What are atomic and molecular properties and structures?
Answer:
Atomic Properties: The electrons associated with atoms are found to have measurable properties which exhibit quantization. The electrons are normally found in quantized energy states of the lowest possible energy for the atom, called ground states. The electrons can also exist in higher "excited states", as evidenced by the line spectra (e.g. the hydrogen spectrum) observed when they make transitions back to the ground states. The existence of these excited states can be demonstrated more directly in collision experiments like the Franck-Hertz experiment .
Other properties associated with the electron energy levels such as orbital angular momentum and electron spin are also quantized and give rise to the quantum numbers used to characterize the levels. These quantized properties are associated with periodic table of the elements, and the requirements of the Pauli exclusion principle on the quantum numbers can be viewed as the origin of the periodicity. The periodic table provides a convenient framework for cataloging other physical and chemical properties of atoms.
While the hydrogen electron energy levels are found to depend only upon the principal quantum number, the energy levels in other atoms are found to have strong dependence upon the orbital quantum number. These levels show a smaller amount of dependence upon the total angular momentum. This dependence may arise from interactions within the atom such as the spin-orbit interaction or may arise only when external fields are applied. When magnetic fields are applied, there is splitting of atomic energy levels from the Zeeman effect, and in response to electric fields there is splitting called the Stark effect.
Molecular properties: While the energy is undoubtedly the fundamental quantity, chemists usually characterize molecules by other properties, for example, the dipole moment or the molecular structure. The ability to accurately calculate these properties is one of the major strengths of modern electronic structure theory. These calculations are made possible by the fact that the properties are responses of the molecule to external parameters such as the nuclear coordinates, applied electric and magnetic fields, etc. These parameters become variables on which a potential energy surface is mapped out. Therefore analytic derivatives of the energy with respect to these variables yield the familiar molecular properties.
The derivatives with respect to nuclear positions give the nuclear forces, which allows rapid minimization of the energy with respect to nuclear coordinates, providing the molecular structure. Second derivatives with respect to nuclear position reveal the force constants, allowing harmonic frequencies to be calculated. These derivatives also allow the classification of stationary points, greatly facilitating the location of transition structures (which will be first order saddle points).
The various derivatives with respect to electric field, magnetic field and nuclear spin allow determination of a range of properties, including: electric polarizability, infrared intensities, magnetic susceptibility, chemical shielding, spin-spin coupling, Raman intensities and hyperpolarizabilities. However they are beyond the scope of this thesis, and these properties will not be discussed here. What is important is that they all result from derivatives of the energy, and thus fast evaluation of the molecular energy is highly desired.
Atomic structure: The integer that you find in each box of the Periodic Chart is the atomic number. The atomic number is the number of protons in the nucleus of each atom. Another number that you can often find in the box with the symbol of the element is not an integer. It is oversimplifying only a little to say that this number is the number of protons plus the average number of neutrons in that element. The number is called the atomic weight or atomic mass.
How can it be that an element must have an averaged atomic weight? The number of protons defines the type of element. If an atom has six protons, it is carbon. If it has 92 protons, it is uranium. The number of neutrons in the nucleus of an element can be different, though. Carbon 12 is the commonest type of carbon. Carbon 12 has six protons (naturally, otherwise it wouldn?t be carbon) and six neutrons. The mass of the electrons is negligible. Carbon 12 has a mass of twelve. Carbon 13 has six protons and seven neutrons. Carbon 14 has six protons and eight neutrons. Carbon 14 is radioactive because, as other atoms with the wrong percentage of neutrons to protons, it is unstable. The nucleus tends to pop apart. The proper ratio of protons to neutrons is about one to one for small elements and about one proton to one and a half neutrons for the larger elements. Types of an element in which every atom has the same number of protons and the same number of neutrons are called isotopes. Carbon 14 is a radioactive isotope of carbon. Any carbon 14 that was made at the time the earth was formed is now almost all gone. Carbon 14 is continuously made from high energy electromagnetic radiation hitting nitrogen atoms in the ozone layer of the earth. This carbon 14 when taken into plants as CO2 will also be taken into animals. We can find out how much carbon 14 that normally is in a living plant or animal and from there we can find the actual amount of carbon 14 left in a plant or animal long dead. We can get a very good idea of how long ago that plant or animal was living from the amount of carbon 14 remaining in the dead body. This process is called ?carbon dating.? The stable, non-radioactive isotopes of carbon play no part in this. As a whole element, carbon has a more or less fixed proportion of the various carbon isotopes. For this reason, we can determine a weighted average of the isotopes for all elements. On a periodic chart you may see some atomic weights that are integers or in parentheses. These are usually on the very large or very rare or very radioactive elements. That is not really an integer atomic weight, but the atomic weight has been estimated to the nearest integer.
Molecular structure: The simple theories of bonding that we learn in General Chemistry are powerful and useful. These theories, which include Lewis structures, VSEPR, and hybridization, are simple models that help predict chemical properties. However, Lewis dot structures and hybridization are approximations that may or may not match reality. We should verify the usefulness of our simple predictions with molecular orbital theory. If the theoretical calculations are done carefully, we can learn a lot about chemical structure by comparing our Lewis structures and hybridization arguments with the molecular orbitals.
The calculations in this database include bond lengths, angles, atomic charges, the dipole moment, bond orders, and molecular orbital energies. The best Lewis structure that fits the molecular orbitals is also calculated, so you can directly compare with your predictions. This best Lewis structure is presented with formal electron pair localized bonds and the hybridization of the atomic orbitals used to form these localized bonds. The Chime plugin is needed to see the 3-D structure of the molecules in these pages. See the link at the bottom of the page for the Chime plugin.
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