ELEMENTS
An element is a pure substance that cannot be decomposed into any pure substance and which forms basic building
blocks of any matter.
Classification of Elements
|
Class |
Position in periodic table |
Properties |
Examples |
|
Noble gases |
Group 'O' |
- Gases
- Colorless
- Chemically Unreactive
- Diamagnetic (weakly repelled by magnetic field)
|
Neon
Argon
Krypton |
|
Representative elements |
Group 'A' |
- Metallic or nonmetallic
- Wide range of chemical behavior
- Range of physical characteristics
- Diamagnetic or paramagnetic (drawn in magnetic field)
|
Nitrogen
Bromine
Boron |
|
Transition elements |
Group 'B' |
- Metallic
- Paramagnetic
- Forms highly colored compounds
|
Titanium
Iron
Zinc |
|
Inner Transition elements |
Bottom of periodic table |
- Properties similar to elements of group III B
- Covered over two periods 6th and 7th
- 6th period elements are called Lanthanide series
- 7th period elements are called Actinide Series
- Metallic
- In general paramagnetic
|
Thorium
Terbium
Curium |
Introduction to the Periodic Table
The Periodic table is an arrangement of elements in the order of their increasing atomic numbers to show that elements
have related properties. Earlier tables, such as those of Dmitry Mandeleev (photograph) (1869) and Lother Meyer
(1869-70) were based on atomic weights which are measured as bulk properties and valency relationships. At that time the
concept of atomic numbers was unknown.
The main purpose of the periodic table was:
Classification elements into groups with similar properties.
- To predict the possibilities of new elements based on their properties.
|
The modern form of the periodic law states that properties of the elements are the periodic function of their atomic
numbers and the properties of the elements depend on their electronic configuration.
The modern periodic table consists of arrangements of elements in three broad categories.
|
- metals (located on LHS)
- non-metals (located on RHS)
- metalloids ( in between) |
Understanding the Periodic Table :
The periodic table (Figure 2) arranges 109 known elements in order of their increasing atomic number. All these elements
are arranged in Groups and Periods.
|
Groups |
Periods |
- The vertical columns are called Groups.
- Eight columns
- 1 to 7 column further divided into 'A' and 'B'
- 8 t h column not divided
- Group zero for noble elements
- Elements in each group have the same number of electrons in the outer shell and hence the same valency.
- Identical Chemical behavior for each group elements
- Lower down the group, number of shells increase by 1; hence atomic radius also increases.
|
The horizontal rows are
called Periods.
Seven rows
No further division
6 t h period includes
Lanthanide series
7 t h period includes
Actinide series
Each period begin with an
alkali metal.
Number of electrons
increase by one, across
the period.
Elements change from
metals (Na) through semi-
metals (Si-Silicon) to non-
metals (Argon-Ar).
Elements change from
reducing agents (Na) to
oxidizing agents (Cl).
Number of shells remain
the same; atom size (radii)
decreases. |
Classification of elements on the basis of electronic configuration:
From the view point of electronic configuration the elements can be subdivided into four major sub groups. They can be
classified as s,p,d and f block elements.
1. s-block elements : These have two atoms and include groups I A & II A. They contain one or two electrons in
their outer S-orbital. Strongly electropositive metals. The group I A elements are alkali metals. Elements with
two electrons in the outer orbit are called alkaline earth metals.
2. p-block elements : The elements from groups III A to VII A are included in this type. These are generally non-metals.
These elements are generally non-conductors with the exception being Silicon and Germanium which
are semi-conductors.
3. d-block elements : These are the elements in which the last electron is placed in d-orbital of penultimate energy
level. These elements are also known as transitional elements. All are metallic and have high melting and boiling
points. Good conductors of heat and energy.
4. f-block elements : The transition elements in which pre-penultimate f-sub orbital is being filled up are called
as inner-transition elements.
There are two series of these elements :
1) Lanthanides & 2) Actinides. Their compounds are colored and
are usually paramagnetic.
5. zero-group elements : Earlier these elements were known as inert or Noble gases. They are chemically unreactive.
They are incapable of forming any chemical compounds.
Reading the Periodic Table :
Figure 2 shows that each box in periodic table contains the following data:
Atomic weight (From 1.00794 to 262.12)
- Atomic number (From 1 to 107)
- Atomic symbol
- Name of the element
- Oxidation states
- Electronic configuration
|
Note the difference between the atomic number and the atomic weight. Thus element 12 is Magnesium (atomic number
is 12) and not Carbon (atomic weight is 12).
