Long Form Of The Periodic Table Or Moseley's Periodic Table

The table which is most commonly used these days is called long form of the periodic table which consists of eight vertical columns and seven horizontal rows. Moseley (1909) studied the frequency of X-rays produced by the bombardment of a strong beam of electrons on a metal target. He found that the square root of the frequency of X-rays (√v) is directly proportional to the number of nuclear charge (z) of the metal. √v = a(Z-b) where a and b are constants. Nuclear charge of metal is equal to the atomic number. So Moseley related the properties of elements with their atomic number and gave the new periodic law.

Moseley's Periodic Law or Modern Periodic Law: Physical and chemical properties of elements are the periodic functions of their atomic numbers. If the elements are arranged in order of their increasing atomic number, after a regular interval, elements with similar properties are repeated.

Characteristics of Periods:

• First period is called shortest period and contains only two elements. Second and third periods are called short periods containing eight elements each. Fourth and fifth periods are long periods containing eighteen elements each. Sixth period is longest period with thirty-two elements. Seventh period is an incomplete period.
• Lanthanide and actinide series containing 14 elements each are placed separately below the main periodic table. These are related to sixth and seventh periods of III B group respectively
• Elements of third period from sodium (Na) to Chlorine (Cl) are called typical elements.
• Valency of an element in a period increases from 1 to 7 with respect to oxygen.

Na₂O MgO Al₂O₃ SiO₂ P₂O₅ SO₃ Cl₂O₇

1 2 3 4 5 6 7

• Elements of second period are called bridge elements.

Characteristic of Groups:

• Moseley’s periodic table contains sixteen groups. These are represented by Roman numerals I, II, III, IV, V, VI, VII, VIII and zero. Groups I to VII are divided into two subgroups A and B, Group VIII consists of three sets, each one containing three elements.
• Inert gases are present in zero group.
• The valency of an element in a group is equal to the group number.
• There is no resemblance in the elements of subgroups A and B of same group, except valency
• The elements of the groups which resemble with typical elements are called normal elements. For example IA, IIA, IIIA, IVA, VA, VIA, VIIA group elements are normal elements or main group elements or representative elements.
• Those elements of the groups which do not resemble with typical elements are called transition elements. For example- IB, IIB, IIIB, IVB, VB, VIB, VIIB, and VIII group elements are transition elements.