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Atomic History and Structure Chapter 4 Early Theories of The Atom DemocritusDemocritus (b. c. 460 BC; d. c. 370 BC) postulated the existence of invisible atoms, characterized only by quantitative properties: size, shape, and motion. Imagine these atoms as indivisible spheres, the smallest pieces of an element that still behave like the entire chunk of matter. Dalton Thomson “The PLUM PUDDING MODEL” Rutherford Rutherford’s Experiment Visit this website and click on the tutorial for “Section 3.2 Rutherford Experiment” Play and watch through all 5 parts Visit this website and see what Rutherford’s Experiment looked like. What is an ATOM? Atom- the smallest particle of an element that retains the properties of that element Subatomic particles Protons – Positive charge, found in the nucleus and have a mass of 1 amu. ( Identify) Neutrons- No charge, found in the nucleus, and have a mass of 1 amu ( Isotopes) Electrons- Negative charge, found in the energy levels outside of the nucleus, have relatively no mass ( Ions) By the Numbers… Atomic number – This determines the elements position in the periodic table ( Identifies the atom) In atoms not ions Atomic # = # protons= # of electron Mass number Mass number = # protons + # neutrons Why are electrons not included in the mass of an atom? You will not look to the periodic table to determine the mass number, the number on the periodic table is an average. Isotopes An Isotope is the same element (at.#) with a different number of neutrons. When naming an isotope you write name of the of the element, dash, then write the mass number Symbol of element Example: Carbon-14 Atomic number from periodic table 14 6 C More about the mass Knowing the mass number you can determine the number of neutrons in a specific atom # of neutrons = mass number - # of protons atomic mass unit (amu)- the mass of the carbon atom. Isotopic Notation Isotope notation C C Determine the number of protons, electrons and neutrons in an isotope. Example #1 Carbon-14 Carbon-12 Examples H H What number is different? H Check your work! Calculating Atomic Mass Atomic mass- weighted average mass of the isotopes of that element. This is the decimal number on the periodic table. To determine the atomic mass you must know what percent of each isotope of the element is found in nature and then it can be calculated. Example: For Chlorine 25% is chlorine-37 75% is chlorine – 35 What is the average atomic mass of chlorine? To Calculate: 25% is chlorine-37 75% is chlorine-35 1. Take mass 37 x .25 = ans A 2. Take mass 35 x .75 = ans B 3. Add ansA + ansB = Average Atomic mass for Chlorine Ions-a charged particle An atom that has either gained or lost an electron. Electrons are lost and gained to make ions When they are gained (-Neg) ions (anion) When they are lost (+ Pos) ions (cations) Ion Examples Example: What is the charge of an ion that has 11 protons and 10 electrons. Write the isotope notation for this atom. Tell the number of P, E and N in the following ions. 24 12 Mg 2 p=12 n=12 e=10 15 7 N -3 p=7 n=8 e=10 History of Periodic Table Dmitri Mendeleev – listed the elements in several vertical columns in order of increasing atomic MASS. He left blanks in the table for elements that were not discovered yet. Henry Mosley – Arranged elements on the periodic table in order of increasing atomic NUMBER. http://www.chemheritage.org/EducationalServices/chemach/ppt/lm04.html Arrangement of Periodic Table Periodic law – the properties of elements are periodic functions of their atomic number Periods – horizontal rows; 7; correspond to energy levels Groups/Families – vertical columns; “group A” Roman numerals correspond to the number of valence electrons Group I – Alkali metals, Group II – Alkaline earth metals, Group VII – Halogens, Group VIII – Noble gases Metals, Nonmetals, and Metalloids Metals – everything to the left of the stairstep; including aluminum; does not include hydrogen Properties: Have luster (shiny), good conductors of heat and electricity, malleable ( able to be pounded into sheets), ductile (able to be pulled into a wire), tend to lose electrons in chemical reactions, most are solids Transition metals – middle block over to stairstep Inner transition metals – bottom 2 rows; sometimes called “lanthanide series” and “actinide series” Metals, Nonmetals, and Metalloids Nonmetals – everything to the right of the stairstep; includes hydrogen Properties – Dull, poor conductors, brittle, tend to gain or share electrons in chemical reactions, most are gases Metalloids – either side of the stairstep; does not include aluminum Periodic Table colored to show metals, nonmetals, and metalloids http://hyperphysics.phy-astr.gsu.edu/hbase/pertab/pertab.html Answers to practice problems 14 6 Example 12 6 C The mass number differs due to the difference in neutrons! Isotopes of Hydrogen 1 1 H 2 1 H 3 1 C H Back to isotopic notation