IX Chemistry - Chapter 4 - Periodicity of Elements and Periodic Table

Definitions
Periodic Table
A table of elements obtained by arranging them in order of their increasing atomic number in which elements having similar properties are placed in the same group is called Periodic Table.

Group
The vertical column of elements in the periodic table are called Groups.

Period
The horizontal rows of elements in the periodic table are called Periods.

Periodicity
The repetition of physical and chemical properties of elements periodically is called Periodicity of Properties.

Periodic Law
Physical and chemical properties of elements are periodic function of their atomic masses.

Metal
Elements which are good conductors of heat and electricity are malleable and ductile and have a metallic luster are called Metals like Sodium, Potassium, Gold, Copper etc.

Non-Metals
Elements which are non or bad conductor of heat and electricity are neither malleable or ductile and have no metallic luster are called Non-Metals like Carbon, Nitrogen, Chlorine etc.

Metalloids
Metalloids are semi metals have the properties which are intermediate between a metal and non-metal like Boron, Silicon, Germanium, Arsenic, Antimony etc.




Law of Triads
A German Chemist, Dobereiner (1829), arranged chemically similar elements in groups of three on the basis of their atomic masses called Triads and it was found that atomic mass of the middle element was approximately equal to the average of atomic masses of other two elements. This is known as Law of Triads.

Drawback or Defect
As very few elements could be arranged in such groups, this classification did not get wide acceptance.




Law of Octaves
An English Chemist Newland (1864) stated that if the elements were arranged in the ascending order of their atomic masses, every eight element will have similar properties to the first. This is knows as Law of Octaves.

Drawback or Defects
1. Noble gases were not discovered at that time and no place was reserved for the undiscovered noble gases.

2. In the same way no blank spaces for the undiscovered elements were present in his table.




Mendeleyv's Period Table and Periodic Law
Russian Chemist, Mendeleyv's (186) who wa working separately from Lother Mayer published a table of elements.

According to Mendeleyv's when the element were arranged in order of their increasing atomic mases, the elements with similar properties were repeated after regular interval and were placed one above the other.A table obtained in this manner is called Periodic Table. Mendeleyv's stated this periodicity in the form of Periodic Law.




Important Features of Mendeleyv's Periodic Table

--------------------------------------------------------------------------------


The important features of Mendeleyv's Periodic table are:

Periods and Groups
The horizontal rows which run from left to right in Periodic Table are called Periods and they are twelve in number.

The vertical rows which run from top to bottom in periodic table are called groups and they are eight in number.

Vacant Spaces
Mendeleyv's left many vacant spaces for the still unknown elements. For example, next to Calcium (40) should be Titanium (48) but it resembled silicon (28) instead of Aluminium (27). He left vacant space for element with atomic mass 44.

Discovery of New Element
Mendeleyv's discovered new elements and also guessed their atomic mass and properties.

Atomic Mass Correction
Mendeleyv's corrected the atomic masses of certain elements on basis of their properties and provided proper place to them in the periodic table.




Defects in Mendeleyv's Periodic Table

--------------------------------------------------------------------------------


The Mendeleyv's Period Table has following defects:

Irregular Position of Some Elements
According to Mendeleyv's Periodic Law Potassium (39) should be placed before Argon (40) but he placed Argon (40) before Potassium (39) which goes against his law.

Position of Isotopes
Mendeleyv's periodic table gives no indication about the position of isotopes.

Structure of Atom
Mendeleyv's Periodic table gives no idea about structure of atoms.

Position of Lanthanides and Actinides
Lanthanides and Actinides have not been given proper place in Periodic Table.

Coinage and Alkali Metals
Alkali metals and coinage metals with different properties are placed in the same group. This defect has been replaced by placing them into two sub groups.




Modern Periodic Law and Modern Periodic Table
Modern Periodic Law
Physical and chemical properties of the elements are periodic function of their atomic number. Mosely (1913) says that atomic mas is not fundamental property. Due to some defects present in Mendeleyv's periodic law, Mosely introduced the concept of anomic number for the elements.

Example
When isotopes were discovered, it was thought advisable to arrange the elements on basis of their atomic number instead o increasing atomic mases. Isotopes were needed different position in the Mendeleyv's periodic table. Hence Mendeleyv's periodic law was modified.


Modern Periodic Table
When Mendeleyv's periodic law was modified and new elements were discovered. This forcd the scientists to change Mendeleyv's periodic law.

The electronic configuration of atoms also played an important role in he arrangement of the modern periodic law. This form of periodic table is called "Long form of Periodic Table" because it contains eighteen groups instead of eight but seven periods instead of twelve.




Group I - The Alkali Metals
The elements of group I are called "Alkali Metals". The word alkali is derived from an Arabic word meaning Ashes.

Elements of Group I
Lithium
Sodium
Potassium
Rubidium
Cesium
Francium
Properties of Group I
1. They are mono atomic.

2. They exist in solid metallic state.

3. Outer most shell of these elements is incomplete having one electron.

4. Elements of this group are highly reactive.

5. Elements of this group have large tendency to form compounds.

6. Elements of this group are strongly electro-positive.




Group II - The Alkaline Earth Metals
The elements of group II are called Alkaline Earth Metals. These elements occur in nature as silicate mineral and their oxides and hydroxides are strongly basic. Therefore these elements are called Alkaline Earth Metals.

Elements of Group II
Beryllium
Magnesium
Calcium
Strontium
Barium
Radium
Properties of Group II
1. They are mono atomic.

2. They exist in solid state.

3. Outer most shell of these elements is incomplete having two electrons.

4. Elements of this group are moderately reactive.

5. Elements of this group have moderate tendency to form compounds.




Group III - The Boron or Aluminium Family
The elements of group III exist in solid state.

Elements of Group III
Boron Metalloid
Aluminium Metal
Gallium Metal
Indium Metal
Thallium Metal
Properties of Group III
1. They are mono atomic.

2. They exist in solid state.

3. Outer most shell of these elements is incomplete having three electrons.

4. Elements of this group are quite reactive.

5. Elements of this group have moderate tendency to form compounds.




Group IV - The Carbon and Silicon Family
Elements of Group IV
Carbon
Silicon
Germanium
Tin
Lead
Properties of Group IV
1. They are mono atomic.

2. They exist in solid state.

3. Outermost shell of these elements is incomplete.

4. Elements of this group are quite reactive.

5. Elements of this group have moderate tendency to form compounds.




Group V - The Nitrogen Family
Elements of Group V
Nitrogen
Phosphorus
Arsenic
Antimony
Bismuth
Properties of Group V
1. Some are mono atomic and some are di-atomic.

2. Some of them exist in gaseous and some are in solid state.

3. Outermost shell of these elements is incomplete having five electrons.

4. elements of this group are quite reactive.

5. Elements of this group have quite tendency to form compound.




Group VI - The Oxygen Family
Elements of Group VI
Oxygen
Sulphur
Selenium
Tellurium
Polonium
Properties of Group VI
1. Some are mono atomic and some are di-atomic.

2. Some of them exist in gaseous and some are in solid state.

3. Elements of this group have quite tendency to form compounds.

4. The tendency of forming covalent bond decreases from oxygen to polonium.

5. There is a gradual decrease in the ionization potential down the group.




Group VII - The Halogen Family
Elements of Group VII
Fluorine Gas
Chlorine Gas
Bromine Liquid
Iodine Solid
Astatine Radioactive
Properties of Group VII
1. They are diatomic except At.

2. Halogens are very active non-metals.

3. Outer most shell of these elements is incomplete having seven electrons.

4. Elements of this group are highly reactive.

5. There is a gradual decrease in the ionization potential down the group.


Transition Elements
Definition
Elements in Group IB, IIB, through VIIB are known as Transition Elements because they show their properties which are transitional between higly reactive and strong electro-positive elements of S-block which form ionic compounds and p-block elements which form largely covalent compounds.

Properties of Transition Elements
1. Transition Elements have incomplete inner electron shells.

2. They show variable valency.

3. They show similar behaviour.

4. They all are metals.

5. They have strong inner atomic bonds.




Group 0, The Noble Gases
The elements of Group VIII A are called "Noble Gases" or "Inert Gases" or "Zero Group Elements".

Elements of Group 0
Helium
Neon
Argon
Krypton
Xenon
Radon
Properties of Group 0
1. They are mono atomic.

2. They exist in gaseous state.

3. Outer most shell of these elements is either complete or contains eight electrons.

4. These elements are mostly chemically non-reactive.

5. These elements have no tendency to form compounds (only a few of these compounds are known).




Atomic Radius
Definition
One half of the distance between the nucleus of two identical atoms when these are in close contact with each other is called Atomic Radius.

Unit
It is measured in angstrom unit A.

Trend in Period
The atomic radii decreases from left to right within a period in the periodic table. This is because nuclear charge increases with the increase of atomic number. But the number of shells remains same within a period.

Trend in Group
Atomic radius increases from top to bottom in a group. This is because, although nuclear charge increases from top to bottom but at the same time on new shell is also added for each successive element down the group.




Ionization Energy (I.E) or Ionization Potential (I.P)
Definition
The minimum energy needed to remove an electron from an isolated, gaseous atom in its ground state is called Ionization Energy.

Unit
It is expressed in electron volts or kilo-joules permole.

1 ev = 96.49kj

Factors Affecting Ionization Energy
The ionization energy of elements depends upon the following factors:

1. Effect of Nuclear Charge on I.E
The greater the nuclear charge the higher is the ionization energy.

2. Effect of Atomic Size
The larger the size of atom the lower is the ionization energy.

Trend of I.E in Period
Ionization energy increases from left to right in a period due to increase in nuclear change and decrease in atomic size.

Trend of I.E in Group
I.E decreases from top to bottom in a group due to increase in atomic size.




Electronegativity
Definition
The tendency of each atom in a covalent molecule to attract a shared pair of electrons towards itself is known as its electronegativity.

Factors Affecting Electronegativity
Electronegativity depends upon the following factors:

Atomic size
Atomic Number
Electron Affinity
Ionization Energy
Trend or Variation in the Period
Electronegativity increases from left to right within a period due to increase in nuclear charge and decrease in atomic size.

Trend or Variation in the Group
Electronegativity values decreases from top to bottom within a group due to increase in atomic size.




Electron Affinity
Definition
The energy change that occurs when an electron is gained by an atom in the gaseous state is known as Electron Affinity.

Electron Affinity for the addition of first electron is negative i.e. energy is released but for further addition of electrons it is positive because energy has to be added to over come repulsion between negative ion and electron.

Unit
It is measured in KJ/mol or in e.v per atom.

Factors Affecting Electron Affinity
Atomic Size
Nuclear Charge
Tend or Variation of Electron Affinity in Group
Down the group in the periodic table, electron affinity decreases because the addition of a new shell to each atom decreases its force of attraction.

Trend or Variation of Electron Affinity in Period
In a period, the electron affinity increases from left to right because the incoming successive atoms have higher nuclear charge and attract electron more towards itself.

Comments :

So far 1
M.BILAL said...
pada hari 

VERY INFORMATIVE

Post a Comment

Share This

Share This

 

© Copyright 2010-2015 Pk-Sites all rights reserved.

A category of .: SitesPk :.