Chemical elementAs discussed at the previous chapter, atoms consist of negative charged electrons and the same number of positive charged protons. Atoms with the same proton number are called chemical elements. Examples are hydrogen (proton number 1) or oxygen (proton number 8).
Chemical reactionA chemical reaction is the process transforming two or more educts to one or more products with different physical and chemical properties. The proportion of the educts to those of the products is always constant. An example for a chemical reaction is the transformation of hydrogen and oxygen to water. At the given example two hydrogen atoms and one oxygen atom react to one water molecule according to the following chemical equation:
The participating elements are labeled with their chemical symbols. The educts are written at the left side of the chemical equation separated by an arrow from the product(s) at the right side. Atoms being bond to each other (forming molecules) are written in one "word" and their proportions are given by inferior numbers. A water molecule consists of two hydrogen atoms and one oxygen atom.
A chemical reaction is called exothermic if energy is released during the process (heat or light). If the reaction absorbs energy from the surroundings (it must be heated or exposed to light) it is called an endothermic reaction. Exothermic reactions can be spontaneous, requiring no input of energy, or non-spontaneous meaning some initial energy is required to start the reaction (e.g. you have to use a spark to start a fire). The energy required to start a non-spontaneous reaction is called activation energy.
Covalent bondA molecule is a group of at least two atoms being connected by chemical bonds (interaction between the electron shells). Those covalent bond or atomic bond is characterized by the sharing of a pair of electrons. The pair of electrons is formed by one valence electron (electron of the outermost shell) of each atom. Two Atoms can be connected to each other by more than just one covalent bonding. Those bondings are called double, triple or (rarely) quadruple bond.
Ionic bondIonic bonds are based on the attractive force between oppositely charged ions. The two electrons being used at a covalent bond are moving inside of the electron shell of both involved atoms. When an ionic bond is formed during a chemical reaction, the electron of one atom is moving permanently into the shell of the other one. Accordingly there are two ions created during the process, one positive charged ion and one negative charged ion. If more than just one electron is exchanged during the chemical reaction, the attractive force between the created ions increases.
Metallic bondingIf the outermost shell of an atom is filled with just one, two or three electrons, the ambition of those elements is high to emit those electrons by forming an ionic bond. The electronegativity of these elements is low. If there are no atoms with a high electronegativity around, those atoms form a metallic bonding among each other. The outermost electrons, interacting just weakly with the atomic core, are released leading to the formation of a lattice of positively charged ions, the so called metallic nuclei. The metallic nuclei are embedded to an "electron sea", called delocalized electrons, conducting electrons or Fermi gas (named after the Italian physicist Enrico Fermi). The delocalized electrons are moving nearly free among the metallic nuclei causing the attractive force of the metallic lattice. This type of bonding is collective in nature. A single "metallic bond" (similar to a covalent bond) doesn't exist.
Chemical propertiesThe chemical properties of a substance (element, ion or molecule) are characterized by the behavior during chemical reactions with other substances. Amongst other things the following reactions are proofed:
The reactivity with oxygen (flammability).
The reactivity with water and oxygen (resistance to corrosion).
The reactivity with chlorine gas.
Furthermore the amount of energy being released (exothermic) or used (endothermic) during the chemical reaction forming the substance is used to characterize the product. The capability to form H+-ions (=acid) or OH--ions (=base) in aqueous solution is a often used chemical property, too.
The reaction between hydrogen and oxygen respectively chlorine gas releases a high amount of energy. Helium, featuring just one more proton than hydrogen, reacts with none of those substances.
In 1852 the English chemist sir Edward Frankland introduced another very important chemical property, the valency or valence number. Originally the valency was the measure of the number of covalent bonds between hydrogen and the given chemical element. Nowadays it is expanded to the "number of bonds" definition (=number of covalent bindings even to other elements but hydrogen) or to the oxidation number. The oxidation number measures the number of electrons being transfered from or to the atom while an ionic bond is formed. If a covalent bond is formed, both shared electrons of the pair are formally assigned to the atom with the higher electronegativity. Electronegativity describes the ability of an atom to attract electrons towards itself. It is increasing with the number of protons and decreasing with the diameter of the atom.