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How is the periodic table arranged?

The periodic table

The periodic table got its structure after some scientists used the information about the atoms and their structure known up to then to create an overview of all atoms.

Historical bases

The physicists Rutherford and Bohr at the turn of the 19th century they were still known today Atomic models and with this had a decisive influence on the design of the periodic table. They found out that an atom consists of the atomic nucleus and the atomic shell. in the core are positively chargedProtons and neutral (uncharged)Neutrons. In the Shell are negatively charged Electronsmoving around the core at different distances. Their trajectories were made clear by means of bowls. The shells, which were labeled with the letters from $ K $ upwards, can each hold a certain number of electrons. A maximum of two electrons fit into the first shell (K shell) until it is completely filled. The second shell (L shell) can hold a maximum of eight electrons and the third shell (M) 18 electrons. The maximum possible number of electrons on a shell is calculated according to $ 2 * n ^ 2 $ (n = shell number). However, if you only consider the main group elements, there are never more than eight electrons in the outermost shell of an atom. The outer electrons of an atom are also called valence electrons.

The number of Valence electrons provides information about the properties of a substance. It has been found that atoms with the same number of outer electrons have similar properties. This knowledge was incorporated into the design of the periodic table.

As you have just learned, there are positively charged protons and negatively charged neutrons in the core of an atom. These are each roughly equally difficult. The electrons, on the other hand, have an incredibly small mass. Because of this, the nucleus defines the mass of an atom. The weight is made up almost one-to-one from the protons and the neutrons. The mass of a proton (or neutron) is given in the atomic mass unit $ u $ (unit). $ 1u $ corresponds to a mass of: $ 0.00000000000000000000166054 = 1.66054 * 10 ^ {- 24} g $

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Oxygen (O) has eight protons and eight neutrons. According to this, an oxygen atom has a mass of 16u.

The main groups of the periodic table

Since the protons of an atom are the only charged particles in the nucleus, they also define the charge of the entire nucleus. The number of protons is also called the atomic number. The atomic number determines the order of the substances in the periodic table from small to large.

In this figure you can see the overview of the atoms that are also in the periodic table.

Main groups in the periodic table


Let's look at what each number mean. At the top left of each field is the Atomic numberthat is in the periodic table too Atomic number is called. The corresponding mass number is located under the abbreviation for the respective element. As you may notice, jumps appear from the 20th ordinal. This is because only the main group elements are listed in this overview. We will look at the missing, so-called subgroup elements later.


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The atomic number indicates the sequence of the elements in the periodic table and is also called the atomic number of the elements. It represents the number of positively charged protons in the atomic nucleus, as well as the total number of negatively charged electrons in the atomic shell of the respective element atom.


Main groups and periods

As you can see in the picture, the periodic table is divided into periods and groups. In each period there are elements whose atoms have the same number of shells. For example, all atoms in the 3rd period have three shells (K, L and M). You can also see that there are eight main groups.

When sorting the substances, the Russian chemist Dmitri Mendeleev noticed that the elements had similar properties at regular intervals. From this he derived the law of octaves, since every eighth substance can be classified into an element family. This is how the eight main groups (I-VIII) of the elements emerged. The number of the main group indicates the number of electrons that are on the outermost shell (the so-called valence electrons, exception: helium with two electrons on the first shell).


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The respective main groups contain elements with similar chemical properties:

1st main group: alkali metals (lithium - sodium - potassium - rubidium - cesium)
2nd main group: alkaline earth metals (beryllium - magnesium - calcium - strontium - barium)
3rd main group: Boron group (boron - aluminum - gallium - indium - thallium)
4th main group: carbon group (carbon - silicon - germanium - tin - lead)
5th main group: nitrogen group (nitrogen - phosphorus - arsenic - antimony - bismuth)
6th main group: chalcogens (oxygen - sulfur - selenium - tellurium - polonium)
7th main group: halogens (fluorine - chlorine - bromine - iodine - astatine)
8th main group: noble gases (helium - neon - argon - krypton - xenon - radon)

Mass and radius of the atoms

The periodic table of the elements gives you information about the Dimensions and the radius of an atom. The height of the mass increases within each main group (from top to bottom) and period (from left to right). For example, lithium (Li) has a higher mass than helium (He) and argon (Ar) has a greater mass than neon (Ne).

You can determine the radius of an atom using the periods. In each period, ascending from top to bottom, the number of bowls increases by one. As a result, the radius of the respective atom is also larger. For the elements just mentioned, this would mean, for example, that a lithium atom has two shells (K and L shell) and an argon atom has three shells (K, L and M shell). Since each additional shell is further away from the atomic nucleus, the greater the radius, the more shells there are.

The subgroups of the periodic table

The following information is not yet of great importance to you, but it belongs to a complete picture of the periodic table of the elements.

As you have already learned, the periodic table shown above skips some ordinal numbers. This is because there are other elements that are not part of the main groups. These elements lie between the second and third main groups, but do not comply with the law of octaves. Therefore they have been separated from the main groups.

Subgroup elements of the periodic table


The Subgroup elements are summarized with ten elements in each period and divided into (sub) groups that run from I to VIII. In the initial chemistry class, the elements of the subgroups initially play a subordinate role.

Now you should know the most important things about that Periodic table and its arrangement knowledge. You can now learn what you have learned using our Exercises deepen. We wish you a lot of fun and success!