1 CHAPTER 5 The Structure of Atoms

2 Fundamental Particles Three fundamental particles make up atoms:

3 The Discovery of Electrons Late 1800’s & early 1900’s Cathode ray tube experiments showed that very small negatively charged particles are emitted by the cathode material – J. J. Thomson Modified the cathode ray tube and measured the charge to mass ratio of these particles. He called them electrons. (Nobel prize in physics, 1906)

4 The Discovery of Electrons 1909 – Robert A. Millikan Determined the charge and the mass of the electron from the oil drop experiment. (The second American to win Nobel prize in physics in 1923) 1910 – Ernest Rutherford Gave the first basically correct picture of the atom’s structure. (Nobel prize in chemistry in 1908)

5 Rutherford’s Atom  The atom is mostly empty space  It contains a very small, dense center called the nucleus  Nearly all of the atom’s mass is in the nucleus  The nuclear diameter is 1/10,000 to 1/100,000 times less than atom’s radius

6 The Discovery of Protons 1913 – H.G.J. Moseley Realized that the atomic number defines the element:  Each element differs from the preceding element by having one more positive charge in its nucleus Along with a number of observations made by Rutherford and some other physicists, this led to the discovery of the proton  The elements differ from each other by the number of protons in the nucleus

7 The Discovery of Neutrons 1932 – James Chadwick recognized existence of massive neutral particles which he called neutrons (Nobel prize in physics in 1935)  The atomic mass of an element is mainly determined by the total number of protons and neutrons in the nucleus  The atomic number of an element is determined by the total number of protons in the nucleus

8 Mass Number and Atomic Number Mass number – A Atomic number – Z Z = # protons A = # protons + # neutrons # protons = # electrons The way we typically write this: full nuclide symbolshort nuclide symbol

9 Isotopes Atoms of the same element but with different masses The same element means that the number of protons is the same, then different masses mean that the number of neutrons differs protium (or hydrogen) deuteriumtritium

10 Isotopes: Example

11 Experimental Detection of Isotopes – Francis Aston Designed the first mass-spectrometer (Nobel prize in chemistry in 1922) Factors which determine a particle’s path in the mass spectrometer: accelerating voltage, V magnetic field strength, H mass of the particle, m charge on the particle, q

12 Mass Spectrometry & Isotopes Mass spectrum of Ne + ions This is how scientists determine the masses and abundances of the isotopes of an element

13 Mass Spectrometry & Isotopes Let’s calculate the atomic mass of Ne using the mass-spectrometry data

14 Atomic Weight Scale A unit of atomic mass (atomic mass unit) was defined as exactly 1/12 of the mass of a 12 C atom Two important consequences of such scale choice: The atomic mass of 12 C equals 12 a.m.u. 1 a.m.u. is approximately the mass of one atom of 1H, the lightest isotope of the element with the lowest mass. The atomic weight of an element is the weighted average of the masses of its isotopes

15 Naturally occurring chromium consists of four isotopes. It is 4.31% 50 Cr, mass = amu 83.76% 52 Cr, mass = amu 9.55% 53 Cr, mass = amu 2.38% 54 Cr, mass = amu Calculate the atomic weight of chromium Isotopes and Atomic Weight

16 Isotopes and Atomic Weight Naturally occurring Cu consists of 2 isotopes. It is 69.1% 63 Cu with a mass of 62.9 amu, and 30.9% 65 Cu, which has a mass of 64.9 amu. Calculate the atomic weight of Cu to one decimal place. A.W.(Cu) = (62.9 amu  0.691) + ( 64.9 amu  0.309) = = 63.5 amu

17 Electromagnetic Radiation Any wave is characterized by 2 parameters: Wavelength ( ) is the distance between two identical points of adjacent waves, for example between their crests It is measured in units of distance (m, cm, Å ) Frequency ( ) is the number of wave crests passing a given point per unit time (for example, per second) It is measured in units of 1/time, usually s -1 1 s -1 = 1 Hz (Hertz)

18 Electromagnetic Radiation The speed at which the wave propagates: c =  The speed of electromagnetic waves in vacuum has a constant value: c = 3.00  10 8 m/s This is the speed of light Given the frequency of the electromagnetic radiation, we can calculate its wavelength, and vice versa

19 Electromagnetic Radiation Electromagnetic radiation can also be described in terms of “particles” called photons Each photon is a particular amount of energy carried by the wave Planck’s equation relates the energy of the photon to the frequency of radiation: E = h  ( h is a Planck’s constant, 6.626· J·s) Max Planck (Nobel prize in physics in 1918)

20 Electromagnetic Radiation What is the energy of green light of wavelength 5200 Å ?