ELECTRONEGATIVITY
The electrons that make up the bond between two atoms in a
covalent compound are not usually equally shared. One method used to determine
which direction the electron pair is pulled is the Electronegativity scale. Electronegativity
is defined as a measure of the ability of an atom in a molecule to draw bonding
electrons to itself. Because Electronegativity is a relative scale, fluorine
is assigned a value of 4.0 and all others are then determined from this element.
The following are the Electronegativities for the Main Group elements.
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H = 2.1 |
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Li = 1.0 |
Be = 1.5 |
B = 2.0 |
C = 2.5 |
N = 3.0 |
O = 3.5 |
F = 4.0 |
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Na = 0.9 |
Mg = 1.2 |
Al = 1.5 |
Si = 1.8 |
P = 2.1 |
S = 2.5 |
Cl = 3.0 |
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K = 0.8 |
Ca = 1.0 |
Ga = 1.6 |
Ge = 1.8 |
As = 2.0 |
Se = 2.4 |
Br = 2.8 |
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Rb = 0.8 |
Sr = 1.0 |
In = 1.7 |
Sn = 1.8 |
Sb = 1.9 |
Te = 2.1 |
I = 2.5 |
For any bond, the absolute difference of the Electronegativities is used. For example, in CCl4, the bond polarity is: 3.0 – 2.5 = 0.5. Since the Electronegativity of Cl is greater than that of C, the electron pair is pulled towards Cl. This in turn makes the bond slightly polar with the Cl end of the bond slightly negative while the C end of the bond is slightly positive.
MOLECULAR GEOMETRY OF MOLECULES
One of the important aspects that influence a lot of the physical properties of molecules is the molecular geometry (shape) of them. The model that we will use to predict the shape of molecules is called Valence Shell Electron Pair Repulsion theory (VSEPR for short).
VSEPR theory is based on the concept that electron pairs will repel each other and, thus, attempt to get as far apart from each other as possible. The number of electron pairs that we can have on our C, N, or O atoms can range from two, three, or four. Note that in the case of a multiple bond (for example CO2), the electron pairs in the double or triple bond occupy the same region of space and, thus, are counted as one electron pair. Also, the electron pairs can be either bonding pairs or non-bonding pairs (also referred to as a lone pair). Thus, let us look at each of the possibilities.
For two electron pairs, the furthest apart that they can get is 180o - also known as a linear geometry. We can get two pairs by a number of combinations. For example, two single bonds with no lone pairs produces this shape.
For three electron pairs, the furthest apart that they can get is 120o - also known as a trigonal planar geometry.
For four electron pairs, the furthest apart that they can get is 109.5o, which is the 3D structure known as a tetrahedron. There are several combinations and geometries that can occur based on this structure. The resulting geometry will be called tetrahedral. Another combination occurs when we have three single bonds and one lone pair. This yields a structure known as trigonal pyramidal. This geometry occurs most often for nitrogen based molecules. Lastly, we can have two bonds and two lone pairs. This yields a structure known as bent. This geometry occurs most often for oxygen based molecules.
See the illustrations below for a visual representation of the Linear, Trigonal Planar, and Tetrahedral shapes as well as the three Molecular Geometries based off of the Tetrahedral arrangement of the electron pairs.
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Herzberg received his early training in Hamburg and subsequently studied physics at the Darmstadt Institute of Technology where in 1928 he obtained his Dr.Ing. degree under H. Rau (a pupil of W. Wien). From 1928 to 1930 he carried out post-doctorate work at the University of Göttingen under James Franck and Max Born and the University of Bristol. In 1930 he was appointed Privatdozent (lecturer) and senior assistant in the Physics Department of the Darmstadt Institute of Technology.
In August 1935 Herzberg was forced to leave Germany as a refugee and took up a guest professorship at the University of Saskatchewan (Saskatoon, Canada), for which funds had been made available by the Carnegie Foundation. A few months later he was appointed research professor of physics, a position he held until 1945. From 1945 to 1948 Herzberg was professor of spectroscopy at the Yerkes Observatory of the University of Chicago. He returned to Canada in 1948 and was made Principal Research Officer and shortly afterwards Director of the Division of Physics at the National Research Council. In 1955, after the Division had been divided into one in pure and one in applied physics, Herzberg remained Director of the Division of Pure Physics, a position which he held until 1969 when he was appointed Distinguished Research Scientist in the recombined Division of Physics.
Herzberg's main contributions are to the field of atomic and molecular spectroscopy. He and his associates have determined the structures of a large number of diatomic and polyatomic molecules, including the structures of many free radicals difficult to determine in any other way (among others, those of free methyl and methylene). Herzberg has also applied these spectroscopic studies to the identification of certain molecules in planetary atmospheres, in comets, and in interstellar space.
Herzberg has been active as President or Vice President of several international commissions dealing with spectroscopy. He was also Vice President of the International Union of Pure and Applied Physics from 1957 to 1963. He held the offices of President of the Canadian Association of Physicists for the year 1956-57 and President of the Royal Society of Canada for the year 1966-67.
Herzberg was elected a Fellow of the Royal Society of Canada in 1939 and of the Royal Society of London in 1951. He was Bakerian Lecturer of the Royal Society of London in 1960 and received a Royal Medal from the Society in 1971. He was George Fischer Baker Non-Resident Lecturer in Chemistry at Cornell University in 1968, and Faraday Medallist and Lecturer of the Chemical Society of London in 1970. He is Honorary Member or Fellow of a number of scientific societies, including the American Academy of Arts and Sciences, the Optical Society of America and the Chemical Society. He is also a Foreign Associate of the National Academy of Sciences in Washington and a member of the Pontifical Academy of Sciences. He is a Companion of the Order of Canada. He has received many other medals and awards and holds Honorary Degrees from a number of universities in Canada and abroad, including one from the University of Stockholm.
Banquet Speech
Gerhard Herzberg's speech at the Nobel Banquet in Stockholm, December 10, 1971
Your Royal Highnesses, Distinguished Guests, Ladies and Gentlemen,
It is very difficult to find appropriate words to say "thank you" for an honour like the Nobel Prize. It is the supreme honour that a scientist can receive. Some of the giants in physics and chemistry have received this prize. Rutherford, the founder of nuclear physics, received the prize in chemistry in 1908. A number of those who have taught me either directly or indirectly are on that list: James Franck, Max Born, Peter Debye, Harold Urey and many others. I should also like to pay tribute to two pioneers in molecular spectroscopy from Sweden, Heurlinger and Hulthén, who accomplished the first difficult analyses of molecular spectra in the 1920's.
In receiving the award this year I think not only of these giants from whom I learned so much but also of my first teacher, Hans Rau, who guided my first steps in research, and to the many collaborators who helped me in my later work. Of them I should like to single out A. E. Douglas, whose quick and critical mind was always ready to help. I also think of my adopted country Canada, which gave a haven to me and my wife when we arrived as refugees. I think of the University of Saskatchewan which supported my work in its early stages and the National Research Council of Canada which provided an atmosphere so conducive to research.
Five years ago I received the great honour of an Honorary Degree from the University of Stockholm here in this building. It was at the same ceremony that His Majesty the King received an Honorary Degree from the Royal Caroline Medico-Chirurgical Institute. I felt doubly honoured that His Majesty was present throughout this ceremony. I could hardly have expected at that time that five years later I would stand here again in this building having received the great and supreme honour of the Nobel Prize from His Majesty's hands. I shall be forever grateful. Tack så mycket.