# The Periodic Table of Elements

### History and development

The periodic table is a table that shows the entire element arrange in an increasing order of their atomic number. The history and development of a systematic pattern for the elements is dated back to the mid-1800s.

John Newlands (1863) proposed the idea of repeating octaves of properties for the elements.
Dimitry Mendeleev and Lothar Meyer around 1869 independently arrived at what could be described as the periodic table or chart. The table was based on the periodic law, which states that the chemical properties of elements are a periodic function of their atomic number or weights… In Mendeleev periodic table the elements were arranged by atomic weights, with recurring properties in a periodic manner.
Henry Mosely  (1900s) re-stated the periodic law by changing the basis of the law from atomic weight to anomic number. The present periodic law is therefore’ The properties of element are a periodic function of their atomic number’.
The present modern day periodic table is based on the Mendeleev authority and priestly Moseley statement of the periodic law. In the table, elements having the same number of electrons in the outermost shell of their atom are placed over one another in vertical columns.
Each of the resulting vertical columns is known as a group or family while each of the resulting horizontal rows is known as period or row.
In the periodic table, there are 18 vertical columns. In the designation before 1984, The Roman numerals were used to represent the groups. Group IA, IIA, IIIA. IVA, VA, VIA, VIIA, IVB, VB, VIB, VIIB and VII are the Subgroups or Short groups.

The group in the periodic table consists of vertical columns of elements arranged in an orderly manner and the Period are horizontal rows of element arranged orderly in  increasing order of their respective atomic number.
For elements to be placed accordingly into the periodic table it will either be placed in the S, P, d or f Orbitals.
Now, element in the s orbital belongs to the s-block and element in the p orbitals belongs to the p block,element in the d-block are the s orbital element and lastly element in the s orbital are the s-block elements.

### Periodicity of Elements in the periodic table

Periodicity of elements is the variation of element in the periodic table both across the period and down the group. This properties tends to determine how elements are been placed either in the group or respective row in the table.
Note, all elements in the same group possess the same or have the same valence electrons in their outermost orbital and exhibit similar chemical and physical properties whereas elements in the same period have varying properties, and their atomic number
increase by plus one.

#### The periodic properties which tends to affect elements are :

1. Ionization energy
2.Electron Affinity
4. Electronegativity
5. Electropositivity
6. Boiling and melting point
7. Electrical conductivity
8. Density and metallic character

Ionization energy refers to the minimum amount of energy requires to remove the most loosely bound electron from the outermost shell of a gaseous atom or element.
Now, not like in  a standard solution, the ionization energy increases across the period and decreases down the group because of the shielding effect, that is the number of shells remain the same when moving from left to right across the periodic table and down the group the number of shells increases arithmetically by plus one.

With this element in group 7 for example fluorine will have the highest ionization value.
Electron Affinity in a clearer view refers to the energy attain when one additional electron is been added to the outermost shell of a gaseous atom to form a univalent ion. Atomic radius is half the distances measure from the center of the atom to the orbit or shell, usually circumference in mathematics.
Now twice the distance between the centers of an atom to the orbit is the Atomic radii.
This periodic property (Atomic or ionic radius) tends to increase down the group from top to bottom
and decrease across the period from left to right majorly because of the shielding and screening effect of the electrons.
How to write down the electronic configuration of elements Electronic configuration
refers to the spatial arrangement of elections in the orbital of element. To effectively write down the electronic configuration of element you need to follow the Hunds rule; where electrons are filled singly before pairing in a degenerate orbital.
For the 10th element which is Neon: we have 1S2 2S22P6.

Thus this element Neon belongs to group 8 and period 2 of the periodic table. Also for the number 17th element which is chlorine will have its electronic configuration as 1s22s22p63s23p5.

Chlorine from the electronic configuration provided above belongs to group 5 period 2 in the periodic table.