 |
| Conceptual Answers of IX level Chemistry Notes on Periodic Table From the Notes Library of H.E.S (Health, Education, and Skills) |
Define the following terms
i. Modern periodic law
ii. Groups
or Families of the periodic table
iii.
Periods of the periodic table
iv. Normal or Representative elements.
v. Transition
elements
1. Modern periodic law
Modern periodic law states that
The physical and chemical properties of the elements are the periodic function of their atomic numbers.
2. Groups of the periodic table
The vertical columns of the elements in the periodic table are called Groups or Families.
3. Periods of the periodic table
The horizontal rows of the periodic table are called Periods of the periodic table.
4. Normal or Representative elements
All the elements of Group-IA to Group-VIIIA (s and p-block) are called Normal or Representative elements.
5. Transition elements
All the elements of Group-IB to VIIIB (d and f-block) are called Transition elements.
What is the need and importance of the periodic table?
Need and importance of the periodic table
In 1800 there were 34 elements known, then in the year 1870, this number becomes almost double, in the year 1974 elements were 105, and today the total number of elements known is about 118. With the increasing number of elements, it was felt necessary that elements should be arranged so they can be studied in an easier way i.e. one can get maximum information in less time.
Briefly describe the history of the periodic table.
History of the periodic table
The periodic table we use and study nowadays is due to the continuous effort of many different scientists. Briefly, the history of the periodic table can be stated as under
1. Al-Razi’s classification
Al-Razi was the first scientist who divided the then-known elements into metals and non-metals.
2. Dobereiner’s classification
In 1817, a German Chemist, J.W.Dobereiner arranged similar elements into groups of three elements. He called these groups “Triads, so the Law of Triads was presented by Dobereiner.
Dobereiner’s statement
The atomic mass of the middle element was the approximate arithmetic mean of the atomic mass of the other two elements.
Example
Chlorine and Iodine have an atomic mass of 35.5 and 127 respectively. The average of these two elements is 81 (Approx), which is the atomic mass of Bromine, so according to Dobereiner’s classification, Bromine should be in the middle of Chlorine and Iodine.
|
Chlorine atomic mass = 35.5 |
Bromine atomic mass= 80 (approx) |
Iodine atomic mass = 127 |
|
The calculation for the atomic mass of the middle element = (Cl+I)/2 = (35.5+127)/2 =81.2 Which is the approximate atomic mass of Bromine, according to Dobereiner’s statement. |
||
3. Newland’s octaves
In 1864, an English chemist, Newland arranged the elements in order of increasing atomic masses. He called it “Octaves” and thus presented the following law of Octaves
Statement
If the elements are arranged in order of increasing atomic masses the properties of every 8th element starting from any point is similar to that of the first.
4. Mendeleev’s periodic table
In 1869, a Russian Chemist, Mendeleev arranged the elements in order of their increasing atomic masses.
Mendeleev’s periodic law
The periodic table presented by Mendeleev is based on a law known as Mendeleev’s periodic law, which can be stated as
The physical and chemical properties of the elements are the periodic function of their atomic masses.
Mendeleev’s periodic table becomes the basis of the modern periodic table.
Write a short note on the Modern Periodic table.
Modern Periodic table
The modern periodic table is also called the Long form of the periodic table. It is based on a law called Modern periodic law which states that
The physical and chemical properties of the elements are the periodic function of their atomic numbers.
The modern periodic table was put forward by Henry Gwyn Jeffreys Moseley, an English physicist.
- Rows of the periodic table are called Periods, while
- Columns of the periodic table are called Groups or Families.
The modern periodic table was recommended by the IUPAC (International Union of Pure and Applied Chemistry) in 1957.
Describe the Periods (rows) of the periodic table.
Periods of the periodic table
The horizontal rows of the periodic table are called Periods of the periodic table.
All the elements in the same period have the same number of electronic shells. There are a total of seven periods in the modern periodic table, their detail is given below
- The first period contains 2 elements that are Hydrogen and Helium. Because of too less number of elements, it is called the shortest period of the periodic table.
- 2nd and 3rdboth contain 8 elements in each period. These two periods are called Short periods of the periodic table.
- 4th and 5th both contain 18 elements in each period. These 18 are further divided into
8 Normal or Representative Elements, whose group number has letter A.
10 Transition elements, whose group number has letter B.
These periods are called long periods of the periodic table.
- 6th period contains 32 elements, this period is further divided into
a. Upper part
This upper part of the 6th period contains 18 elements, of which 8 are Normal and 10 are transition elements.
b. Lower Part
This lower part of the 6th period contains 14 elements which form a series called the Lanthanide series or Rare earth elements. So there is a total of 32 elements in the 6th period (18 + 14 = 32) which is so called the longest period of the periodic table.
- 7th period contains 23 elements, which are further divided as
a. Upper part
The upper part of the 7th period contains 9 elements only.
b. Lower part
The lower part of the 7th period contains 14 elements which form a series called the Actinide series. As there are 23 elements in the 7th period so is called an incomplete period of the periodic table.
The number of valence electrons of the elements in the same period increases gradually by one, across the period from left to right.
Describe the Groups (columns) of the periodic table.
Groups of the periodic table
The vertical columns of elements in the periodic table are called Groups or Families.
Relationship between group number and electronic structure (Top to Bottom)
The periodic table is arranged in such a manner that the elements in the same group have the same number of electrons in the outermost shell of their atoms i.e. they have the same number of valence electrons.
Because of the similarities in the number of valence electrons, the elements show resemblance in their chemical properties in the same group.
Each group of periodic table is assigned its own names which are as
- The Group-IA elements are called Alkali metals e.g. Lithium (Li), Sodium (Na), and Potassium (K) are alkali metals. These elements have one valence electron and also show similar chemical properties i.e. they are very reactive, electropositive, and show metallic characters.
- The Group-IIA elements are called Alkaline earth metals e.g. Beryllium (Be), Magnesium (Mg), and Calcium (Ca) are alkaline earth metals. They have two valence electrons and are less electropositive than alkali metals.
- The Group-IIIA elements are called the Boron family. These elements have three valence electrons.
- The Group-IVA elements are called the Carbon family. These elements have four valence electrons.
- The Group-VA elements are called the Nitrogen family. These elements have five valence electrons and need three more to complete their octet.
- The Group-VIA elements are called the Oxygen family or Chalcogens. These elements have six valence electrons and need two more to complete their octet.
- The Group-VIIA elements are called the Halogens family. These elements have seven valence electrons and need one more to complete their octet.
- The Group-VIIIA elements are called Noble gases, Inert gases, or Zero group. These elements have eight valence electrons (except Helium which has two valence electrons) and have complete doublet and octet.
Describe the blocks of the periodic table.
Blocks of the periodic table
The periodic table is divided into four blocks on the basis of the involvement of orbital in the bond formation, these blocks are
a. s-block elements
The elements of Group-IA
(Alkali metals) and Group-IIA (Alkaline earth metals) are called s-block
elements because of the involvement of their s-orbital in the bond formation.
b. p-block elements
The elements of Group-IIIA to Group-VIIIA are called p-block elements because of the involvement of their p-orbital in the bond formation.
c. d-block elements
The elements of Group-IB to Group-VIIIB are called d-block elements because of the involvement of their d-orbital in the bond formation.
d. f-block elements
The elements of the Lanthanide and Actinide series are called f-block elements because of the involvement of their f-orbital in the bond formation.
Define and explain the term Periodicity of properties.
Periodicity of properties
The repetition of properties of elements in the periodic table is called Periodicity of properties. (OR)
The process/pattern by which there is the repetition of properties in all the groups and periods after a certain regular interval is called periodicity of properties.
Explanation
The word periodicity means repetition at regular intervals. The modern periodic table is arranged in such a way that the properties of elements are repeated after a certain regular interval of every 8th element, this repetition of properties is called Periodicity of properties.
Following are the Periodicity of some properties
- Atomic Size
- Ionization energy
- Electron Affinity
- Shielding Effect Electronegativity.
0 Comments:
Post a Comment