The Modern periodic table, developed by an English physicist Henry Mosely, is based on the the studies through X-ray diffraction, which revealed that atomic number– an indicator of the no. of protons in the nucleus, which is unique for any element – is the more fundamental criteria than atomic mass. He stated the modern periodic law as – properties of elements are a periodic function of their atomic numbers. Keeping atomic number as the base also enables us to predict how many more elements are yet to be found out in between the two known elements in the Modern periodic table. Later when noble gases were invented, they could be easily accommodated in the table.

Based on the modern periodic law, the elements are arranged into rows and columns based on their atomic number. There are 18 columns, called groups and 7 rows, called periods. We know that electrons are distributed in the various shells around the nucleus. The period represents the outermost shell to which electron is added.

The metallic and non-metallic character of elements can be largely attributed to the number of valence electrons and valence. Metallic character is characterised by the ease of elements lose electrons to attain octet. Elements having lesser than 4 electrons in valence shell are hence metallic, which means metals are found towards the left of the table.

i. As we go down a group, the valence electrons are much farther away from the nucleus, which means they can be released easier, increasing the metallic character. As for non-metals, they are found towards the right of the table; and are generally electron acceptors so as to complete their octet. Down a group, accepting an electron becomes a tougher job, decreasing non-metallic character.

ii. Similarly, as we move from left to right in a period, the metallic character and atomic size decreases as the nuclear charge increases so valence electrons are attracted with greater force.