Periodic Classification of Elements [Class 10] Chapter Notes by VIVEK

Introduction

When a group of similar characteristics combined so that the properties of one of them if known then the other elements can be co-related. This is known as periodic classification of elements.

Placing Similar groups and species together is known as Classification.

 

NEED FOR CLASSIFICATION OF ELEMENTS

Classification is necessary to easily understand the properties of different elements in a periodic table in an easier, systematic way and also remember the properties of each element very easily.

 

Dobereiner’s Triads

Dobereiner’s law states that when the three elements in triad were written in the order of increasing atomic mass, the atomic mass of the middle element was roughly the average of the atomic mass of the other two element.

Ex.- Triad of Li, Na, K.

 

ADVANTAGES OF TRIAD LAW

1.       1. He was the first scientist who successfully classified the elements into Triad.

2.       2. His attempts of classification encouraged other chemists to correlate the properties of elements with their atomic masses.

 

LIMITATIONS OF TRIAD LAW

1.       1. He could not arrange all the elements known at the time into Triad.

 

Newland’s Law of Octaves

Newland’s law of octaves states that when the elements are arranged in the order of increasing atomic masses then every eighth element had the properties similar to that of the first.

 

ADVANTAGES OF LAW OF OCTAVES

1.       1. This law worked well with the lighter elements only.

 

LIMITATIONS OF LAW OF OCTAVES

1.       1. It is applicable only upto Calcium and after Calcium the 8^th element did not posses the similar property to that of the first.

2.       2. He placed two elements in the same slot in a particular group, having different properties.

3.       3. He assumed that only 56 elements were existed in the nature and no more elements were likely to be discovered in the future.

4.       4. He placed the similar elements far away from each other.

 

Mendeleev’s Periodic Law

Mendeleev’s periodic law states that the properties of elements are the periodic functions of their atomic masses.

 

FEATURES OF MENDEELEV’S PERIODIC TABLE

● Twelve horizontal rows, which were condensed to 7, known as periods.

● Eight vertical columns known as groups.

● Groups I to VII subdivided into A and B subgroups.

● Group VIII doesn’t have any subgroups and contains three elements in each row.

● Elements in the same group exhibit similar properties.

 

ADVANTAGES OF MENDELEEV’S LAW

1.       1. A very systematic study of the elements.

2.       2. Prediction of new elements and their properties.

3.       3. Correction of some doubtful atomic masses.

4.       4. Placing the noble gases in a separate group without disturbing the main table.

 

LIMITATIONS OF MENDELEEV’S LAW

1.       1. Position of hydrogen: Hydrogen resembles both, the alkali metals (IA) and the halogens (VIIA) in properties, so, Mendeleev could not justify its position.

2.       2. Position of isotopes: Atomic weight of isotopes differ, but, they were not placed in different positions in Mendeleev’s periodic table.

3.       3. Anomalous pairs of elements: Cobalt (Co) has higher atomic weights but was placed before Nickel (Ni) in the periodic table.

4.       4. Placement of like elements in different groups: Platinum (Pt) and Gold (Au) have similar properties but were placed in different groups.

5.       5. Cause of periodicity: He could not explain the cause of periodicity among the elements.

 

Modern Periodic Law

This law states that the properties of the elements are the periodic function of their atomic number. This law was proposed by Henry Moseley in 1913.

 

FEATURES OF MODERN PERIODIC TABLE

1.      Periods in Modern Periodic Table

·         Elements present in the same period have the same number of shells which is equal to the period number.

·         On moving from left to right in a given period, the number of electrons in the valence shell increases from one to eight while the number of shells remains the same.

2.      Number of Elements in a Period

·         The first period contains only two elements ₁H and ₂He and is known as the shortest period.

·         The second period (₃Li to ₁₀Ne) and the third period (₁₁Na to ₁₈Ar) contain 8 elements each and are known as short periods.

·         The fourth period (₁₉K to ₃₆Kr) and the fifth period (₃₇Rb to ₅₄Xe) contain 18 elements each and are called long periods.

·         The sixth period contains 32 elements (₅₅Cs and ₈₆Rn) and is also known as the longest period.

·         Seventh period has 28 elements from Fr to atomic number 114 called incomplete period.

(After the recent discoveries of the new elements and their addition to the periodic table, the seventh period is officially complete)

·         14 elements each of the sixth and seventh periods from La to Lu are placed separately at the bottom of the table, are called Lanthanides and 14 elements from seventh period from Ac to Lc are similarly placed separated and are called Actinides.

3.      Groups in Modern Periodic Table

·         The modern periodic table contains 18 vertical columns known as groups.

Group 1 elements are known as alkali metals.
Group 2 elements are known as alkaline earth metals.
Group 15 elements are known as pnicogens.
Group 16 elements are known as chalcogens.
Group 17 elements are known as halogens.
Group 18 elements are known as noble gases.

4.    Position of Elements in Periodic Table

The position of an element in the periodic table determines its chemical nature. Based on the position of the elements in the periodic table, they can be classified as follows: 

              Noble gases: These are a group of elements placed in group 18, which are tasteless, odourless monoatomic gases that have very low chemical reactivity. They are also referred to as inert gases and due to their inertness, they are suitable to be used where reactions are not required. 

             Normal elements: All the elements that are placed in groups 1 to 7 are included in this.

          ·Alkali metals: The elements in group 1, namely Lithium (Li) - Francium (Fr), except Hydrogen (H) are termed alkali metals as they tend to form hydroxide with water which is strong alkalis. Thus, alkali metals are very reactive and react quickly with water or air. Example - Sodium (Na) reacts violently with oxygen in the air, so it is stored in mineral oil.

    Alkaline earth metals: These include group 2 elements starting from Beryllium (Be) - Radium (Ra). They are less reactive than the alkali metals that are found as a compound

           Transition elements: These include the elements from groups 3 to 11. These are so named as they exhibit a transition in their properties from the left to the right, including an increase in atomic size, ionization energy, electronegativity. 

          Inner transition elements: These are elements with similar properties, placed at the end of group 3 in periods 6 and 7. These are called the lanthanide series starting from Lanthanum (La) - Lutetium (Lu) in period 6 with 14 elements. Period 7 contains 14 elements starting from Actinium (Ac) - Lawrencium (Lr). 

          Halogens: These include a group of elements in group 17 that are generally non-metals that can exist in solid, liquid and gas form. They react with the metals to form salts

 

Classification of Modern Periodic Table

The modern periodic table is also classified into metals, non-metals and metalloids.

Metals

Metals are electropositive as they form bonds by losing electrons.

In general cases, oxides of metals are basic in nature.

Non-metals

Nonmetals are electronegative as they form bonds by gaining electrons.

In general cases, oxides of non-metals are acidic in nature.

Metalloids

The elements which show the properties of both metals and nonmetals are called metalloids or semimetals.

For example – Boron, silicon, germanium, arsenic, antimony, tellurium and polonium.

 

 

ADVANTAGES OF MODERN PERIODIC TABLE

1.       1. It is very systematic and easy to remember and reproduce.

2.       2. Since the isotopes have same atomic numbers so position of elements allotted is justified.

3.       3. The positions of elements are linked with electronic configuration.

 

Trends in the Modern Periodic Table

• Valency and Valence Electrons - On moving left to right in a period, valency increases and then it decreases. But it remains same down in a group. As we move from left to right in a period, valence electron increases and remain same as we go down the group.
  
• Atomic Size - It decreases left to right in a period as the nuclear charge increases due to large positive charges on the nucleus. Atomic size increases down in a group due to decrease in nuclear charges and addition of new shell.
• Metallic Character - Ability of atom to lose the electron is known as Metallic Character. Metallic character decreases from left to right in a period. This is due to increase in nuclear charge. But non-metallic character increases left to right in a period. And metallic character increases down the group as the size increases it can easily lose electron.
• Ionization Energy is the energy required to remove an electron from an isolated gaseous atom. Ionization energy increases as we move left to right in a period. This is due to increase in nuclear charge as we move left to right in a period. But down in a group ionization energy decreases due to decrease in nuclear charge but there are some exceptional cases.
• Electropositive Character decreases from left to right in a periodic table and increases down the group. This is due to decrease in metallic character from left to right in a period.
• Basic Character of Oxides increases down the group as atomic radius increases and ionization energy decreases. This is due to increase in metallic character or electro positivity of elements. Acidic character of oxides decreases as non-metallic character of elements decreases from top to bottom.

 

 

Subscribe to our Newsletter

Get notified about the new notes releases, doubt solving questions, and many more at VIVEK.

🛩️ Fly to our official Telegram Group.

  SUBSCRIBE