In the periodic table all chemical elements are classified according to their atomic number and their chemical properties in main group elements (columns IA to VIII A) and transition group elements (IB to VIII B). The number of protons increases continuously from left to right. In addition to this horizontal classification, the periodic table is divided vertically into periods. These periods are not chosen randomly but correspond in the shell model to the electron shell introduced by Bohr (K, L, M, N, O, P and Q).

Elements in a certain group are all showing similar chemical behavior due to their identical amount of electrons in their outermost shell (this accounts only for elements in the main group).

The periodic table arranges elements into groups with similar chemical properties and periods with identical number of shells!

As the number of period increases a new electron shell is added. Therefore  the atoms within a certain group grow larger from top to bottom of the periodic table. On the other hand the atomic radius decreases from left to right oft the periodic table. This is due to the increasing number of protons that comes along with the atomic number. The number of electrons in the shell will increase as well . The more protons an atomic nulceus contains the higher its charge and the higher the charge of the electron shell. However, a higher charge results in an stronger force of attraction between the nucleus and the shell. Since the number of shells will not increase within a period the stronger force of attraction will bind the shell much stronger to the nucleus.

The size of an atom will increase within a group from top to bottom but will decrease within a period from left to right!

Within each period the element on the rightmost side of the periodic table will have the highest force of attraction between its nucleus and its shell. This configuration makes the element extremely stable. Since the elements on the rightmoste side are gaseous they are referred to as noble gases (or inert gases).

The classification of the periodic table into main groups and transition groups is due to their different distribution of electrons in atomic orbitals (electron configuration). For the same reason a further division can be made into lanthanides and actinides (the term actionide derives from the fact that all these elements are radioactive).

In the main group the s and p orbitals of the respective atoms are occupied by electrons (“s-block” or “p-block”), while in the transition group an electron is added in the d-orbital of the respective atom (“d-block”). In case of lanthanides and actinides the occupation of the f-orbital (“f-block”) takes place.

The main group elements can be further subdivided according to their physical and chemical behavior. This is usually done as follows:

• non-metals
• alkali metals
• alkaline earth metals
• metals
• metalloids (sometimes misleading called semimetals)
• halogens and
• noble gases.

Note that alkali metals and alkaline earth metals are “metals” in the true sense. Between the group of the alkaline earth metals and the metals is the transition group which is not shown in this figure. The reason for the word “transition” now becomes clear and since all the elements within the transition group are metals they are also referred to as transition metals.  Thus, about 80 % of the existing elements are metals!

A few elements have proporties of metals as well as of nonmetals. Those are referred to as metalloids. However, there is no cear definition of a metalloid! Usually the metalloids include:

• boron (B)
• silicon (Si)
• germanium (Ge)
• arsenic (As)
• antimony (Sb)
• bismuth (Bi)
• selenium (Se)
• tellurium (Te)
• polonium (Po)

The number of outer electrons of an atom (also referred to as valence electrons) significantly determines the chemical properties of the respective element. For main group elements, the number of valence electrons corresponds directly to the main group number. For example, potassium (Ka) belongs to the main group number 1 and therefore has one electron in its outer shell; so does  sodium (Na) and cesium (Cs). Accordingly to the fifth main group, elements like nitrogen (N), phosphorus (P) and arsenic (As) do have five valence electrons.

The number of the main group corresponds to the number of valence electrons of the elements assigned therein! The chemical behavier is mostly influenced by the number of valence electrons!

The chemical similarity due to the common number of valence electrons is particularly evident in the case of the alkali metals (1st main group, with the exception of hydrogen), the alkaline earth metals (2nd main group), the halogens (7th main group) and the noble gases (8th main group) ,

However, this relatively simple determination of the valence electrons based on the group number only works for the main group elements. In the transition group, however, this principle fails. Thus, the entire transition metals have only one or two outer electrons. Consequently, all transition elements have similar chemical properties.

First, you can distinguish between pure substances and mixtures. Pure substances contain only one type of particle. In the simplest case, particle means a chemical element. For example hydrogen (H), pure iron (Fe) or graphite (carbon C). Not only single atoms but also whole molecules can form pure substances. These substances are characterized by a certain atomic ratio and are referred to as chemical compounds. Pure compounds for example are water (H₂O), carbon dioxide (CO₂), acetone (C₃H₆O) or cementite (Fe₃C).

In contrast to mixtures, pure substances have fixed atomic ratios (only in the specific case of elements does a pure substance consist of one type of atom)!

If matter consist of several types of particles (atoms or molecules) with no specific atomic ratio due to chemical bonds, this will be referred to as mixtures. Such mixtures can be further classified into heterogeneous mixtures and homogeneous mixtures.

Homogeneous mixtures do have a uniform distribution of the different particles. Homogeneous mixtures for example are gasmixtures like air as well as solutions. But in contrast to a gas mixture, the state of matter of a solution is liquid. Therefore homogeneous mixtures can further by classified into gasmixtures and solutions. The group of solutions includes, for example, sugar water, salt water or carbonated soda. In addition to gases and liquids, solids can also form homogeneous mixtures. This is the case with some alloys such as copper-nickel alloys. So this could be a third classification of homogeneous mixtures.

Mixtures of substances with an diverse distribution of the containing particles are referred to as heterogeneous mixtures. A mixture of a solid and a liquid is called a suspension, for example, iron sludge, quicksand or liquid concrete. Heterogeneous mixtures of different liquids, which can not be mixed homogeneously, will be referred to as emulsions (e.g. oil-water-mixture, milk, mayonnaise, etc.). For heterogeneous mixtures of two or more solids, such as iron ore, granite or marble, one speaks of a solid sol. The last group of heterogeneous mixtures are the aerosols. These are mixtures of solids or liquids in gases. Examples of aerosols are cigarette smoke, water fog or car exhaust.

A suspension is a mixture of solid particles dissolved in a liquid. A mixture of two different liquids, which can not be dissolved homogeneously, is called emulsion.

Atomic structure

Matter is made up of microscopic units called atoms. Chemical elements are composed of atoms of a certain type. The classification of the elements takes place according to the periodic table. If several atoms (chemical elements) react with each other and form a stable unit by chemical bonds, these are called molecules.

Molecules are stable compounds of atoms by using chemical bonds.

For example, water consists of the elements hydrogen and oxygen. In each case, two hydrogen atoms (H) and one oxygen atom (O) join together in order to form a stable H₂O molecule. Atomic units such as molecules, atoms, protons, neutrons, electrons, etc. are simply referred to as particles. In this connection one often speaks of the so-called particle model of matter.

The particle model of matter means that matter is made up of particles without making a difference between atom, molecules, etc.

According to Rutherford’s atomic model, an atom consist of an positively charged atomic nucleus (lat. nucleus = “core”) and an negatively charged electron shell. The nucleus contains positively charged protons. These protons are the reason for the positive charge of the atomic nucleus.

The repulsive force between the protons due to their identical charges is compensated by the strong attraction of the neutrons,  who are also present in the atomic nucleus. The neutrons themselves are electrically neutral, but they still exert a strong attraction to the protons. In this way the protons are held together stably in the nucleus.

The particles in the nucleus (neutrons and protons) are also referred to as nucleons.

The attraction between the protons an the neutrons can not be caused by a electrostatic field because neutrons do not carry electric charges and theirfore can not be affected by such a field. It is rather another type of force. This force is called strong nuclear force or strong interaction. In addition to the electromagnetic force, the gravitational force (gravity) and the weak interaction (weak nuclear force), the strong interaction is one of the four fundamental forces of physics.

The interaction of the strong nuclear force is very limited in range, but at small distances as in atomic nuclei , that force can be extremely strong. The strong interaction between the protons and neutrons is the reason why this nuclear force outweighs the repulsive electrostatic forces of the protons and thus holds the nucleus together. The strong interaction is the “clue” for the nucleus so to speak.

The strong nuclear force (strong interaction) between the nucleons holds the atomic nucleus together.

The electron shell is located around the positively charged nucleus of an atom. It is formed by the negatively charged electrons. In a simplistic notion, the electrons in this imaginary shell orbit the positive nucleus. The electrostatic forces of attraction between the positive nucleus and the negative electrons ensure that the orbiting electrons are held stably on their path around the atomic nucleus, so that the atom does not fall apart.

The electron shell is an imaginary shell where the electrons orbit the nucleus.

Atomic number

Characteristic for a particular type of atom or for a chemical element is the number of protons in the nucleus! The number of protons essentially determines the chemical behavior of the element and is responsible for the order in the periodic table. Therefore, the number of protons is often referred to as atomic number. For example, a hydrogen atom always has one proton in its nucleus. If it contained two or three protons in the nucleus, it would no longer be a hydrogen atom but a helium atom (2 protons) or a lithium atom (3 protons).

The number of protons in an atom’s nucleus (atomic number) determines the element.

Isotopes

In contrast to the number of protons, the number of neutrons is not characteristic for a chemical element. For example, a lithium atom usually has four neutrons in its nucleus. However, this only applies to 92.5% of all lithium atoms. The remaining 7.5% contain only three neutrons in the nucleus. Such modifications of atoms with different numbers of neutrons but of course still having the same number of protons (otherwise it would be another element) are referred to as isotopes. The lithium atom thus has two (stable) isotopes.

Isotopes do have the same number of protons but different number of neutrons.

The hydrogen atom even has three isotopes. The most abundant hydrogen isotope (99.98%) has only one proton in its nucleus and no neutron. Becaus of only having one proton that hydrogen isotope is also called protium (symbol: H).

If the hydrogen atom has a neutron in addition to the proton, this isotope is called deuterium (symbol: D). Deuterium is represented by only 0.015% of all naturally occurring hydrogen atoms.

Another hydrogen isotope even has two neutrons in the nucleus and is called tritium (symbol: T). This isotope accounts for only a tiny fraction of total hydrogen in nature. However, unlike protium and deuterium, tritium is not stable and decomposes with a half-life of approximately 12 years. Due to the decomposition tritium is radioactive.

Ions

In the electrically neutral state, there are just as many positively charged protons in the nucleus as there are electrons in shell. The electric charge of an electron and a proton is identical in magnitude, but with the opposite sign. In the macroscopic point of view, the electrostatic effects cancel each other out. In this state, the particle is electrically neutral. However, if this neutral state is disturbed by absorbing or removing electrons, the atom is called an ion. The process of absorbing or losing electron is called ionization.

With excess of electrons, the atom is negatively charged. A negatively charged ion is referred to as an anion. An electrically positively charged ion is  called a cation. Since the number of protons determines the element, an ion can only be obtained by donation of electrons, but not through the release of a proton (changing the number of protons would create a completely different element).

An ion is an electrically charged atom (or a group of atoms). A negatively charged atom is called anion and a positively charged atom is called cation.

Note, that anions are larger in size than their respective atom due to the excess of electron and cations are smaller in size because of the loss of electrons.