3. Modern Periodic Table

The Modern Periodic Table:

Periodicity. The properties which reoccur after a regular intervals in a periodic table are called periodic properties and the phenomenon is called periodicity of element.
Henry Moseley (1913) exhibited that the atomic number of an element is a more fundamental property than its atomic mass. Mendeleev’s periodic law was modified and atomic number was adopted as the basis of the modern periodic table.

The Modern Periodic Law states that:
The physical and chemical properties of the elements are the periodic function of their atomic numbers.
It means that if the elements are arranged in order of increasing atomic numbers, the elements with similar properties recur after regular intervals.
Many new forms of periodic table have been proposed in recent times with modern periodic law as the guiding principle, but the general plan of the table remains the same as proposed by Mendeleev. The most commonly known periodic table is the Long form of the periodic table.Few Points to remember:

• Modern periodic table contains 18 vertical columns known as groups and 7 horizontal rows known as periods.
• Elements in a group have the same number of valence electrons.
• Number of the shells increases as we go down the group.
• Elements in a period have same number of shells.
• Number of elements placed in a particular period depends upon the fact that how electrons are filled into various shells.
• Maximum number of electrons that can be accommodated in a shell depends on the formula 2n² where n is the number of the given shell. For example, in K shell the number of electrons is 2 x (1)² = 2 in the first period, L shell – 2 x (2)² = 8 elements in the second period.
• Position of the element in the periodic table tells about its reactivity.

Trends in the Modern Periodic Table
Valency: Number of valence electrons present in the outermost shells. Valency remains the same down a group but changes across a period.
Atomic Size: Atomic size refers to radius of an atom.
Atomic size or radius decreases in moving from left to right along a period due to increase in nuclear charge. Atomic size increases down the group because new shells are being added as we go down the group.
Metallic Character: Metallic character means the tendency of an atom to lose electrons.
Metallic character decreases across a period because the effective nuclear charge increases, that means the tendency to lose electrons decreases. Metals are electropositive as they tend to lose electrons while forming bonds.