7.Classification of Elements

Mendeleev’s periodic table

Mendeleev’s periodic rule states, “The physical and the chemical properties of elements are a periodic function of their atomic weights.”

The merits of Mendeleev’s periodic table are:

1. Incorrect atomic weights of some of the arranged elements were corrected.

2. Existence of some undiscovered elements was predicted and Mendeleev left gaps for them.

The defects of Mendeleev’s periodic table are:

1. Hydrogen was placed in the group I with alkali metals like Li, Na etc but it could be also placed in the position of halogens.

2. The position of isotopes should be separated according to Mendeleev’s periodic rule but they were kept within the same group.

3. There were no suitable places for Lanthanides and Actinides series.

Modern periodic table

Modern periodic rule states that “The physical and the chemical properties of elements are a periodic function of their atomic number.”

Modern Periodic Table has many advantages like Elements are arranged in 4 different blocks, they are arranged in the increasing number of atomic number, position of alkali metals, alkali earth metals, halogens, inert gases etc are separated. So it is superior than Mendeleev’s periodic table.

The features of Modern Periodic Table are:

1. The wrong position of some elements like Argon and Potassium, Cobalt and Nickel were rearranged by their atomic number.

2. Isotopes of the same element can be placed within the same group due to the same atomic number.

3. The controversy of Hydrogen was explained.

4. Elements have been classified into 4 different blocks.

Periods are the horizontal rows of elements in the periodic table. There are seven periods.

Characteristics of periods

The number of electron in the valance shell increases as we move from the left to right.

The valency of the electron increases from 1 to 4 then decreases to 0.

Atomic size of an atom decreases from left to right due to increase in nuclear charge as extra electron is added.

Ionization energy and electro positivity decreases as we move from left to right in periodic table.

Electro negativity and metallic character increases as we move from left to right.

Groups are the vertical columns of elements in periodic table. There are 18 groups arranged vertically.

Characteristics of groups

Atoms of the element in the group have the same number of the electron in the outermost shell.

Elements in the group have same valency except for the group 0, which do not take part in the chemical reaction.ar

Atomic size increases as we move down as one new shell is added.

Ionization increases as we move down the group.

Metallic character, chemical reactivity of metals increases but not metallic character and chemical reactivity decreases as we move down the group.

Sub-shells

Each shells consists number of sub-shells in which electrons are distributed .

s-block

It includes alkali and alkali earth metals which forms positive ions by losing one or two electron of the outermost shells.eg Ca, Mg

p-block

It includes metals, metalloids   non metals and inert gases .they may have 1-6 valence electrons in the outermost shells.The completely filled p-orbital are noble gases.

d-block

It includes the transition element which lies in between s and p block elements. E.g. Ag

  The valance electron lies in the d sub shells.

f- block

It includes the elements of lanthanide and actinide.

Aufbau principle

This principle explains how the atoms are being arranged in orbital. According to this principle, the sub shells of lowest energy is filled first then higher energy level are filled so on.

Valency

The total number of the electrons in the outermost shell of an atom, either shared or gained or lost in chemical combination is called valency. The outermost electron determines the chemical reactivity of the element.eg sodium has one valence electron that it transfers to the shell of other atom and becomes positively charged.

.so the valency of sodium is 1.