Periodic Classification of Elements - Class 10th Science

The Modern Periodic Table

Henery Moseley, in 1913, showed that the atomic number of an element is a more fundamental property than its atomic mass.

Modern Periodic Law

"Properties of elements are a periodic function of their atomic number ".

The Modern Periodic Table: At a Glance

The long form of Periodic Table is known as the Modern Periodic Table.

In Modern Periodic Table elements are arranged in the increasing order of their atomic number.

Since, Hydrogen (H) has atomic number equal to 1 (one), it is placed at the first place in the Periodic Table.

There are 18 groups (vertical columns) in the Modern Periodic Table.

There are seven periods (Horizontal rows) in the Modern Periodic Table.

In 1st group Alkali metals are kept.

In 2ndgroup Alkaline earth metals are kept.

In last group, i.e. in 18th group, Inert gases are kept.

At the left side of Modern Periodic Table, metals are kept except hydrogen which is a non-metal.

At the right side of the Modern Periodic Table, metalloids and non-metals are placed.

There are two separates periods (horizontal rows) are placed at the bottom of Modern Periodic Table, in which Lanthanides and Actinides are placed separately. Lanthanides and Actinides are known as Rare Earth Metals also.

Group number in the Modern Periodic Table denotes the valence electrons of an element.

The period number shows the number of orbits present in an element.

Position of Elements in the Modern Periodic Table

All alkali metals are kept in the 1st group.

All alkaline earth metals are kept in 2nd group.

All inert gases are kept in the 18th group.

Lanthanides are placed separately at the bottom of modern periodic table.

Similarly, other elements are kept in separate groups according to their properties.

On the other hand elements having same number of occupied shells are placed in the same period. For example: Sodium (Na), Magnesium (Mg), Aluminium (Al), Silicon (Si), Phosphorous (P), Sulphur (S), Chlorine (Cl) and Argon (Ar) are palced in the same third period.

How to know the position of a Element in the Modern Periodic Table

By knowing the atomic number and electronic configuration of an element, one can find the position of element in the Modern Periodic Table.

What is the need to find the position of an element in the Modern Periodic Table?

Since, elements are classified according to their properties in the Periodic Table, thus by knowing the position, we can understand the properties of an element easily.

Steps to find the position of an element in the Modern Periodic Table

(a) First know the atomic number.

(b) Write the electronic configuration of element

(c) Find out the number of valence electrons (Valence electrons are the electrons present in outermost orbit of an element)

(d) Find the number of orbit

Now, decide the Position of Element in the Modern Periodic Table

Number of Valence electrons = Group Number

If valence electron is more than two, then add ten to it to find out the group number in the long form of Modern Periodic Table.

Note: But this rule will be applied correctly to the elements having atomic number equal upto 20. To finding the group number of many of the elements having atomic number more than 20 (twenty) is slightly complex and you will learn about that in higher classed.

Number of Orbit = Period Number

Example:

(a) What is the position of Hydrogen (H) in Modern Periodic Table?

Atomic number of Hydrogen (H) = 1

Electronic configuration of Hydrogen (H) = 1

Valence electrons of Hydrogen (H) = 1

Number of orbit present in Hydrogen (H) = 1

Group= Number of valence electrons = 1

Period = Number of orbit = 1

Thus, position of Hydrogen (H) in Modern Periodic Table is Group 1st and Period 1 (One).

(b) What is the position of Sodium (Na)?

Atomic number of sodium (Na) = 11

Electronic configuration of Sodium (Na) = 2, 8, 1

Valence electrons of Sodium (Na) = 1

Number of orbit in Sodium (Na) = 3

Group= Number of valence electrons = 1

Period = Number of orbit = 3

Thus, position of Sodium (Na) in Modern Periodic Table is Group 1st and Period 3 (Three).

(c) What is position of Oxygen (O) in Modern Periodic Table?

Atomic number of Oxygen (O) = 8

Electronic configuration of Oxygen (O) = 2, 6

Valence electrons of Oxygen (O) = 6

Number of orbit in Oxygen (O) = 2

Now, if valence electron is more than two, then add 10 (ten) to find the group number in long form of Modern Periodic Table.

Here, valence electrons = 6

∴ Group number = 6 +10 = 16

Period = Number of orbit = 2

Thus, oxygen is in the 16th group and second period in the Modern Periodic Table.

(d) What is the position of Chlorine (Cl) in the Modern Periodic Table?

Atomic number of Chlorine (Cl) = 17

Electronic configuration of Chlorine (Cl) = 2, 8, 7

Valence electrons of Chlorine (Cl) = 7

Number of orbit in Chlorine (Cl) = 3

Now, if valence electron is more than two, then add 10 (ten) to find the group number in long form of Modern Periodic Table.

Here, valence electrons = 7

∴ Group number = 7 +10 = 17

Period = Number of orbit = 3

Thus, Chlorine (Cl) is in the 17th group and 3rd (Third) period in the Modern Periodic Table.

Maximum Number of Elements in a Period in Modern Periodic Table

Number of maximum elements is decided by the number of maximum electrons a shell can have.

The maximum number of electrons in a shell or orbit is calculated by a formula 2n2, where n is number of given shell from the nucleus.

A period can have maximum number of element equal to the maximum number of electron in a shell for which that period is specified.

(a) Maximum number of electrons in K shell = 2 x (1)2 = 2

Since, maximum number of electrons in K shell is equal to 2, thus, first period has maximum number of elements equal to 2.

(b) Maximum number of electrons in L shell = 2 x (2)2 = 8

Since, maximum number of electrons in L shell is equal to 8, thus, second period has maximum number of elements equal to 8.

(c) Maximum number of electrons in M shell = 2 x (3)2 = 18

Since, maximum number of electrons in K shell is equal to 18, but in outermost period maximum number of electrons cannot exceeds 8, thus, third period also has maximum number of elements equal to 8.

Trends in Modern Periodic Table

Valency and Valence Electrons

Valency of an element is determined by the number of valence electrons present in outermost shell of

its atom.

Valence electrons remain the same in a group. Valence electrons become zero in 18th group.

But valence electrons first increase moving from left to right in a period and then decreases.

Atomic Size

Atomic size is the radius of an atom, which is visualized as the distance between centre of the nucleus and the outermost shell of the atom of an element.

Atomic size increases when we increase from top to bottom in a group because of increase in number of shell or orbit.

Atomic size of the atom of an element decreases as we move from left to right in a period. This happens because with the increase in atomic number, positive chare in nucleus which tends to pull the electrons closer to the nucleus and reduces the size of the atom.

Metallic and Non-metallic Properties

Metallic properties of elements decreases by moving from left to right in a group. At left most part of Modern Periodic Table, metals are placed and at the right side non-metals are placed.

Electropositive nature or electropositivity

The nature to lose electrons while forming bond is called Electropositive nature of an element.

The Electropositive nature decreases by moving from left to right in a period.

The Electropositive nature increase by moving top to bottom in a group.

Electronegative Nature or Electronegativity

The nature to accept electrons while forming bond is called Electronegative nature or electronegativity of the atom of an element.

The electronegativity increases as we move from left to right in a period and vice versa.

These trends of Electropositivity and Electronegativity of atom of an element help to predict the nature of oxides formed by the elements because the oxides of metals are basic and that of non-metal are acidic in nature.

Electropositivity and electronegativity of an element also tell the metallic and non-metallic nature of an element. Electropositive elements are metals while electronegative elements are non metals.

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