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THE DISCOVERY OF THE ELECTRON • BY 19TH CENTURY, SCIENTISTS DISCOVERED THAT MATTER IS COMPOSED OF INDESTRUCTIBLE BUILDING BLOCKS CALLED ATOMS CATHODE RAYS – IN THE LATE 1800'S AN ENGLISH PHYSICIST NAMED J.J.THOMSON PERFORMED EXPERIMENTS TO PROBE THE PROPERTIES OF CATHODE RAYS. CATHODE RAYS...... • He constructed a partially evacuated glass tube called cathode ray tube • Applied high electrical voltage between the two electrodes Findings- 1. Cathode rays travel from the negatively charged electrode to the positively charged electrode 2. The particles that compose the cathode rays travel in straight lines 3. The are independent of the composition of the material from which they originate 4. They carry a negative electrical charge J.J.THOMSON CATHODE RAY EXPERIMENT CATHODE RAY EXPERIMENT • The electrical charge is the fundamental property of some of the particles that compose atoms and that results in attractive and repulsive forces • Electric Field - The area where the attractive and repulsive forces exists • J.J.Thomson measured the charge to mass ratio of the cathode ray particle by deflecting them using electric and magnetic fields • The value measured was –1.76×108 Coulombs per gram • ROBERT MILLIKAN Performed an “oil-drop experiment” to determine an accurate value for the charge (and therefore the mass also) of the electron. ROBERT MILLIKAN • Atomizer produces tiny drops of oil • Drops fall through a very small hole. fall through at a time. Only a few drops can ROBERT MILLIKAN • X rays produce charges on the oil drops. • Charged plates above and below. Measure quantity of charge required to stop oil drop from falling. ROBERT MILLIKAN • Three forces determine the motion of the oil drops in this experiment: • Gravitational (downward) • Buoyant (upward) because drops VERY small • Electric (upward) ROBERT MILLIKAN • Density of the oil is known. • Observe radii of drops with a microscope. (This is a measure of volume.) • With these two, mass of each drop can be determined. • Gravitational and buoyant forces dependent on mass, so those known too. ROBERT MILLIKAN • Record electric field needed to suspend oil drop (stop it from dropping). • The amount of electric charge needed to stop the oil drop is based on the total amount of charge carried by the oil drop. ROBERT MILLIKAN • By repeating the experiment for many droplets, they confirmed that the recorded charges were all multiples of some fundamental value, and calculated it to be 1.5924 × 10−19 Coulombs. They proposed that this was the charge of a single electron. • Their value was within one percent of the currently accepted value of 1.602176487 × 10−19 coulombs. THE STRUCTURE OF THE ATOM • Atoms are charged neutral. • The discovery of electron raised a question about the presence of a positive charge, to neutralize the negative charge of the electron. • J.J.Thomson proposed that electrons are held inside a positively charged sphere – Known as the plum pudding model PLUM PUDDING MODEL DISCOVERY OF RADIOACTIVITY • The discovery of radioactivity by Henri Becquerel and Marie curie led to the experimental view of the structure of atom. • Radioactivity – Emission of small energetic particles from the core of certain unstable atoms. • Scientists discovered three types of particles emitted during radioactivity • They are α particles,β particles and γ particles RUTHERFORD'S GOLD FOIL EXPERIMENT • Rutherford worked under Thomson to confirm his plum pudding model, conducted an experiment known as the gold foil experiment. • He used α particles , which is positively charged, and passed through an ultra thin gold foil. • Findings – Most of the α particles pass through with little or no deflection • Some particles were deflected • Some bounced back • Rutherford created a new model which suggests the existence of a small dense nucleus NUCLEAR MODEL - RUTHERFORD RUTHERFORD'S NUCLEAR MODEL OF ATOM • Rutherford observed that the mass and charge of the atom must be concentrated in a space smaller than the size of the atom itself. • Nuclear Theory has three basic parts 1. Most of the atom's mass and its charge are concentrated at the center of the nucleus 2. Most of the volume of the atom is empty throughout which tiny negatively charged electrons are dispersed 3. The atom is electrically neutral . It has positively charged as well as negatively charged particles. RUTHERFORD'S MODEL - INCOMPLETE • Rutherford's model seemed as incomplete – Compare H and He . H has one proton and He has 2. But mass of H seems to be ¼ th of mass of He. • Rutherford's student James Chadwick discovered that the uncounted mass is due to the presence of neutrons within the nucleus. • Mass of neutron is similar to that of a proton but it doesn't have a charge • The He atom is 4 times massive than H atom, since it contains 2 protons and 2 neutrons. SUBATOMIC PARTICLES • Subatomic particles – Protons, neutrons and electrons compose atoms and the are called subatomic particles • Protons and neutrons has almost the same mass • Mass of Proton = 1.67262* 10 -27kg • Mass of neutron = 1.67493* 10 -27kg • This mass can be expressed in atomic mass units (AMU) • AMU - 1/12 th the weight of 1 carbon atom containing 6 protons and 6 neutrons PROTONS, NEUTRONS AND ELECTRONS • The mass of proton and neutron is approximately 1 amu and that of an electron is 0.00055amu. • Both protons and electrons have a charge. Protons are assigned to have a charge of +1 and electrons are assigned to have a charge of –1. • The charge of proton and electron are equal in magnitude but opposite in sign. • Neutron has no charge MATTER IS CHARGED NEUTRAL • Matter is usually charged neutral - protons and electrons are present in equal number • When matter acquires some charge imbalances, it tries to equalize quickly • Example : The shock you receive by touching a door nob during dry weather is an equalization of charge that you developed as walked across the carpet. • Can you imagine matter as a sample with only protons or only electrons? - Matter would have been unstable with extraordinary repulsive forces WHAT MAKES THE ELEMENTS DIFFERENT FROM ONE ANOTHER? THE NUMBER OF PROTONS DEFINE THE ELEMENT • Example: An atom with 2 protons in its nucleus is He atoms • An atom with 6 protons is C atom • An atom with 92 protons is U atom SUBATOMIC PARTICLES Particle Symbol Relative Relative Location Mass Charge Proton p+ 1 amu 1+ Inside Nucleus neutron n 1 amu 0 Inside Nucleus Electron e - 1/2000 amu 1- Outside Nucleus ATOMIC NUMBER AND MASS NUMBER •“Z” Atomic Number- This is equal to the number of protons in your atom. •“A” Mass Number- Protons + Neutrons ELEMENTS • Each element is identified by its atomic number • Each element is represented by a chemical symbol • He – Helium • C – Carbon • Th – Thorium • Elements derived their names from Latin, Names of places, Names of scientists etc ELEMENTS AND THEIR NAME ORIGIN • Sodium – Latin word Natrium – Na • Tin – Latin word Stannum – Sn • Chlorine – Greek word Chloros meaning pale green – Cl • Europium , Polonium , and Berkilium are derived from the places where they discovered or the place of the discoverers • Curium, Einsteinium and Rutherfordium are named after name of scientists ISOTOPES Isotopes: naturally occurring atoms of the same element that vary in their number of neutrons Examples: 35Cl & 37Cl Isobars- atoms with the same mass number but different atomic numbers Example C-14 and N-14 RELATIVE ABUNDANCE Relative (percent or natural) abundance: how often the isotope occurs in nature; expressed in a percentage Example: 35Cl has an abundance of 75% & 37Cl has an abundance of 25% THE PERIODIC TABLE All elements up to #118 have been synthesized in lab by research scientists. CONTENTS OF EACH BOX Atomic number Symbol Element name Atomic mass Different periodic tables provide different amounts of info and in different orders Who developed the Periodic Table? • Dimitri Mendeleev: used atomic mass to order the elements • Henry Mosley: current arrangement by atomic number Vertical columns are called groups or families. Horizontal rows are called periods. WHY THE NAME? Properties of elements change as you move across a period. The same pattern of properties repeats when you move from one period to the next. So the properties occur “periodically”. Elements with similar physical and chemical characteristics end up in same family. Ex: Group 1A elements are all very reactive with water. NOTICE THAT THERE ARE TWO NUMBERING SYSTEMS FOR THE FAMILIES: A AND B GROUPS DISTINCTION IUPAC CONSECUTIVE NUMBERING SYSTEM “A” Columns 1A 8A 2A 3A 4A 5A 6A 7A “B” Columns 3B 4B 5B 6B 7B 8B 9B 10B 1B 2B IUPAC Consecutive Numbering System 1 18 2 13 3 4 5 6 7 8 9 10 11 12 14 15 16 17 THERE ARE THREE TYPES OF ELEMENTS ON THE PERIODIC TABLE. METALS - elements to the left of the stairstep - EXCEPT FOR HYDROGEN! PROPERTIES OF METALS • Lustrous • Conduct electricity • Ductile (can be drawn into wires) • Malleable (can be pounded into sheets) • High boiling/melting points • Usually solids NONMETALS - elements to the right of the stairstep and hydrogen PROPERTIES OF NONMETALS • Dull appearance • Non or poor conductors of electricity • Usually gases or liquids • Not malleable or ductile METALLOIDS - elements ON the stairstep - EXCEPT FOR ALUMINUM! PROPERTIES OF METALLOIDS Properties are more variable. Properties are intermediate between those of metals and those of nonmetals. KNOW THE NAMES OF THE FOLLOWING REGIONS ON THE PERIODIC TABLE ALKALI METALS Alkali Metals ALKALINE EARTH METALS HALOGENS Alkali Metals Alkaline Earth Metals Halogens Alkali Metals Alkaline Earth Metals NOBLE GASES Halogens Alkali Metals Alkaline Earth Metals TRANSITION METALS Noble Gases Halogens Alkali Metals Alkaline Earth Metals Transition Metals Noble Gases INNER TRANSITION METALS PLEASE NOTE: THE INNER TRANSITION METALS ARE PART OF THE PERIODIC TABLE. To see where they fit in, look at the atomic numbers. To see an “intact” periodic table, go to http://www.ptable.com/ Then click on the box beside “Wide” One reason the periodic table is drawn with the inner transition metals separate is so the table fits better onto a single piece of paper. Halogens Alkali Metals Alkaline Earth Metals Transition Metals Noble Gases INNER TRANSITION METALS Halogens Alkali Metals Alkaline Earth Metals “Other” Metals Transition Metals Noble Gases INNER TRANSITION METALS SOME ELEMENTS ARE REFERRED TO AS “REPRESENTATIVE ELEMENTS” • Most of the A-Group Elements • Their properties very clearly illustrate the periodic law REPRESENTATIVE ELEMENTS All of Column 1. REPRESENTATIVE ELEMENTS All of Column 1. All of Column 2. REPRESENTATIVE ELEMENTS All of Column 1. All of Column 2. All the nonmetals. Representative Elements All of Column 1. All of Column 2. All the nonmetals. And Aluminum. REPRESENTATIVE ELEMENTS • Properties very clearly represent the periodic law • Number of valence electrons can be determined by looking at group number • Valence electrons- electrons in the highest occupied energy level of an atom • Elements in a group have similar properties BECAUSE they have the same number of electrons SIGNIFICANCE OF % ABUNDANCE In any sample of Cl, 75.77% of the atoms have a mass number of 35 and 24.23% of the atoms have a mass number of 37 These isotopes are considered when average atomic mass is calculated ATOMIC MASS • The mass reported in the periodic table is the weighted average of all naturally occurring isotopes • Remember they are usually written as 1735Cl with the atomic number on the bottom and the mass number on the top. AVERAGE ATOMIC MASS To Calculate Average Atomic Mass: 1) Change the percent into the decimal form. 2) Multiply the percent abundance by the mass for each isotope. 2) Add these numbers together to determine the average atomic mass EXAMPLE Magnesium has two naturally occurring isotopes: 24Mg and 25Mg. 24Mg has a mass of23.985042 amu and a percent abundance of 79% and 25Mg has a mass of24.985837 amu and a percent abundance of 21%. Determine the average atomic mass. PRACTICE PROBLEM The three naturally isotopes of neon, their percent abundances, and their atomic masses are: neon-20, 19.99244amu, 90.51%; neon-21, 20.99395amu, 0.27%; neon-22, 9.22%, 21.99138amu. Calculate the weighted average atomic mass of neon.