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Topic 2 Atomic Structure IB Chemistry SL Coral Gables Senior High School Ms. Kiely Prepare for Lab Quiz: Put everything away except for a pen (no pencil) and a calculator. Bell-Ringer This is an example of a paper 3 question. In preparation for an experiment, a student attempts to make a solution that contains 1 mole of hydrated copper(II) sulfate crystals. He calculates that he will need [(1 x 63.55) +(1 x 32.07) + (4 x 16.00)] grams of the copper(II) sulfate to appropriately make the solution. Outline the major error in his calculation. Atomic Models: Atomic Models: Democritus The word atom comes from the Greek word atomos. Coined by a Greek philosopher named Democritus in 400 B.C., the word atomos means indivisible. Democritus believed all matter was composed of microscopic, solid, indivisible particles. According to Democritus’ model, the atom would be a fundamental particle: the smallest particle of matter. Does Democritus’ model of the atom agree with our current understanding of the atom? Atomic Models: John Dalton Model John Dalton was an English chemist who, similar to Democritus, proposed that all matter was made up of small, indivisible spheres called atoms. Unlike Democritus, though, Dalton actually had empirical reasoning for this; he performed many experiments with gases, which led him to his conclusions. Dalton’s Atomic Theory: All matter is composed of tiny indivisible particles called atoms Atoms cannot be created or destroyed Atoms of different elements are different Atoms can combine together in small numbers to form molecules and compounds ● Atoms of the same element are alike in every way ● ● ● ● Was Dalton Right? True or False: “All matter is composed of tiny indivisible particles called atoms” False! The atom is NOT fundamental. It is made up of protons, neutrons and electrons, and therefore is divisible. Subatomic particles are SO SMALL that instead of describing them by their actual values we describe their masses by relative values. That is why these values have no units. Protons and neutrons are relatively the same size, they have a relative mass of 1. Electrons are about 2,000 times smaller than protons and neutrons, and are therefore given a relative mass of 0.0005. Protons are positive with a relative charge of +1; electrons are negative with a relative charge of -1; neutrons are neutral and therefore have no charge. Was Dalton Right? True or False: “Atoms cannot be created or destroyed” True! The Law of Conservation of Mass states that mass can neither be created nor destroyed. Mass is of course referring to atoms. Atoms are not destroyed and created during chemical reactions; they are only rearranged. Was Dalton Right? True or False: “Atoms of different elements are different” True! The atomic number, (number of protons in the nucleus), differentiates the atoms of one element from another. Was Dalton Right? True or False: “Atoms can combine together in small numbers to form molecules and compounds” True! Atoms chemically combine together in whole-number ratios to form compounds and molecules. i.e. H₂O and CO₂ Was Dalton Right? True or False: “Atoms of the same element are alike in every way” False! Even though every atom of one element has the same atomic number, (the same number of protons in its nucleus), they do not necessarily have to have the same number of neutrons! Isotope: isotopes are atoms that have the same number of protons but different numbers of neutrons. Each of these nuclei represent hydrogen because they each only have 1 proton; however, they are not alike in every way since they each have different amounts of neutrons. Isotopes are different atoms of the same element with different mass numbers; i.e. different numbers of neutrons in the nucleus. Isotopes have the same chemical properties (they react in exactly the same way), but have different physical properties, such as different melting points, boiling points, densities, masses, etc. Isotopes react in the same way because they have the same number of electrons, and chemical reactions depend only on the number and arrangement of electrons and not on the composition of the nucleus. Isotopes have different physical properties because, for example, their different masses mean that their atoms move at different speeds. Mass Number of an Atom We determine the mass of an atom by taking a look at its nucleus. Mass Number = # of protons + # of neutrons * Electrons are SO SMALL that they are not counted when considering the mass of an atom. Only protons and neutrons determine the mass of an atom. Example: An atom of Gold (Au) has a mass number of 175. How many protons, neutrons, and electrons are there in this atom of gold? P = 79, N= 96, E= 79 AZ Notation & Mass Numbers A mass number X chemical symbol Z atomic number Test yourself! 1. Information is given about four different atoms. Which two atoms are isotopes? 2. State the amount of protons, neutrons, and electrons in the following species: i) ⁴⁰Ca ii) ⁵⁰V iii) ³³P Relative Atomic Mass (Ar) Relative Atomic Mass (Ar): the average mass of an element, taking into account all the mass numbers of all the different types of isotopes that exist for the element and their respective percent abundances. This value if relative to (one atom of) 12-C. (The definition of RAM would be a 2 mark IB question) Be careful with the terms mass number and relative atomic mass! Mass number refers to the mass of ONE atom. It is calculated by summing the protons and neutrons in the nucleus. RAM is an average of all the mass numbers present for an element. Relative Atomic Mass (RAM) How do you calculate the RAM of an element? To calculate the RAM of an element, multiply the mass of each isotope by its natural abundance (in decimal form), and then add the products. (Mass # of isotope 1 x Abundance of isotope 1) + (Mass # of isotope 2 x Abundance) + (etc…) This is done for ALL isotopes that exist for that particular element. Test Yourself Consider an element Z that has two naturally occurring isotopes with the following percent abundances: the isotope with a mass number of 19.0 is 55.0% abundant; the isotope with a mass number of 21.0 is 45.0% abundant. What is the relative atomic mass for element Z? You should always calculate RAM values to two places after the decimal. Test Yourself The element vanadium has two isotopes, , and a relative atomic mass of 50.94. State and explain which isotope is more abundant.