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Description of the Modern Periodic Table

  1. The elements are arranged in the increasing order of their atomic number.
  2. There are seven periods.
    1st period contains only 2 elements, H (1) and He (2)—shortest period.
    2nd period contains 8 elements, from Li (3) to Ne (10)—short period.
    3rd period contains 8 elements from Na (11) to Ar (18)—short period.
    4th period contains 18 elements from K (19) to Kr (36)—long period.
    5th period contains 18 elements from Rb (37) to Xe (54)—long period.
    6th period contains 32 elements from Cs (55) to Rn (86)—longest period.
    7th period contains 31 elements from Fr (87) to Uuo (118)—incomplete period.
  3. There are 18 vertical columns.
  4. The elements belonging to groups 1–2 and 13–17 are called normal or representative elements. Those belonging to groups 3–12 are called transition elements.
    Elements of group 1 are called alkali metals, while the elements of group 2 are called alkaline earth metals. Elements of group 17 are called halogens and those of group 18 are called noble or inert gases.
  5. The two rows at the bottom of the table consist of lanthanides [from Lanthanum (57) to Lutetium (71)] and the actinides [from Actinium (89) to Lawrencium (103)]. La and Ac belong to group 3.
  • Metals become more reactive as we go DOWN a group and move LEFT in a period. Thus, metals such as Li, Na, K, Rb and Cs are the most reactive metals (1st and 2nd groups).
  • Non-metals are more reactive UP in a group and RIGHT in a period. Thus, non-metals such as F, Cl, Br and I are among the most reactive of non-metals (17th group).
  • Fluorine is a pale yellow gas. Chlorine is a yellow green gas. Bromine is a dark red liquid. Iodine is a violet-coloured solid. Astatine is a dark black solid.
  • Noble gases are relatively non-reactive. This is because they have a complete valence shell. They have no tendency to gain or lose electrons except xenon. They are all gases at room temperature.
  • Certain metals such as gold, silver and platinum are generally found in free state.
  1. In this table, a complete separation of metals and non-metals has been achieved. The elements to the left of the staircase line are mostly metals while those on the right side of it are non-metals. Some of the elements that lie right next to the staircase line possess properties intermediate between those of metals and non-metals. These are called metalloids. These are boron, silicon, arsenic, antimony etc.
  2. The periodic table is divided into four blocks:
  1. s-block elements: These are elements belonging to groups 1 and 2 and are called s-block elements.
  2. p-block elements: These are elements of groups 13–17 and are called p-block elements.
There are 38 radioactive elements. Elements technetium (43), promethium (61) and elements from atomic numbers 84–118 are radioactive.
  1. d-block elements: These are elements of groups 3–12 and are called d-block elements. They are also called transition elements.
  2. f-block elements: These are elements placed at the bottom of the periodic table and are called f-block elements. They are also called inner transition elements.
    Elements up to uranium (Z = 92) are found in nature, and elements beyond uranium with Z > 92 are not available in nature but are produced synthetically. These are, therefore, called synthetic elements or transuranic elements.
    Each group is known by a characteristic name. Figure 3.2 gives the name of the different groups belonging to the periodic table.
Some information about elements:
  • Hydrogen (H)—Most abundant element in the universe.
  • Oxygen (O)—Most abundant element in the earth’s crust.
  • Nitrogen (N)—Most abundant element in the earth’s atmosphere.
  • Aluminium (Al)—Most abundant metal in the earth’s crust.
  • Mercury (Hg)—Liquid metal at room temperature.
  • Gallium (Ga)—Brittle solid at low temperatures. Liquefies when kept in the palm.
  • Lithium (Li)—Lightest metal.
  • Osmium (Os)—Heaviest metal.
  • Helium (He)—Lowest melting and boiling point.
  • Tungsten (W)—Highest melting and boiling point.
  • Silver (Ag)—Best conductor of electricity at STP.
  • Germanium (Ge), Silicon (Si)—Semiconductors, i.e. electrical conductivity lies between those of conductors and insulators.
  • Diamond—Naturally occurring hardest substance.

Characteristics of Groups and Periods

  1. Elements within a group have similar chemical and physical properties due to their similar valence shell of electrons.
  2. Elements in the same period have different valence electrons and hence show different properties.
  3. Each group has its own generalised electronic configuration. By knowing the number of valence electrons and generalised electronic configuration, we can identify the group to which an element belongs.
    For example, electronic configuration of fluorine is 1s2 2s2 2p5. Its general electronic configuration is ns2 np5 and hence it belongs to the 17th group.
  4. The highest energy level will indicate the period to which that element belongs.
    For example, electronic configuration of aluminium is 1s2 2s2 2p6 3s2 3p1. The highest energy level is 3. Hence, aluminium belongs to the third period.

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