2Defining the AtomOBJECTIVES:Describe Democritus’s ideas about atoms.
3Defining the AtomOBJECTIVES:Explain Dalton’s atomic theory.
4Defining the AtomThe Greek philosopher Democritus (460 B.C. – 370 B.C.) was among the first to suggest the existence of atoms (from the Greek word “atomos”)He believed that atoms were indivisible and indestructibleHis ideas did agree with later scientific theory,and was not based on the scientific method – but just philosophy
5Dalton’s Atomic Theory (experiment based!) All elements are composed of tiny indivisible particles called atomsAtoms of the same element are identical. Atoms of any one element are different from those of any other element.John Dalton(1766 – 1844)Atoms of different elements combine in simple whole-number ratios to form chemical compounds
6Sizing up the AtomElements are able to be subdivided into smaller and smaller particles – these are the atoms, and they still have properties of that elementIf you could line up 100,000,000 copper atoms in a single file, they would be approximately 1 cm longDespite their small size, individual atoms are observable with instruments such as scanning tunneling (electron) microscopes
7Structure of the Nuclear Atom OBJECTIVES:Identify three types of subatomic particles.
8Structure of the Nuclear Atom OBJECTIVES:Describe the structure of atoms, according to the Rutherford atomic model.
9Structure of the Nuclear Atom One change to Dalton’s atomic theory is that atoms are divisible into subatomic particles:Electrons, protons, and neutrons are examples of these fundamental particlesThere are many other types of particles, but we will study these three
10Discovery of the Electron In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle: the electron
11Modern Cathode Ray Tubes TelevisionComputer MonitorCathode ray tubes pass electricity through a gas that is contained at a very low pressure.
12Mass of the ElectronMass of the electron is9.11 x gThe oil drop apparatus1916 – Robert Millikan determines the mass of the electron: 1/1840 the mass of a hydrogen atom; has one unit of negative charge
13Conclusions from the Study of the Electron: Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons.Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electronsElectrons have so little mass that atoms must contain other particles that account for most of the mass
14Conclusions from the Study of the Electron: Eugen Goldstein in 1886 observed what is now called the “proton” - particles with a positive charge, and a relative mass of 1 (or 1840 times that of an electron)1932 – James Chadwick confirmed the existence of the “neutron” – a particle with no charge, but a mass nearly equal to a proton
15Mass (g) / Relative mass Subatomic ParticlesParticleChargeMass (g) / Relative massLocationElectron(e-)-19.11 x 10-281/1840(negligible)Electron cloudProton (p+)+11.67 x 10-241NucleusNeutron(n)
16Thomson’s Atomic Model J. J. ThomsonThomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.
18Ernest Rutherford’s Gold Foil Experiment - 1911 Rutherford was a student of Thomson and decided to see if there was more evidence for the 'plum pudding /blueberry muffin' theory.His idea: to bombard the atom with dense alpha particlesalpha particle = He atom with no electrons- this was like shooting bullets at a blueberry muffin
19Ernest Rutherford’s Gold Foil Experiment - 1911 this was like shooting bullets at a blueberry muffin…. So what did Rutherford expect?
20Surprising result of Rutherford’s Gold Foil Experiment - 1911
21Rutherford’s Findings Most of the particles passed right throughA few particles were deflectedVERY FEW were greatly deflected“Like gun shells bouncing off of tissue paper!”
22Rutherford’s Findings and deductions Most of the particles passed right throughan atom was much more than just empty space and scattered electronsA few particles were deflected,very few were greatly deflectedSince alpha particles are relatively heavy, positively charged particles, the fact that the occasional particle would be deflected by either a small or large extent, an atom must have a positively charged center that contains most of its mass (which Rutherford termed as the nucleus).
24An illustration that compares the plum pudding model of an atom to what Rutherford observed in his experiment.The top structure shows how the alpha particles would have passed through the gold foil atoms if the plum pudding model was correct in its assumptions.The bottom structure shows what Rutherford and his collogues observed and is the true depiction of an atom's structure.
25The nucleus as described by Rutherford: The nucleus is smallThe nucleus is denseThe nucleus is positively charged