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Atoms, Molecules and Ions Chapter 2, Part 2 September 2nd, 2004 Exam #1 on Thursday, 16th of September Chapters 1,2 and wherever we stop from Chapter 3 on Tuesday the 14th of September. More History • In 1909 Robert Millikan, at the University of Chicago calculated the charge of the electron by performing his famous oil-drop experiment. • Henri Becquerel discovered accidentally, the existence of radioactivity in 1896. • In 1911 Ernest Rutherford performed his famous gold foil experiment (which he later described as shooting at a sheet of paper with a cannon and having the shell return to you) leading to the development of the idea of the nucleus and the neutral atom. 14 1 The Neutral Atom • Neutral = No charge • # of protons (positive charges) = # of electrons (negative charges) • Atoms of different elements have different numbers of protons, neutrons and electrons. • The number of protons determine the identity of the element. 15 Atomic Symbol Atomic Mass Number (A) = The sum of # of protons and the # of neutrons Atomic number (Z) = The # of protons 12 6 C In any neutral atom, the atomic number = # of electrons in one atom of that element Atomic Mass Number – Atomic number = Number of neutrons. 16 2 Isotopes • Dalton, had proposed that all atoms of the same element must have the same mass. • One of the 2 flaws in Dalton’s theory was, atoms of the same element can have DIFFERENT masses. • The mass of an atom is due to the mass of the protons and the neutrons in that atom. • Isotopes are atoms of the same element which have the same number of protons but different number of neutrons. The word isotopes is from the Greek words for “equal place”. 17 Isotopes • Isotopes have the exact same physical properties but different chemical properties. (Some are RADIOACTIVE and can emit particles and energy). • All naturally occurring elements have 2 or more isotopes. • Isotopes exist in different amounts (called isotopic abundance) and have different lifetimes. • A mass spectrometer is the most direct and accurate means of determining atomic weights and the existence of isotopes. 18 3 Example of Isotopes 12 6 C 13 6 C 14 6 C 19 Atomic Mass • The mass of an atom is decided by the number of protons and the number of neutrons present in the atomic nucleus. • Since atoms are so tiny, using an ordinary unit such as gram or kilogram is inconvenient and the atomic masses are usually reported in a special unit called atomic mass unit (amu). • One amu = 1.661 x 10-24 g 20 4 How and Why of the amu • Dalton had proposed an atomic scale based on the mass of one Hydrogen atom, which he had assigned the mass of 1. Several of his reported masses were later proven wrong. • The Hydrogen based scale was then replaced with an Oxygen based scale and then by the now accepted Carbon – 12 based scale. • One amu = 1/12th of the mass of one carbon atom. • The mass of one proton = 1 amu. • The mass of one neutron = 1 amu. 21 Atomic Masses and Isotopes • The sum of the atomic masses of the protons and the neutrons present in an atom should add up to give an exact round number and be the same as the atomic mass number “A”. • The presence of isotopes makes matters slightly complicated. • Each isotope of an element is present in certain percent abundance, which has to be accounted for when it comes to calculating the atomic masses. 22 5 Importance of % Abundance 23 Using % abundance • The % abundance is factored in the calculation for finding the atomic mass of any element. • Sample Exercise 2.4 shows how to make use of this idea. • Consider the following example: 24 6 Isotopes of Neon • Neon has 3 naturally occurring 1. Change each % value to a isotopes. The masses and fraction. natural abundances of these isotopes are as follows. Calculate the atomic mass of Neon. 1 19.99 amu 90.48 % 2 20.99 amu 0.27 % 3 21.99 amu 9.25 % 2. Multiply the isotopic masses by these fractions and then add the results together. Atomic mass of Neon = (Mass of isotope #1 x fraction of #1) + (Mass of isotope #2 x fraction of #2) + (Mass of isotope #3 x fraction of #3) 25 Thus, the average atomic mass of Neon is 20.18 amu 26 7 Compounds, Molecules & Ions Compounds Molecular Ionic Molecules composed of 2 or more elements Cations with positive charges Anions with negative charges There are more than 19 million registered compounds with no end in sight as to how many more will be prepared. Each compound can be identified and distinguished by its characteristic properties. 27 The Periodic Table • A systematic arrangement of elements according to increasing atomic numbers. Comic book periodic table (http://www.uky.edu/Projects/Chemcomics/) 28 8 The Periodic Table • A systematic arrangement of elements according to increasing atomic numbers. • The vertical columns are known as groups. 29 The Periodic Table • The horizontal rows are known as periods. 30 9 Metals, Metalloids and Non-metals • Metals: – Solids at room temperature (except Hg). – Hard, shiny. – Good conductors of electricity and heat. – Malleable (from Latin for a hammer). – Ductile. – High melting point and density. – Typically located on the left side of the periodic table. – Metals will typically combine with non-metals to form ionic compounds. 31 Metalloids • Have properties that are in between the metals and non-metals. • Certain metalloids are used as the raw materials for the manufacture of semiconductor devices. 32 10 Non-metals • Solids, liquids (Br) or gases. • Dull, non-malleable and non-ductile. • Low melting points and densities. • Poor conductors of heat and electricity. • Located on the right of the periodic table. 33 Elements that exist as diatomic molecules • There are 7 known elements that exist as diatomic molecules: H2, F2, O2, N2, Cl2, Br2, I2 • When any of these elements are mentioned it is assumed that they exist as diatomic molecules unless specified otherwise. • 2 other elements that are commonly polyatomic are Phosphorus (P4) and Sulfur (S8) 34 11 Chemical Formulas • Show the symbols and the ratios in which the atoms of the elements in a compound combine. • Do not show the arrangement of atoms in the compound (structural formulas). • If the formula contains one atom of an element the symbol of the element represents the atom. The number 1 is not used as a subscript. The formula of water is H2O, not H2O1. • When the formula contains more than one atom of an element the number of atoms is indicated by a subscript written to the right of the atom. The number 2 to the right of the H indicates 2 atoms of hydrogen present in H2O. 35 12