Estructura periodica y periodicidad quimica

Electronic Structure and Chemical Periodicity

Periodic Law
The periodic law was discovered independently in the mid 1800’s by Dimitri I. Mendeleev of Russia and Lothar Mayer of Germany. The periodic law states that when the elements are arranged in order of increasing atomic number (the original used atomic mass), the properties repeat at “regular” intervals. If we look at the periodic table,this means that all of the elements in a given column have similar chemical properties. For example, all of the group IA elements react with water to produce a metal hydroxide and hydrogen gas. The chemical reactions are very similar, there are different amounts of energy released in each reaction but the over chemical equation is pretty much the same. Mendeleev used the periodic law to predictthe existence of a couple of elements. He predicted the atomic mass and the chemical formula of the oxide of the element. All of this from knowing the properties of the element that would be above it in the periodic table.

The Periodic Table
The periodic table is a chart representing the elements that are known. Each element occupies a single spot in the table. It can be thought of as agraphical representation of the periodic law. The periodic table has rows, which are called periods, and columns, which are called groups. Some of the groups have special names:

Group IA II A VII A VIII A

Name Alkali metals Alkaline Earth Metals Halogens Noble gases

The noble gases on some older periodic tables are called the inert gases because, at the time, it was thought that they did notform compounds.

The Energy of an Electron
When we talk about the energy of an electron in an atom, we need to resort to the branch of physics called quantum mechanics. Quantum mechanics deals with the way in which

very small objects behave. With this theory, we can determine the motion or energy of a particle. In quantum mechanics, we find that the energy of an electron is a quantizedproperty. In other words, it can have only certain, discreet values. An electron cannot have just any energy. It can only have an energy that it is allowed to have. This is a fundamental restriction by nature. It is not imposed by anything. It is just the way it is. Electrons are found to have energies that place the electrons in shells around the nucleus. A shell is a mathematical construct that allowsus to understand the behavior of the electrons. It is the region around the nucleus where the electrons have approximately the same energy and are approximately the same distance from the nucleus. The shell number, n, tells us in which shell the electrons are. The lowest shell number is 1. There is no highest shell number. The number of electrons a shell can hold is given by the formula, 2n2. Thefirst shell can hold 2 electrons (2(1)2 =2). The second can hold 8 (2(2)2 = 8), and so on. Within each shell exist one or more sub-shells. The number of sub-shells is given by the shell number. The first shell has one sub-shell and so on. The sub-shells are given labels of s for the first sub-shell in a shell, p for the second, d for the third and f for the fourth. The sub-shells can hold a numberof electrons based on the kind of sub-shell it is:

Sub-shell s p d f

# of electrons 2 6 10 14

All of the electrons in a given sub-shell have the same energy. Electrons in different subshells in the same energy level have slightly different energies. Each of the sub-shells is further divided into orbitals. Each orbital, which again is a mathematical construct, can hold, at most, 2electrons. The number of orbitals in a subshell is 1 orbital in an s sub-shell, 3 orbitals in a p sub-shell, 5 orbitals in a d sub-shell and 7 in an f sub-shell. All of the orbitals in a given sub-shell have the same name as the subshell. For instance, the orbitals in a 3p sub-shell (the second sub-shell in the third energy level) are called 3p orbitals. The orbitals have a shape depending on the kind...