Atomic Theory

I. Electromagnetic Radiation
A. Electromagnetic radiation is defined as waves that travel through space. Waves have three primary characteristics: wavelength, frequency, and speed. Wavelength is the distance between two consecutive peaks or troughs in a wave, the frequency is defined as the number of waves per second that pass a given point in space. All waves, of course, travel at the speed of light. The SI unit for wavelength is meters and for frequency hertz.

II. The Nature of Matter
A. Planck is a famous scientist that disproved the theory that energy is continuous and found that energy actually exists at certain levels. Energy can be gained or lost only in wh0ole-number multiples of the quantity hv, where h is a constant called Planck's constant with a value of 6.626e-34 J*S. The change in energy in a system can be defined by DE=hc/l. It is clear that energy is in fact quantized and can only occur in discrete units of size hv. Each of these small "packets" of energy is called a quantum.
B. Einstein suggested that EMR can be viewed as a stream of "particles" called photons. The energy of each photon is given by E=hc/l. Einstein also derived the famous equation E=mc2 in a related study. Thus, a wave also exhibits particle properties and has mass.
C. De Broglie's came up with the idea that particles also exhibit wave properties. This conception is outlined by these two formulas: m=h/lv and l=h/mv. C.J. Davisson and L.H. Germer carried out the experiment that proved the theory that the electron has wave properties at Bell Laboratories in 1927. They figured that since X rays produce diffraction patterns in crystal that are attuned to their wavelengths then electrons would follow suit with their particular crystals. They shot electrons into a nickel crystal and successfully found a regular diffraction pattern, thereby proving the theory. To summarize, everything is energy; matter is only a form of energy and thereby has both wave and particle properties.

III. The Atomic Spectrum of Hydrogen
A. The continuous spectrum of a prism contains all the wavelengths of visible light. In contrast, the hydrogen emission spectrum has only a few specific wavelengths in the visible region. This is called a line spectrum.
B. Niels Bohr developed a quantum model for the hydrogen atom. Bohr proposed that the electron in a hydrogen atom moves around the nucleus only in certain allowed circular orbits. Classical physics actually said that an electron would lose energy because it is constantly accelerating and fall into the nucleus. Bohr said classic physics was wrong in this point and assumed that the angular momentum occurred in certain increments. This new model of the Hydrogen electron was consistent with the line spectrum. The energy of any electron in a certain level is given by the formula E=-2.178e-18 J(Z2/n2). The energy for moving a electron up and down energy levels can simply be found by adding and subtracting energies of levels. Bohr's model is fundamentally incorrect because it does not work with any other elements.

IV. The Quantum Mechanical Model of the Atom
A. The study of the structure of the atom is known as quantum mechanics. Only certain circular orbits have a circumference into which a whole number of wavelengths of the standing electron wave will "fit." All other orbits would produce destructive interference of the standing electron wave and are not allowed. A wave function is a function of the coordinates of the electron (in its wave state). A specific wave function is called an orbital. Because of the wave properties of an atom the speed and position of an atom can never be defined at the same time. This is known as Heisenberg uncertainty principle and is defined as Dx*D(mv)>h/4p.
B. This all means there is never a clear location that the electron is at. The electron can only be said to exist in a certain area 90% of the time. This is shown in an electron density map.

V. Quantum Numbers
A. Quantum numbers define the energy, size, shape, orientation, and spin. The principle quantum number is n and defines the energy and size of the orbital. The angular momentum quantum number l defines the shape of the orbital. The values of l is 0 to n-1. The magnetic quantum number ml defines the orientation of the orbital and its value is -l to l. The electron spin is defined as ms and its values are -1/2 and 1/2.
B. The different spins of two electrons in an orbital create different magnetic fields that do not repel each other. Any more electrons would repel in the because of the same magnetic field. This means that only two electrons can exist in one orbital.
C. All orbital with the same value of n have the same energy--they are said to be degenerated. The lowest energy state of an electron is the ground state and when the electron goes up to a higher energy level it is in a excited state.

VI. Polyelectronic Atoms
A. Each electron in a polyelectronic atom is moving in a field of charge that is the net result of the nuclear attraction and the average repulsion of all the other electrons. The effective nuclear charge is designate Zeff and is equal to the actual charge minus the effect of electron repulsion. Essentially, the electron repulsion of the other electrons "shield" the highest energy electron from the force of the nucleus. However, shielding by the other electrons is not totally effective because of the following principle. Electrons in orbitals are not always away from the nucleus and can "penetrate" it's own orbital. This penetration causes the other lower energy electrons to have no effect at all to shield this electron. The lower energy orbitals, of course, have more penetration.
B. The energy required to remove the highest energy electron from the atom is called the ionization energy. Electrons in the same type of orbital are not very effective in shielding each other. On the other hand, the core electrons closer to the nucleus do provide effective shielding for outer electrons.

VII. The Periodic Table
A. Patterns in discovered elements were eventually noticed and certain scientists arranged them into a table. The most notable of these scientists is Medeleev who is the essentially the creator of our modern periodic table. Mendeleev actually predicted the existence and properties of yet unknown elements for his time. As protons are added one by one to the nucleus to build up the elements, electrons are similarly added to these hydrogenlike orbital. This is called the aufbau principle. Hund's rule states that the lowest energy configuration for an atom is the one having the maximum number of unpaired electrons allowed by the Pauli pinciple in a particular set of degenerate orbitals.
B. Valence electrons are the electrons in the outermost principal quantum level of an atom. Valence electrons are the most important electrons to chemists because they are involved in bonding, as we will see in the next two chapters. The (n+1)s orbitals always fill before the nd orbitals.
C. Trends in the periodic table are as follows. Ionization energy increases across a period from left to right. This is consistent with the idea that electrons added in the same principal quantum level do not completely shield the increasing nuclear charge caused by the added protons. On the contrary, ionization energy decreases in going down a group. This is due to the fact that there are more electrons and they orbit farther away from the nucleus. Electron affinity is the energy change associated with the addition of an electron to a gaseous atom. The more negative the energy, the more electron affinity the atom has. Electron affinity shows the same trend as ionization energy (increase left to right, bottom to top). Atomic radii decrease in going from left to right across a period. This decrease can be explained in terms of the increasing effective nuclear charge (decreasing shielding) in going from left to right. This means that the valence electrons are drawn closer to the nucleus, decreasing the size of the atom. Of course, atomic radii increase down a group.

VIII. Properties of Alkali Metals
A. The essence of the periodic table is that the groups of representative elements exhibit similar chemical properties that change in a regular way. The most basic division of the elements in the periodic table is ino metals and nonmetal. The areas between these two types are the metalloids or semi-metals.
B. There is a overall increase in density in going down Group 1A. This occurs because atomic mass generally increases more rapidly than atomic size. Group 1A elements are very reactive because they are metals, and metals readily lose their electrons. Of course, the atoms that lose their electron more readily reduce much better with most elements. With water, however, lithium is the best reducing agent. This is because Li is much smaller and thus its charge density is also much greater. This means that the polar water molecules are more strongly attracted to the small Li ion. Sodium and Potassium react much more vigorously with water. This is because of the relatively high melting point of lithium. When sodium and potassium react with water, the heat evolved causes them to melt, giving a larger area of contact with water.