2
Fundamental Particles
Particle Mass (amu) Charge
Electron (e-) 0.00054858 - 1
Proton (p,p+) 1.0073 +1
Neutron(n,n0) 1.0087 0
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.
1897 – J. J. ThomsonModified 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 RutherfordGave 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
EAZ
The way we typically write this:
Cl3717
Cl37
full nuclide symbol short 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
H11 H21 H31
protium(or hydrogen)
deuterium tritium
11
Experimental Detection of Isotopes
1919-1920 – Francis AstonDesigned 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 12C atom
Two important consequences of such scale choice: The atomic mass of 12C 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 is4.31% 50Cr, mass = 49.946 amu83.76% 52Cr, mass = 51.941 amu9.55% 53Cr, mass = 52.941 amu2.38% 54Cr, mass = 53.939 amuCalculate 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% 63Cu with a mass of 62.9 amu, and 30.9% 65Cu, 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 crestsIt 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 = 3.00108 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, 6.626·10-34 J·s)
Max Planck(Nobel prize in physics in 1918)