of knowing do we use to interpret indirect evidence, gained through the use of technology? UNDERSTANDINGS/KEY IDEA 2.1.A Atoms contain a positively charged dense nucleus composed of protons and neutrons (nucleons). UNDERSTANDINGS/KEY IDEA 2.1.B Negatively charged electrons occupy the space outside the nucleus. Protons – found in nucleus Neutrons – found in nucleus Electrons – surround the nucleus in energy levels Element – substance that cannot be broken down into simpler substances by a chemical reaction. Atom – smallest particle (species) of an element that retains the properties of that element. Compound – the chemical combination of two or more elements MODELS OF THE ATOM John Dalton – Dalton’s atomic theory JJ Thomson – plum pudding model and discovered the electron Ernest Rutherford – gold foil experiment and discovered the proton Niels Bohr – solar system model where the electrons orbit the nucleus Quantum Mechanical Model – modern theory where electrons exist in cloud shapes or “orbitals” DALTON’S ATOMIC THEORY 1. All elements are composed of atoms. 2. Each element has atoms that are different from the atoms of any other element. 3. Atoms cannot be subdivided, created or destroyed. 4. Atoms of different elements combine in simple ratios to form chemical compounds. 5. One type of atom cannot be changed into another type of atom by a chemical reaction. MODERN ATOMIC THEORY Not all aspects of Dalton’s theory proved to be correct. Atoms can be subdivided. Atoms can have different masses (isotopes). Important parts that are still relevant: All matter is composed of atoms. Atoms of one element differ in properties from those of every other element. JJ THOMSON’S MODEL PLUM PUDDING MODEL Discovered the electron with its negative charge Adapted the Dalton model to display negative electrons suspended in a positive “fluid” Positive gooey stuff Looks like a chocolate chip cookie! Negative electron, Also called the “Plum Pudding” model. held in place RUTHERFORD’S MODEL Discovered the positively charged, dense nucleus Contained most of the mass of the atom Electrons surrounded the central nucleus Most of the atom was empty space Positive nucleus Negative electrons Empty space There’s a problem with the Rutherford Model… What do positive and negative charges do? They attract to each other! So, in this model, why don’t the e- just move into the nucleus? BOHR’S MODEL Neils Bohr had a possible solution… Instead of the electrons just hanging out around the nucleus (which would lead them to crash into it)… Maybe the electrons had energy, and maybe they “orbited” the nucleus like planets orbit the Sun! Bohr came up with the idea that the size of an electron’s orbit was related to how much energy the electron had. the energy level of an electron would determine how far away from the nucleus the electron would be. Energy Levels Energy levels are like the steps on a ladder: You can’t stand between the steps on a ladder, and electrons cannot hang out between energy levels. Number the energy levels: n = 1, 2, 3, 4, … Energy levels are different from the steps on a ladder because they are NOT evenly spaced! n=5 n=4 n=3 n=2 Increasing Energy! Energy Levels n=1 (lowest energy an e- can have) Nucleus Nice, normal ladder Energy level ladder (ground floor) Quantum Mechanical Model Now, we know that electrons do not follow in specific paths around the nucleus Instead, we currently believe that they pop in and out of existence, so fast it’s crazy. • Like camera flashes going off when a superstar walks in! Quantum Mechanics is used to explain this crazy behavior It’s based on probabilities (chances) that something will be true. QUANTUM MECHANICAL MODEL Quantum Mechanics uses a “cloud” model to describe where the electron is likely to be found. These clouds take on particular shapes based on where an electron with a specific energy is most likely to be found. There is a 90% chance that the electron is somewhere in here. Quantum Mechanical Model - Orbitals Quantum Mechanics keeps the idea of energy levels – these are actually the rows on the periodic table. It also adds sublevels, known as “atomic orbitals” These orbitals are referred to as s, p, d, f The shapes of atomic orbitals depend on the energy levels. APPLICATION/SKILLS Be able to use the nuclear symbol A X to deduce the number of Z protons, neutrons and electrons in atoms and ions. Atomic number – number of protons in the nucleus Mass number – number of protons plus neutrons in an atom Isotope – atoms with the same number of protons but different numbers of neutrons (in other words different mass numbers) SHORTHAND NOTATION X MASS NUMBER (A) = PROTONS + NEUTRONS A Z ATOMIC NUMBER (Z) = PROTONS Ion – atom that has lost or gained an electron Cation – positive ion formed by the loss of one or more electrons Anion – negative ion formed by the gain of one or more electrons Given shorthand notation, isotopic information or an ion, you should be able to figure out how many protons, neutrons and electrons are present. Remember the proton number identifies the element. To be neutral, electrons and protons must equal. If you have an ion, your electrons will be either more or less than the protons depending upon the charge. ISOTOPE SYMBOLS Chlorine exists as 2 isotopes: 35Cl and 37Cl These can also be written as chlorine35 and chlorine-37. The difference is the number of neutrons. EXAMPLES 1. Chlorine-35 has 17p, 17e, and 18n 2. Al3+ has 13p, 10e and 14n 3. F- has 9p, 10e, and 10n GUIDANCE Relative masses and charges of the subatomic particles should be known. The mass of the electron can be considered negligible. Masses and charges of sub-atomic particles RELATIVE MASS RELATIVE CHARGE 1 +1 ELECTRON 0.0005 -1 NEUTRON 1 0 PARTICLE PROTON UNDERSTANDINGS/KEY IDEA 2.1.C The mass spectrometer is used to determine the relative atomic mass of an element from its isotopic composition. THE MASS SPECTROMETER The mass spectrometer is used to measure the masses of different isotopes and their relative abundance. It has 5 basic operations. MASS SPECTRA The results of the mass spectrometer are presented in the form of a mass spectrum. The mass spectra for Molybdenum looks like this. There are 7 isotopes shown with their % abundance. RELATIVE ATOMIC MASS The relative atomic mass (Ar) of an element is the average mass of an atom of the element taking into account all its isotopes and their relative abundance. This is why the atomic mass is not a whole number. APPLICATION/SKILLS Be able to calculate non-integer relative atomic masses and abundance of isotopes from given data, including mass spectra. The masses of atoms of all elements actually range from 1x10-24 to 1x10-22g. These numbers are difficult to manage so we use relative values. To use relative values, a standard has to be agreed upon. The carbon-12 isotope was chosen as the standard in 1961 and was given the relative mass of 12.000 exactly. The masses of all other elements are measured relative to 12C. EXAMPLE 1 What is the relative atomic mass of chlorine if it has two isotopes with the following abundances: 35Cl at 75% and 37Cl at 25%? Multiply the isotope mass by the abundance and add them together. (35 x .75) + (37 x .25) = 35.5amu EXAMPLE 2 Boron exists in 2 isotopic forms, 10B and 11B. Use your periodic table to find the abundances of the two isotopes. You must recognize that the atomic mass for Boron is 10.81 so it should make sense to you that more of 11B exists since 11 is closer to 10.81 than 10 is. Let x atoms be 10B, therefore 11B would be 1 – x. Remember you have to multiply the isotope mass by the abundance to get total mass. 10x + 11(1-x) = 10.81 10x + 11 – 11x = 10.81 11 – x = 10.81 11-10.81 = x .19 = x So the abundances are 10B = 19.00% and 11B = 81.00% EXAMPLE 3 Determine the average atomic mass of the following element from the mass spec data. .813(10) + .187(11) = 10.19 amu www.youtube.com Compare the properties of the isotopes of an element. Isotopes show the same chemical properties as their parent element since neutrons do not affect how they react. Isotopes with more neutrons are heavier and move more slowly at a given temperature. This can be used as a means to separate them. The difference in neutrons does affect physical properties like boiling and melting points, mass, density and rate of diffusion for gases. Remember a physical property is something that can be measured without changing the chemical composition of the substance. Citations International Baccalaureate Organization. Chemistry Guide, First assessment 2016. Updated 2015. Brown, Catrin, and Mike Ford. Higher Level Chemistry. 2nd ed. N.p.: Pearson Baccalaureate, 2014. Print. Most of the information found in this power point comes directly from this textbook. The power point has been made to directly complement the Higher Level Chemistry textbook by Catrin and Brown and is used for direct instructional purposes only.