10/9/20131Fri, Oct 11• Lecture 7 (Representation)– Introduction to Modern Concept of Atomic Structure (4.5‐6)– Isotopes (4.7)– Element Properties (4.8‐9)– Ions (4.10)• Questions we’ll answer:– What is an atom made of?– What are the consequences of atomic composition?A Brief History of Charge• 600 BCE: Greek philosopher Thales observes that amber could become “charged” when rubbed with fur. – The term “electricity” is derived from the Greek word for amber, elektron.• 1733: C.F. du Fay proposes that there are two separate types of electricity that cancel each other.• 1750s: B. Franklin proposes a “one‐fluid” theory of electricity. Electricity can “flow” from one insulating object to another if they are rubbed together.• 1839: M. Faraday shows that there is one type of electricity with two “polarities,” positive and negative.10/9/20132What do atoms look like?Cathode Ray Tube: http://youtu.be/7YHwMWcxeX81897: J.J. Thompson performed a series of experiments on cathode rays, a type of electrical emission that occurs when a high electrical potential is applied across a vacuum.Thompson’s main results:• Cathode rays can be deflected from a straight line path by an applied electric or magnetic field.• The properties of cathode rays are the same no matter what substance they are derived from.Conclusion: Although atoms are electrically neutral, they contain charged particles.J.J. Thompson (1856‐1940)Cathode (–)Anode (+)Cathode rays travel in a straight line from the cathode, unless an external electric or magnetic field is applied.10/9/20133The “Plum‐Pudding” Model of the Atom• Experiments indicated that atoms were neutrally‐charged, and that they contained negatively‐charged particles called electrons.• To reconcile these two observations, Thompson proposed a model of atomic structure the atom is composed of discrete, negatively‐charged electrons embedded in a cloud of uniform positive charge.(an actual, spherical plum pudding)• Cathode rays are obtained when electrons are released from atoms and accelerated by an electric potential. • Since the electrons are repelled by negative charge, and attr acted by positive charge, electrons must be negatively‐charged.“Plum‐Pudding” Modelof atomic structure6What do atoms look like?E. Rutherford(1871‐1937)1909: Ernest Rutherford set out to test Thompson’s Plum‐Pudding Model.According to the Plum‐Pudding model, an atom’s mass and positive charge are diffuse. Therefore, particles of sufficient momentum should be able to pass straight through a thin layer of atoms.Rutherford performs an experiment in which a beam of high‐velocity alpha () particles are aimed at a piece of very thin gold foil. Most  particles do pass straight through. But occasionally, a particle is deflected backwards from the foil.particles are released during radioactive decay, are four times more massive than H, and carry a positive charge.10/9/20134n+7The Nuclear Atom1911 –Ernest Rutherford demonstrated the nuclear nature of the atom in which the empty space is 10,000 to 100,000 times larger than the size of the nucleus.Expected Result Actual ResultRutherford’s Experiment: http://youtu.be/Zd6_zVdMgJkConclusion~300 pm~0.01 pmWhat do atoms look like?• 1913: As part of his CRT work, Thompson also looked at how a stream of ionized (positively‐charged) neon gas was affected by electric and magnetic fields.• Whereas all electrons have identical charge and mass, and therefore would all be deflected the same amount, the beam of positively‐charged neon ions exhibited two deflection points in the detector.• Other measurements showed that the neon ions all had the same charge. “atomic mass unit” …an atomic‐scale unit of mass.Conclusion: Neon atoms must differ in the mass of their nuclei…these are called isotopes.Isotopes for many other elements were discovered as well.10/9/201359Finally…Modern Atomic Structure1. Every atom contains small, dense nucleus.2. All of the positive charge and most of the mass are concentrated in the nucleus.3. The nucleus is surrounded by a large volume of nearly empty space that makes up the rest of the atom.4. The rest of the atom is thinly populated by electrons, the total charge of which exactly balances the positive charge of the nucleus.5. Research in the decade or so following the gold‐foil experiment suggested that the nucleus contained two kinds of particles:If an atom was the size of a baseball stadium, the nucleus would be the size of a fly on home plate.~300 pm~0.01 pm10What roles do the different particles play?• # Protons = chemical identity of the atom (which element is it?)– In an electrically‐neutral atom, the number of protons in the nucleus is exactly balanced by the number of electrons.• # Electrons = ionic character of the atom. An ion has either more or fewer electrons than the electrically‐neutral atom.– anion = more electrons, so ion is negatively‐charged– cation = fewer electrons, so ion is positively‐charged• # Neutrons