1. The discovery of a large number of elements and the study of their properties revealed between some certain similarities. This prompted chemists to search for a classification of the elements not only to facilitate their knowledge and description, but more importantly, for research leading to new advances in our understanding of matter.
2. First attempt at classification: Döbereiner triads. Between 1817 and 1829, J. W. Döbereiner, Professor of Chemistry at the University of Jena, stated his law of triads, grouping elements with similar properties.
3. Second attempt at classification: Law of the octaves of Newlands. In 1864, the English chemist J. A. R. Newlands noted that the elements arranged in ascending order of their atomic weights after every seven elements in the eighth repeated the properties of the first and by analogy with the musical scale enunciated his law of octaves.
4. Third attempt at classification: Periodic Table Mendelejeff. It was the Russian chemist Dmitri I. Mendelejeff which established the periodic table of elements comprising the scope of the periodic law.
Mendelejeff early work dating from 1860 and 1869 findings were read in the Russian Chemical Society. Himself summarized his work on the following assumptions:
1. If items are sorted according to their atomic weights, show a clear periodicity.
2. Items similar chemical properties have atomic weights like (K, Rb, Cs).
3. The placement of items in order of their atomic weights corresponds to its valence.
4. The most widespread in nature are small atomic weight. These elements have well defined properties. Are typical elements.
5. The value of the atomic weight and characterized an element to predict their properties.
6. You can expect the discovery of yet unknown elements.
7. In some elements the atomic weight can be corrected if you know the the adjacent elements.
Here is a clear and comprehensive synthesis of not only the construction of the table, but also its chemical significance.
The modern periodic table has seven periods and eight groups.
Periods: Each horizontal strip.
Each vertical stripe group.
Family: A group of elements having similar properties.
Advantages of Mendelejeff
1. Corrected the atomic weights and valences of some elements do not have room on your table as they were considered so far.
2. He noted the properties of some unknown elements, including three which he called eka-boron, eka-aluminum, eka-silicon.
3. Ramsy discovered in 1894 that a gas called argon. Is monatomic, no chemical reactions and had no place in the table. Immediately assumed that there might be other similar gas properties that together form a group. Indeed, shortly after they discovered the other noble gases and the group assigned zero.
4. All holes left blank were filled with the discovery of the corresponding elements. They had properties similar to those assigned by Mendelejeff.
Defects table Mendelejeff
1. It has a fixed place for hydrogen.
2. It highlights a single valence.
3. The set of elements called rare earths or rare (lanthanide) has no location in the table or need to put them all together in one place, like a single element, which is not true.
4. There was no explanation to the fact that some will have periods of 8 elements: others 18, others 32, etc..
5. The distribution of elements is not always in order of increasing atomic weights.
Modern periodic table
In this century it was discovered that the properties of elements are periodic function of atomic weights, but their numbers vary periodically with atomic or nuclear charge. Here’s the real modern periodic law which is governed by the new system: “The properties of the elements are periodic function of their atomic numbers”
In modern times, the periodic table is represented horizontally elongated enough for periods of 18 elements form a single series. This disappear disturbances caused by the child groups. The periodic length is the most accepted, the classification of Werner, to appreciate more easily the periodicity of the properties of the elements.
Regular and periodic properties of chemical elements
Are periodic properties of chemical elements which emerge from the valence electrons or electrons chain outermost floor and most of the physical and chemical properties.
The distance of the outermost electrons to the nucleus. This distance is measured in Angstroms (A = 10-8) within a Periodic Table group, with increasing atomic number of the members of a family increases the density and atomic mass that grows more than the atomic volume, F color (yellow-green gas), Cl (green gas), Br (red liquid), I, solid (black purple), the lumen and the atomic radius, the metallic character, the ionic radius, although the ionic radius of the elements metal is less than its atomic radius.
The electron, ion energy released by an electron gas receiving and passing a gaseous atoms, the value is equal to the ionization potential and decreases with increasing atomic number of the members of a family. Electronegativity is the tendency of an atom to attract electrons. In a family decreases with atomic number in a period increases with the atomic number.
Atoms are composed of the nucleus and the cortex and the number of positive charges in the first equals the number of electrons in the crust, hence its electronegativity. If the electronic bark of a neutral atom loses or gains electrons are called ions.
Ions are atoms or atomic groups that have an excessive number of electrons or inadequate to compensate the positive charge of the nucleus.
In the first case the ions are negatively charged and are called anions, and the latter are positively charged and are called cations.
Electropositive and electronegative elements
Electropositive elements are called those who have a tendency to lose electrons becoming cations, and in that group are metals.
Electronegative elements are taking electrons easily transformed into anions, this group includes the metalloids.
Most electropositive elements are located on the left side of the periodic system, are called alkaline elements. As you progress through each period to the right decreases the electropositive character come, finally, the electronegative halogen strong character.
The bond between the atoms is performed by electrons outside the last layer, which are called valence electrons.
The union consists of one or more valence electrons of some atoms are introduced into the electronic sphere of the other.
The noble gases have eight electrons in its last layer, except helium which has two. This electronic configuration and chemical inertness tells them great stability.
All atoms have a tendency to transform its electronic system they possess and acquire the noble gases, because this is the most stable structure.
Electrochemical valence is called the number of electrons lost or gained an atom to become ion. If the number of electrons lost or gained is 1, 2, 3, etc.. It is said that the ion is monovalent, bivalent, trivalent, etc..
When joining or bonding of atoms can occur if:
1. Ionic bond, if there is electrostatic attraction.
2. Covalent bond, they share electrons.
3. Coordinate covalent bond when the pair of electrons is given only by one.
4. Metallic bond, the valence electrons so belongs in common to all atoms.
Ionic or electrovalent
It was proposed by Kossel W in 1916 and is based on the transfer of electrons from one atom to another. The definition is: “electrovalence is the ability of atoms to attract electrons to transfer or acquire a stable configuration, thus forming ion combinations where two opposites.”
Excepting only the noble gases to combine all the elements tend to acquire the same electronic structure the nearest noble gas. The atom that gives up electrons becomes positively charged ion (cation), while the gains that causes the negative ion (anion).
General Properties of Ionic Compounds
In general, ionic compounds have boiling and melting points too high, then to separate the molecules have to undo all the building lens, which has a high lattice energy.
Normal covalent bond
Is defined as follows: “It is the chemical phenomenon whereby two atoms come together to share one or more pairs of electrons, so they do not lose or gain electrons, but the share.”
An atom can complete its outer shell sharing electrons with another atom.
Each pair of electrons shared by two atoms is called double mail. This class is called covalent chemical bond, and is in all the molecules consist of non-metals, binary combinations of these elements together form such as gaseous hydrides and most carbon compounds.
Each double electron (represented by the sign:) Sandwiched between the symbols of the atoms, indicating a single covalent bond and is equivalent to the script structure of formulas.
In covalent bond can be: single, double or triple as a share, two or three pairs of electrons.
Coordinate covalent bond
Is defined as follows: “It is the link occurs when two atoms share a pair of electrons, but this comes only a couple of atoms combined.
In this case the dative covalent bond is called or coordinated. The atom that gives the pair of electrons is called the donor, and which receives acceptor. When we want to simplify the electronic formula puts an arrow going from donor to acceptor.
The crystal structure of metals and alloys explain a lot of its physical properties.
The metal lattice is made of atoms (atomic lattice) that occupy the nodes of the network is very compact with several others.
In most cases the atoms are arranged in cubic lattice, held by forces from the valence electrons, but the valence electrons are not very subject, but form an electron cloud that moves with ease when it is driven by action of an electric field.