Classfication of elements and periodicity in property

classification of elements

Arranging the known element in such a way that similar elements are placed together and dissimilar elements are separated from one another is known as 'classification of elements.

Periodic Table

 it may be defined as the table which classifies the elements into systematic grouping According to their properties

Mendeleev's periodic law

 According to this law: "the physical and chemical properties of the elements are periodic functions of their atomic masses."
This means that if the elements are arranged in the order of their increasing atomic masses, elements with similar properties are repeated after regular intervals. A table was prepared on the basis of this law and was called as Mendeleev's periodic table.

Modern periodic law (given by Moseley)

according to this law, "The physical and chemical properties of elements are periodic functions of their atomic numbers."
Therefore, Moseley arranged the elements in increasing order of their atomic numbers. Then, the elements with similar properties were repeated after regular intervals.
The table so formed is called a Long form of the periodic table or the modern periodic table.

Characteristics of the modern periodic table

1. The arrangements of elements in this table are based upon modern periodic law.
2. It consists of 18 vertical columns called as groups or families.
3. There are 7 horizontal rows called as periodic or series.
4. There are 4 blocks s, p, d, and f.

Type of elements ( classification )

1. S - block elements

 the elements in which the last electron enters in s-orbital are called as s-block elements. Elements of group 1 & 2 belong to s-block. They have 1 and 2 electrons in their outermost shell respectively.

2. P-block elements

 The elements in which the last electron enters in p-orbital are called as p-block elements. Elements of group 13, 14, 15, 16, 17 & 18 belong to p-block. They have 3, 4, 5, 6, 7 & 8 electrons in their outermost shell respectively.

3. D-block elements

 the elements in which the elements enter in d-orbital are called as d-block elements. Elements of the 3rd group to 12th group are in d-block.

4. F-block elements

 the elements in which the last electron enters in d-orbital called d-block elements on inner transition elements. This block consists of two series of 14 elements each and is shown at the bottom of the periodic table.

                    The first (or 4f) inner transition series is called lanthanides ( or lanthanoids), because they follow Lanthanum in the table. Similarly, the second (or 5f) inner transition series is called Actinide ( or actinoids) because they follow actinium in the periodic table.
Periodicity of properties: Reoccurrence or repetition of chemical properties of elements at regular intervals when arranged in a definite order is called periodicity of properties.
For example, the properties of Li, Na, K etc are similar.
Cause of periodicity: to explain the cause of periodicity, let us study the electronic configuration of elements of group-1
.
2.
We observe that all elements of a particular group have a similar outer electronic configuration.
Thus the cause of periodicity is Reoccurrence of similar outer electronic at certain regular interval. These regular in group-1 are 2, 8, 8, 18, 18, 32. These numbers are called magic numbers.

Advantages of the long form of periodic table

1. lt relates the position of an element in the to its electronic configuration more clearly.
2. lt is easy to remember.
3. metal has been separated from non-metals.
4. on the basis of electronic configuration, the elements are classified into s,p,d, and f-blocks. thus their properties can be studies easily.
5. This table has been able to explain the cause of the periodicity of properties of elements on the basis of similar outer electronic configuration.

The disadvantage of the long form of the periodic table

1. The position of hydrogen is not settled.
2. Lanthanides and Actinide do not get the place within the main periodic table.
3. No place is given to isotopes of elements.

The relation between no. Of elements in a period and electrons in energy shells:

Thus the first period contains 2 elements, second & third period contains 8 elements each, 4th and 5th period contains 18 elements each and 6th and 7th period contains 32 elements.

Prediction of the period, group and block of an element

The period, group or block of an element can be predicted from the electronic configuration of elements as below:

1. The period of an element corresponds to the principal quantum no. Of the valence shell.
2. The block of elements corresponds to the subshell which receives the last electron.
3. The group of an element is predicted from the no. Of electrons in the balance shell or/ and penultimate shell as follows:

For s-block elements, group no. It is equal to the no. Of valance electrons.
For p-block elements, group no. It is equal to 10+ no. Of electrons in the balance shell.
For s-block elements, group no. It is equal to no. of electrons in (n-1)d and ns subshell.

Example:

Example: for atomic no. 104, the name may be written as:
Un(1)+nil(0)+quad(4)+ium= Unnilquadium

The recommended and IUPAC names of these elements are:

Periodic properties

 The properties of elements that vary gradually in the same period or group with the increase in atomic no. Are called as periodic properties.

Same of these properties are:

1. Atomic size
2. Ionization energy ( or ionization enthalpy)
3. Electron affinity ( electron gain enthalpy)
4. Electronegativity

Atomic size

 If the atom is assumed to be spherical, the atomic size is given by the radius of the sphere and is called an atomic radius.
Atomic radius is defined as time distance from the center of the nucleus up to the outer most shell of electrons.

Variation of Atomic Radius:

In a period: as we move from left to right in a period, the atomic radius decrease. This is because along a period, the nuclear charge goes on increasing and thus outer shell electrons are attracted more strongly towards the nucleus & hence atomic radius decrease along a period.

In a group: As we move from top to bottom, the atomic radius increase. This is due to the increase in no. Of shell and screening effect of inner electrons.
      However, the effect of the increase of nuclear charge is very small as compared to the effect of these two factors, thus atomic radius increases along with a group.

Ionic radius: the size of the ion is expressed in terms of ionic radius. The ion is formed as a result of the addition or removal of electrons from the outer most shell of the atom.
      The ion formed by the loss of electron get +be charge and is called as the cation.
The ion formed by the gain of electron gets -ve charge and is called as the anion.

The radius of the cation is smaller than its atom: A cation is formed by loss of one or more electrons from the gaseous atom. As a result, the outer shell of electron is completely removed and this the size decreases.
Example 
Na+ < Na
Na = Na+   + e-
2,8,1.    2,8