Later, Henry Gywn-Jeffreys Moseley showed that the atomic number of an element is numerically equal to the number of electrons round the nucleus. The number of electrons in turn is equal to the number of protons in the nucleus. He suggested that atomic number is a more fundamental property of an element than its atomic mass. When the elements are arranged in the increasing order of their atomic number, most of the defects of Mendeleev's classification get rectified.
Remember:-
Relation between Mass Number and Atomic Number
Atomic Number (Z) is the number of protons in the nucleus of an atom. It is also equal to the number of electrons since the atom is electrically neutral.
Mass Number (A) is the total number of neutrons and protons present in the nucleus of an atom.
The periodic law given earlier is now modified and followed today. It states that "the properties of the elements are periodic functions of their atomic numbers."
The modern periodic table is also known as the long form of the periodic table or the extended form of the periodic table.In this table, the elements are arranged in the order of increasing atomic numbers in such away that elements with similar properties fall in the same vertical column. These vertical columns are referred to as 'Groups'. There are eight groups, which are further sub-divided into eighteen sub-groups. The table has seven horizontal rows known as 'Periods'.
Noble (inert) gases are placed in the VIII A, which is at the extreme right of the table. Halogens are placed in group VII A, just ahead of the noble gas elements. The alkali metals (Na, K, etc.) are placed in group I A and the alkaline earth metals (Ca, Ba, etc.) are placed in II A which is at the extreme left of the table. Transition elements such as copper, iron, etc., are placed in the B groups, which occur at the middle of the table. The inner transition elements, lanthanides and actinides are placed in two separate series at the bottom of the main body of the periodic table.