Modern periodic law


As per the modern periodic law, the elements are arranged in increasing order of atomic numbers of elements since their properties are the periodic function of the atomic number. This law was postulated by Henry Mosely in 1913 using X-ray analysis. This table consists of 18 groups and 7 periods wherein, period indicates the principal quantum number of the valence shell of the element and group indicates the physical properties such as metallic, nonmetallic, inert etc.

Table 1: Classification of groups in the modern periodic table.

Group number Group name Chemical properties
IA (group 1) Alkali metals Form strong alkalis with water
IIA (group 2) Alkali earth metals Form weaker alkalis with water than group 1
IIIA (group 13) Boron family Contains three valence electrons
IVA (group 14) Carbon family Contains four valence electrons
VA (group 15) Nitrogen family Contains nonmetals and metalloids
VIA (group 16) Oxygen family Also called chalcogens
VIIA (group 17) Halogen family Form salt with metals
Group 18 Zero group Contains Inert noble gases

Since the third group is too long to fit into the third column, it is broken and flipped into an island containing elements 57 through 71. Elements in this group called Lanthanides are characteristic for their silvery white color and they tarnish on contact with air. The Actinide line in the bottom row consists of element 19-103. Out of these elements, only thorium and uranium occur naturally on Earth while the others are synthetic and all of these elements are radioactive.

Under group 3 through 12 lies the transition metals that are hard but malleable shiny and possess good conductivity of heat and light. Some post-transition metals are identified in group 13 to group 17. These elements have classic characteristics of transition metals but they are softer and they conduct poorly than the other transition metals. This group includes aluminum, gallium, indium, tin, lead and bismuth.

The metalloids form a staircase that transitions from metals to nonmetals. Metalloids behave as semiconductors and some of the examples include Boron silicon germanium arsenic antimony tellurium and polonium.

Also, the number of the period indicates the number of shells present in the atom. Likewise, the first period of the modern periodic table contains 2 elements: hydrogen and helium. Another point to note is that the first element in every group behaves considerably different from the rest of the elements in the same group. This is probably due to its smaller size and non-availability of d orbitals for bonding. The anomalous behaviour also corresponds with a higher electronegativity of these elements (Singh, 2011).

This periodic table made the position of isotopes, hydrogen, nickel and cobalt well explained and placed appropriately in the periodic table. Hence, all the anomalies in the Mendeleev’s classification were solved. Also, elements with similar electron configuration were placed in the same group causing a repetition that resulted in periodicity and presentation of periodic trends within the table (Huheey, Keiter, Keiter, & Medhi, 2006).

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