Classification of Elements

Classification of elements in groups provides us with a fixed pattern in which the elements change their properties periodically. Periodic tables made the study of the physical and chemical properties of elements simple and organized.

Mendeleev’s periodic table:

Mendeleev's law is the law that properties of the elements are periodic functions of their atomic numbers. Originally the statement that the chemical and physical properties of the elements recur periodically when the elements are arranged in the order of their atomic weights.

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. The 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 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 the Lanthanides and Actinides series.

Modern periodic table: 

The modern periodic table is used to organize all the known elements. Elements are arranged in the table by increasing atomic numbers. In the modern periodic table, each element is represented by its chemical symbol. Columns of the periodic table are called groups. Elements in the same group have similar properties.

The 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, the position of alkali metals, alkali earth metals, halogens, inert gases, etc are separated. So it is superior to Mendeleev’s periodic table.

The features of the 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:

Periods in the periodic table. In each period (horizontal row), the atomic numbers increase from left to right. The periods are numbered 1 through 7 on the left-hand side of the table. Elements that are in the same period have chemical properties that are not all that similar.

Characteristics of periods

1. The number of the electron in the valance shell increases as we move from the left to right.
2. The valency of the electron increases from 1 to 4 then decreases to 0.
3. The atomic size of an atom decreases from left to right due to the increase in nuclear charge as the extra electron is added.
4. Ionization energy and electro positivity decrease as we move from left to right in the periodic table.
5. Electronegativity and metallic character increase as we move from left to right.

Groups:

 The modern periodic table of the elements contains 18 groups or vertical columns. Elements in a group have similar chemical and physical properties because they have the same number of outer electrons.

Characteristics of groups

1. Atoms of the element in the group have the same number of the electron in the outermost shell.
2. Elements in the group have the same valency except for group 0, which does not take part in the chemical reaction.
3. Atomic size increases as we move down as one new shell is added.
4. Ionization increases as we move down the group.
5. Metallic character, chemical reactivity of metals increases but not metallic character, and chemical reactivity decreases as we move down the group.

Sub-shells: 

A subshell is a subdivision of electron shells separated by electron orbitals. Subshells are labeled s, p, d, and f in an electron configuration. Each shell consists of the number of sub-shells in which electrons are distributed.

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

p-block: It includes metals, metalloids nonmetals, 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-betweens and p block elements. E.g. Ag. The valance electron lies in the d subshells.

f- block: It includes the elements of lanthanide and actinide.

Aufbau principle

The Aufbau principle, simply put, means electrons are added to orbitals as protons are added to an atom. The term comes from the German word "Aufbau", which means "built-up" or "construction". Lower electron orbitals fill before higher orbitals do, "building up" the electron shell.

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

Valency:

The total number of electrons in the outermost shell of an atom, either shared or gained or lost in a 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 another atom and becomes positively charged. so the valency of sodium is 1.