Class 11th science chemistry (Classification of Elements and Periodicity in Properties)

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In this chapter, we will discuss the development of the periodic table and periodic law. The arrangement of elements in the modern periodic table is in eight vertical columns (groups)and seven horizontal rows (periods) based on their atomic number. On the basis of electronic configuration, elements can be classified into four types in the periodic table viz s-block, p-block, f-block, d-block element. More than 70-80% of the known elements are Metals. Elements located at the top in the periodic table are nonmetals and are less than 20. In a group, as the atomic number increases, the Metallic character increases. In a period, there is a decrease in the Metallic character from left to right. Also, the chemical, as well as the physical properties, vary with their atomic number. Elements of the same group exhibit similar chemical properties because of the similarvalence shell electronic configuration.

Periodic Trends In Properties Of Elements

  • Atomic sizes – With the increase in atomic number the atomic radii in the group will increase. On the other hand, the atomic radii decrease from left to right in a period.
  • Ionization enthalpies – Decreases down a group but increases across a period.
  • Electron gain enthalpies – Tends to become less negative down a group and more negative across a period.
  • Electronegativity – Electronegativity decreases down a group but increases across a period.
  • Chemical reactivity – Chemical reactivity is lowest in the centerAnd is highest at the two extremes of a period.


Earlier scientists assumed that the properties of elements are periodic functions of their atomic masses. On the basis of this assumption, Mendeleev placed 63 elements in a vertical column called groups and in horizontal rows called periods. This method was rejected as it could not explain the position of certain elements, rare earth metals, and isotopes. A scientist named Henry Moseley removed these defects and put forward the modern periodic table with the modern periodic table

Moseleys Periodic law:

He stated that the properties of elements are periodic functions of their atomic number.

Modern Periodic Table:

A tabular arrangement of elements in groups and periods which highlights the regular trends in properties of elements is defined as the periodic table.
modern periodic table

Features of Modern Periodic Table

There are eighteen vertical columns known as groups in the modern periodic table which are arranged from left to right and seven horizontal rows which are known as periods.
Group number
Group name
Property
Group 1 or IA
Alkali metal
They form strong alkalis with water
Group 2 or IIA
Alkaline earth metals
They also form alkalis but weaker than group 1 elements
Group 13 or IIIA
Boron family
Boron is the first member of this family
Group 14 or IVA
Carbon family
Carbon is the first member of this property
Group 15 or VA
Nitrogen family
This group has non-metals and metalloids
Group 16 or VIA
Oxygen family
They are also known as chalcogens
Group 17 or VIIA
Halogen family
The elements of this group form salts.
Group 18
Zero group
They are noble gases and under normal conditions they are inert.

Classifications Of Elements in the periodic table

The elements of group 1, 2, 13, 14, 15, 16, and 17 are known as the main group elements or normal elements. The elements of groups 3, 4, 5, 6, 7, 8, 9, 11 and 12 are known as the transition elements. The group 18 is called the noble gases or inert gases. Their outermost shell is completely filled. Due to this stable electronic configuration, they generally don’t react with the other elements.
When we talk about the periods of a modern periodic table, one should keep in mind that the number of shells present in an atom determines its period number. The elements of period one will have only one shell, elements of period two will have two shells and so on. The first period of the modern periodic table is the shortest period as it contains only two elements. The period number two and three consists of eight elements each and is known as short groups. The period four and five have eighteen elements and are known as the long group. In the modern periodic table, the group number 3 of period six contains the lanthanide series which are the rare earth elements. We have radioactive elements (actinides) present in group 3 of period seven.


The tendency of an atom in a molecule to attract the shared pair of electrons towards itself is known as electronegativity. It is a dimensionless property because it is only a tendency. It basically indicates the net result of the tendencies of atoms in different elements to attract the bond forming electron pairs. We measure electronegativity on several scales. The most commonly used scale was designed by Linus Pauling. According to this scale fluorine is the most electronegative element with a value of 4.0 and caesium is the least electronegative element with a value of 0.7.
Electronegativity of elements depends on the following factors:
  1. Size of an atom: A greater atomic size will result in less value of electronegativity, this happens because electrons being far away from the nucleus will experience lesser force of attraction.
  2. Nuclear charge: A greater value of nuclear charge will result in greater value of electronegativity. This happens because increase in nuclear charge causes electron attraction with greater force.
Trends in electronegativity
As we move across a period from left to right the nuclear charge increases and the atomic size decreases, therefore the value of electronegativity increases across a period in the modern periodic table. There is an increase in atomic number as we move down the group in the modern periodic table. Nuclear charge also increases but the effect of increase in nuclear charge is overcome by the addition of one shell. Hence the value of electronegativity decreases as we move down the group. For example, in the first group the value decreases as we move from lithium downwards to francium.
It is a general observation that metals show a lower value of electronegativity as compared to the non-metals. Therefore metals are electropositive and non-metals are electronegative in nature. The elements in period two differ in properties from their respective group elements due to the small size and higher value of electronegativity.
The elements in second period show resemblance to the elements of the next group in period three. This happens due to a small difference in their electronegativities. This leads to the formation of a diagonal relationship.

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