Models to Describe Atoms
Bohr Model of Atoms - electrons occur in discrete, or quantized, energy levels - electrons orbit the nucleucs of the atom - electrons are distributed in shells (n=1..n), with orbits K,L, M, and N.
One small problem - this model only adequately describes hydrogen!
Schrodinger model of Atoms - wave theory better describes electron-shells of quantum number higher than 1 - three numbers specify the position of an atom in 3D space (as coefficients in a wave equation):
1) Principal quantum number (n, the volume of the electron
orbital, 1, 2, .. etc.) which also represents the energy level of the shell.
The shells are also designated K, L, M, and N;
2) Azimuthal quantum number, (I, general shape of the region; which may
be designated as 0,1,2,...etc. or by s, p, d, f, g, h, I, .... Note that
the number refers to the dimension, or axes necessary to describe the orbital
shape;
3) Magnetic quantum number, restricts the orientation and shape of the
orbital;
4) A fourth quantum number, the Spin quantum number, defines the direction
of electron spin.
Each electron in an atom can be described by four quantum numbers, and no two electrons can have the same numbers (this is the Pauli exclusion principal).
Periodic Table and Orbital Configurations
The elements are arranged in columns in the periodic table, I-VIII, which represents the number of electrons in their outermost shells. Most elements are metals, with high conductivity, ductility, melleability; nonmetals have poorer conductivity, etc. The far right column has noble gases, with either two (He) or eight electrons in the outermost orbitals the horizontal rows of the periodic table
The periods, or rows in the chart, are determined by the the energy level and principal quantum number of the outer electron shell (K=1, L=2, M=3, N=4, etc.). From left to right the shell is filled by electrons, into orbitals with shape s, p, f, and d.... The transition elements have partly filled d or p orbitals. most elements occur naturally, but those with Z= 43, 61, and 93-103 are synthetic.
Valence Electrons: the ordered, basic chemical properties of elements depends on the number and configuration of electrons in the outermost shells, the valence electrons. Atoms with similar valence tend to behave similarly, and to substitute for each other in minerals.
Differences in valence state result when different numbers of electrons can be lost from an atom: For example, the transition element iron can have either 2 or three free electrons in its outermost shels, and thus it may have the valence of Fe+2 and Fe+3.
Bond-mechanisms and the periodic table
The major types of bonds are:
Ionic: a form of electron capture or "swapping"
between atoms of greatly differing valence (e.g., NaCl)
Covalent: electron sharing between two or
more atoms of similar, or identical, valence (Diamond)
Metallic: Electron sharing whereby atoms move
freely through a structure (native metals)
Van der Waal's: Weak polarized attraction
among atoms, no electron transfer (graphite)
Ions are atoms which have a net electrical charge: electronegativity is a measure of the ability of an atom to attract electrons to it's nucleus. Metals have low electronegativity and are electron donors. They tend to become cations. Nonmetals have high values of electronegativity and are electron receptors, and tend to be anions. The difference in electronegativity can be used to estimate the 'bond character', which can be thought of as the degree of ionic and covalent character to a particular atomic bond.
Ionic Radii: The size, and radius, of an atom is not a fixed quantity, but rather depends upon the size of the atom in a neutral state, the valence of the atom in a compound, and the coordination of the atom (number of other atoms it is joined to) in a compound.