Elements | Chemistry Section | Class 10 (SEE) Science Notes

Classification of Elements

The process of putting different elements together into different groups or classes according to their characteristic is known as the classification of elements. There are different historical backgrounds of the classification of elements.

Although not only a single scientist could be given credit for the classification of elements, but Russian scientist Dmitri Mendeleev's contribution in this field is the greatest among other scientists. He had classified 63 elements discovered at that time by making a scientific periodic table.

Note: Antony Lavoisier was the first scientist to classify elements into metals and non-metals.

Periodic table

A table in which elements are classified into various blocks, periods and groups on the basis of their similarities and dissimilarities is called periodic table.

Mendeleev's Periodic table

The periodic table in which elements are arranged according to the increasing order of their atomic weight is called Mendeleev's periodic table.

State Mendeleev's periodic law.

Mendeleev's periodic law state that, "the physical and chemical properties of the elements are the periodic function of their atomic weight

Characteristics of Mendeleev's Periodic table

1. In this table elements were arranged in increasing order of their atomic number.

2. This table consists of seven horizontal rows called periods and eight vertical columns called groups.

3. Some gaps were left for the undiscovered elements like scandium, gallium, germanium, etc.] which should be similar to Born, Aluminium and Silicon, respectively. So, he suggested the names of those undiscovered elements as Eka-Boron, Eka-Aluminium, and Eka-Silicon.

4. In this table, he had not included noble or inert gases, as they were not discovered at that time.

5. The group of Mendeleev's periodic table were not further divided into sub-groups.

Advantages or merits of Mendeleev's periodic table

1. In the history of the classification of elements, Mendeleev's periodic table was the first scientific as well as the systematic table of elements

2. This table made the study of elements easy, fast, systematic and more convenient.

3. In this table, there were some gaps for undiscovered elements, which shows the pathway for other scientists to discover them.

4. It helps to correct the doubtful atomic mass of some elements like gold, platinum, uranium, etc.

Disadvantages or demerits of Mendeleev's Periodic table

1. In Mendeleev's Periodic table, hydrogen was placed along with metal like Li, Na, K, etc. which was not properly explained.

2. He himself had dishonoured his own law by not giving separate positions for isotopes.

3. He could not give the correct position for lanthanides and actinides.

4. He had disobeyed his own law in some cases such as Argon ( having atomic mass 39.9) was placed before potassium ( having atomic mass 39.1) and Co (atomic mass = 58.9) before Ni ( atomic mass = 58.6).

5. Highly reactive alkali metals were grouped together with less reactive coinage metals.

6. Similar elements like Cu and Hg were placed in different groups whereas dissimilar elements like manganese and halogens were

placed at the same group.

7. His periodic table was unable to explain the atomic properties like valency, reactivity, etc.

Isotopes

The atoms of the same element which have the same atomic number but different atomic masses are called isotopes.

For example Protium, Deuterium and Tritium are the three isotopes of hydrogen.

Isobar

Those elements having different atomic numbers but the same atomic masses are called isobar.

For example Argon and Calcium both contain the same atomic mass but different atomic numbers.

Modern Periodic Table

The periodic table in which elements are arranged according to the increasing order of their atomic number is called the modern periodic table. It is also known as the long form of the periodic table. The modern periodic table is based on the atomic number as the fundamental property of elements.

Although Mendeleev's periodic table made history on the classification of demerits. His work also had been criticized more due to drawbacks. So, in the course of finding the solution, Henry Moseley and his friends in 1913 AD proposed another periodic table known as the modern periodic table.

State modern periodic law.

Modern periodic law state that, "the physical and chemical properties of the elements are the periodic function of their atomic number."

Properties or characteristics of the modern periodic table

1. In the modern periodic table, elements are arranged in the increasing order of their atomic number.

2. In this periodic table there are seven horizontal rows called periods and eighteen vertical columns called groups.

3. The 14 rare earth elements i.e. Lanthanides (atomic number 58 — 71) and 14 Transuranium elements i.e. actinides (atomic number 90-103) are in separate rows at the bottom of the periodic table.

4. Metals are kept on the left-hand side and non-metal are kept on the right side of the table.

5. Inert gases like He, Ne, Ar, Kr, le and Rn are kept in zero groups at the extreme right side of the table.

Advantages or merits of the modern periodic table

1. Hydrogen has atomic number one with only one electron in its k-shell. Thus, they kept hydrogen in the first period and IA group along with Li, Na and K.

2. The anomalies related to isotopes have automatically been solved since all isotopes of one element have the same atomic number.

3. When elements are arranged based on the increasing atomic number, the wrong position of Ar, and K or Ni and Co has been corrected automatically.

4. More reactive alkali metals are kept under the IA group and less reactive coinage metals are kept under the IB group of the modern periodic table.

5. Lanthanides and actinides are placed in a separate box below the main table to avoid the undue sidewise expansion of the periodic table.

6. The representative, transitional, inner-transitional and noble elements have been separated.

7. The elements are classified into s-block, p-block, d-block and f-block according to the orbital, in which the last electron enters. Thus, the study of elements has become more specific.

Disadvantages or demerits of the modern periodic table

1. Position of hydrogen is still controversial.

2. Lanthanides and actinides ap separated from the main body of the periodic table.

3. According to the electronic configuration the position of helium is not appropriate.

4. In group 'VIII B', there are three vertical columns without any justification.

Periods

The horizontal rows of the periodic table where elements having a gradual change in atomic number and characteristics are kept are called periods.

S.N.	Period	Number of elements	Types of period
1.	First	2	Very short period
2.	Second	8	Short period
3.	Third	8	Short period
4.	Fourth	18	Long-period
5.	Fifth	18	Long-period
6.	Sixth	32	Very long period
7.	Seventh	26	Very long but incomplete period.

Characteristics of periods

i. They are horizontal rows of the periodic table.

ii. These elements have a k-radial change in the atomic number as we move in a particular period.

iii. These elements have gradual changes in characteristics as we in the particular period.

iv. In a period, the atomic size decreases as we move from left to right in a particular period.

v. In a period, valency first increases up to 4 and then decreases up to 0 as we move from left to right in a particular period.

Groups

Vertical columns of the periodic table where elements having almost similar characteristics are kept are called groups. Characteristics of groups are:

i. They are the vertical column of the periodic table.

ii. All the elements in a particular group have almost the same characteristics.

iii. All the elements in a particular group have the same valance electrons.

iv. The atomic size increases as we move from top to bottom in a particular group.

v. The valency remains the same for all elements in a particular group.

Electronic configuration

The arrangement of electrons in an orbital or a shell is called electronic configuration.

Shells are named as K-shell, L-shell, M-shell, N-shell and so on depending upon the distance between the nucleus and the orbits or shells. On the basis of 2n2 formula, duplet means two electrons in the number of shells ( for example helium) and octet means eight electrons in the outermost shell ( for example Ne, Ar etc.). But the electronic configuration of all the elements can not be explained by this formula. So, the concept of sub-shells or orbitals is assumed. Here, s, p, d, and f are the orbitals or sub-shells.

Orbitals or Sub-Shells

The region around the nucleus where there is a maximum probability of finding electrons is called orbital or sub-shell. There are 4 orbitals. They are s, p, d and f. Each orbital has its own capacity to hold the electrons. The capacity of holding electrons by each orbital is as given below

Name of orbital	Capacity of holding electrons
s-orbital	2 electrons
p-orbital	6 electrons
d-orbital	10 electrons
f-orbital	14 electrons

The shape of 's' orbital is spherical whereas the shapes of p, d and f orbitals have dumb-bell shapes.

Here, K-orbit has only one orbital and it is denoted by ls. L-orbit has two orbitals denoted by 2s and 2p. M-orbit has three orbitals denoted by 3s, 3p and 3d. N-shell has four orbitals denoted by 4s, 4p, 4d and 4f. Diagram of different orbitals having different shapes: "s, p, d. f"

State Aufbau principle

Aufbau principle states that "the orbitals of lower energy level are filled at first with the electrons and only then the orbital of higher energy level is filled".

Note: Aufbau is a German noun that means construction or building up. It was formulated by Niels Bohr and Wolfgang Pauli. Examples of Electronic configuration in Sub-shells or Orbitals:

S.N.	Elements	Symbols	Electronic configuration in sub-shell
1.	Hydrogen	H	1s¹
2.	Helium	He	1s²
3.	Lithium	Li	1s²,2s¹
4.	Beryllium	Be	1s²,2s²
5.	Boron	B	1s²,2s² 2p¹
6.	Carbon	C	1s², 2s² 2p²
7.	Nitrogen	N	1s²,2s² 2p³
8.	Oxygen	O	1s²,2s² 2p⁴
9.	Fluorine	F	1s²,2s² 2p⁵
10	Neon	Ne	1s²,2s² 2p⁶
11.	Sodium	Na	1s²,2s² 2p⁶3s¹
12	Magnesium	Mg	1s²,2s2 2p6 3s²
13	Aluminium	Al	1s²,2s2 2p6 3s² 3p¹
14	Silicon	Si	1s²,2s2 2p6 3s² 3p²
15	Phosphorus	P	1s²,2s2 2p6 3s² 3p³
16	Sulphur	S	1s²,2s2 2p6 3s² 3p⁴
17	Chlorine	Cl	1s²,2s2 2p6 3s² 3p⁵
18	Argon	Ar	1s²,2s2 2p6 3s² 3p⁶
19	Potassium	K	1s²,2s2 2p6 3s² 3p⁶ 4s¹
20	Calcium	Ca	1s²,2s2 2p6 3s² 3p⁶4s²

1. Classification based on sub-shells in which the last electron enters.

On the basis of orbitals, in which the last electron enters, the elements are classified as,

s-block

The elements whose last electrons enter into 's' sub-shell are called s-block elements. In the modern periodic table, groups IA and IIA are the s-block elements. For example Sodium, Magnesium etc. are the elements of s-block.

p-block

The elements whose last electrons enter into 'p' sub-shell are called p-block elements. In the modern periodic table, the elements of groups IIIA, IVA, VA, VIA, VIIA and zero groups are the p-block elements. For example Boron, Nitrogen, Oxygen etc. are the elements of p-block.

d-block

The elements whose last electron enters into 'd' sup-shell are called d-block elements. In the modern periodic table, the elements of groups IB, IIB, IIIB, IVB, VB, VIB, VIIB, and VIIIB are the elements of d-block. For example Iron, Nickel, Copper etc. are the elements of d-block. The d-block elements are also known as transitional elements.

f-block

The elements whose last electron enters into 'f' sub-shell are called f-block elements. In the modern periodic table, it consists of two series of 14 elements known as lanthanides and actinides. These elements are placed at the bottom of the periodic table. They are also known as inner-transitional elements.

Classification based on the number of incomplete shells in an atom

The elements also are classified into four types depending upon the number of incomplete shells in an atom. They are:

a. Representative elements

b. Transitional elements

c. Inner-transitional elements

d. Noble Gases or Inert elements

Some important things to remember from periods and groups.

1. The size of atoms goes on increasing on moving from top to bottom in a group as the number of shells goes on increasing.

2. While going from left to right in a period, the atomic size decreases because the number of shells remains constant but the internuclear forces of attraction between the nucleus and the valance electron increase. It is due to an increase in the number of protons in the nucleus and the number of electrons in the last shell.

2. Atomic size decreases left to right in the period.

3. Valency remains the same for the particular group but first increases up to four and decreases up to zero along in the period.

4. While going from top to bottom in a group, the reactivity of metals goes on increasing because the number of shells or atomic radii goes on increasing due to this, the internuclear force of attraction between the nucleus and valance electron decreases. Hence, the loss of electrons becomes easy and reactivity increases as metals always donate the electrons during the chemical reactions.

5. While going from top to bottom in a group, the reactivity of non-metals goes on decreasing because the number of shells or atomic radii goes on increasing due to this, the internuclear force of attraction between the nucleus and valance electron decreases. Hence, the gain of electrons becomes difficult and the reactivity decreases as non-metal receive or gain the electron during the chemical reaction.

6. While going from left to right in a period, the reactivity of metals decreases because the numbered shell remains constant but the internuclear force of attraction between the nucleus and the valance shell increases due to an increase in the number of protons in the nucleus and electrons in valance shell. Hence, the atomic size decreases and it becomes too difficult to lose electrons during the chemical reactions.

7. While going from left to right in a period, the reactivity of non-metals increases because the numbered shell remains constant but the internuclear force of attraction between the nucleus and the valance shell increases due to increase in the number of protons in the nucleus and electrons in valance shell. Hence, the atomic size decreases and it becomes easy to gain electrons during the chemical reaction.

Reactivity increases left to right in the period.

Uses of Periodic table

i. It makes the study of elements easy, fast, clear, systematic and scientific

ii. It gives the correct position of elements.

iii. It helps to study the properties of elements.

Explain the position of hydrogen in the modern periodic table.

Hydrogen has only one electron and one proton. Sometimes it loses its electron to make hydrogen ion (Hi) similar to the alkali metals of group IA groups and sometimes it gains one electron to make hydride (fr) similar to halogens of the VIIA group. Due to this reason, it is very difficult to keep it at a particular place. But due to one atomic number, it is kept under the IA group at the top of the modern periodic table.

Differentiate between

S.N.	Modern periodic table	S.N.	Mendeleev's periodic table
1.	In this table, elements are arranged on the basis of the increasing order of atomic numbers.	1.	In this table, elements are arranged on the basis of the increasing order of atomic weight.
2.	It has seven periods and eighteen groups.	2.	It has seven periods and eight groups.
3.	The position of Lanthanides and actinides is justified.	3.	The position of Lanthanides and actinides is not justified.
4.	Highly reactive alkali metals and less reactive coinage metals are kept in the same group.	4.	Highly reactive alkali metals and less reactive coinage metals are kept in one group.

S.N.	Groups	S.N.	Periods
1	The vertical columns of elements in the periodic table are called groups.	1.	The horizontal rows of elements in the periodic table are called periods.
2.	All the elements of a group have the same valency.	2.	The elements of a period have different valency.
3.	The size of atom increases from top to bottom.	3.	The size atom decreases from left to right.
4.	The element of same group have similar properties.	4.	The elements of the same period have different properties.

S.N.	S-block	S.N.	P-block
1.	The elements whose last electrons enter into 's' sub-shell are called s-block elements.	1.	The elements whose last electrons enter into 'p' sub-shell are called p-block elements.
2.	Elements of group IA and IIA belong to s-block.	2.	Elements of group IIIA, IVA, VA, VIA, VIIA and zero group
3.	It is present at the left side of the periodic table.	3.	It is present at the right side of the periodic table.
4.	It includes only metals (i.e. alkali metals and alkaline earth metals).	4.	It includes metals, metalloids, non-metals and inert gases.

S.N.	Representative elements	S.N.	Transition elements
1.	The elements of sub-group 'A' in the periodic table are called representative elements.	1.	The elements of sub-group 'B' in the periodic table are called transition elements.
2.	They include both metal and non-metal.	2.	They are all metals
3.	Only the valance shell is incompletely filled.	3.	The last and second last shells are incompletely filled.
4.	They have single definite valency.	4.	They have variable valency.

S.N.	Alkali metals	S.N.	Halogens
1	They are highly reactive metals.	1.	They are highly reactive non-metal.
2.	They are good conductors of electricity.	2.	They are non-conductor of electricity.
3.	They form electrovalent compounds with non-metals.	3.	They form electrovalent compounds with metals and covalent compounds with hydrogen.
4.	They are the elements of group IA in the periodic table.	4.	They are the elements of group VIIA in the periodic table.

Different types of elements according to their group in the modern periodic table

Alkali metal

Elements of group IA of the modern periodic table except hydrogen are called alkali metals. They are highly reactive metals except for hydrogen. Elements of group IA are called alkali metals because they react with water to give strong alkali.

Alkaline earth metal

Elements of group IIA of the modern periodic table are called alkaline earth metals.

Elements of group IIA are called alkaline earth metals because their oxides react with water to form alkali and their oxides exist in the earth even at high temperatures.

Halogens

Elements of group VIIA of the modern periodic table are called halogens. They are also called highly reactive non-metal.

Elements of group VIIA are called halogens because they form salt after reacting with the elements of group IA except for hydrogen and they form acid after reacting with hydrogen.

Inert gases

Elements of group VIIA or Zero group of the modern periodic table are called inert gases.

Elements of group VIIA or zero groups are called inert gases or noble gases because they do not take part in any chemical reaction.

Different types of elements on the modern periodic table according to their block in the periodic table

Representative elements or Normal elements

Those elements which lie under s-block and p-block of the modern periodic table except zero group elements are called representative elements or normal elements.

Elements of s-block and p-block except zero group are called representative elements because their outermost orbit is incompletely filled with single valency.

Transitional elements

Those elements which lie under the d-block of the modern periodic table are called transitional elements.

Elements of d-block are called transitional elements because their second last and last outermost orbits are incompletely filled with electrons.

Inner-transitional elements

Those elements which lie under the f-block of the modern periodic table are called inner-transitional elements.

Elements of f-block are called inner-transitional elements because their third last, second last and last outermost orbits are incompletely filled with electrons.

Lanthanides

The 14 elements starting from atomic number 58 to atomic number 71 of the modern periodic table are called lanthanides. They are rear earth elements.

Actinides

The 14 elements starting from atomic number 90 to atomic number 103 of modern periodic are called actinides. They are transuranium.