MAKING ORDER OUT OF CHAOS – THE MODERN PERIODIC TABLE

MAKING ORDER OUT OF CHAOS – THE MODERN PERIODIC TABLE

  • In 1913, Henry Moseley showed that the atomic number of an element is a more fundamental property than its atomic mass as described below.
  • Accordingly, Mendeléev’s Periodic Law was modified and atomic number was adopted as the basis of Modern Periodic Table
  • In the modern periodic table elements are arranged in the increasing order of their atomic numbers in the form of a table
  • table having 7 horizontal rows of elements called periods and 18 vertical rows of elements called groups
  • Cause of periodicity – It is due to the repetition of same outer shell electronic configuration at a certain regular interval
  • Isotopes are placed at one place in the same group.
  • There is no element between hydrogen and helium as atomic masses always comes in whole numbers.
  •  Atomic number is represented by Z, and it is equal to the number of protons in the nucleus of the atom.
  • It also consists of 18 vertical columns known as Groups and 7 horizontal rows known as Periods.
  •  Elements having same number of valence electrons are placed in the same group 1

basic properties of the modern periodic table

  • The horizontal rows of elements in a periodic table are called periods.
  • There are seven periods in the long form of periodic table.
  • The elements in a period have consecutive (continuous) atomic numbers.
  • The first period has 2 elements H and He called the very short period.
  • Second period has 8 elements Li to Ne called short period.
  • Third period has 8 elements Na to Ar called short period.
  • Fourth period has 18 elements K to Kr called long period.
  • Fifth period has 18 elements Rb to Xe called long period.
  • Sixth period has 32 elements Cs to Rn called very long period.
  • Seventh period has 28 elements from Fr to atomic number 114 called incomplete period.
  • 14 elements each of he sixth and seventh periods are placed separately at the bottom of the table.
  • The 14 elements of the sixth period from La to Lu are called Lanthanides and the 14 elements of the seventh period from Ac to Lr are called Actinides

basic properties of modern periodic table

 

  • The vertical columns in a periodic table are called groups.
  • There are 18 groups in the long form of periodic table.
  • These groups are numbered as 1 to 18.
  • Group 1 is on the left side of the periodic table whereas group 18 is on the extreme right side of the periodic table.
  • The elements in a group do not have consecutive atomic numbers
  • Elements having 1 valence electron are placed in group 1,
  • Elements having 2 valence electrons are placed in group 2,
  • Elements having 3 valence electrons are placed in group 13,
  • Elements having 4 valence electrons are placed in group 14,
  • Elements having 5 valence electrons are placed in group 15,
  • Elements having 6 valence electrons are placed in group 16,
  • Elements having 7 valence electrons are placed in group 17, and
  • Elements having 8 valence electrons (or 2 valence electrons in K shell) are placed in group 18.

  • Hydrogen element has been placed at the top of group 1 , above the alkali metals because the electronic configuration of hydrogen is similar to those of alkali metals.
  • Both, hydrogen as well as alkali metals have 1 valence electron each.
  • Since hydrogen atom is very small in size, many properties of hydrogen are different from those of alkali metals.
  • Therefore, while discussing the alkali metals of group 1, hydrogen is never included.
  • In some of the periodic tables, however, hydrogen is not placed in any group.
  • Hydrogen is treated as a very special element and placed alone at the head of the periodic table.

NOBLE GASES

  •  The elements in the eighteenth group, helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and the radioactive radon (Rn) are called noble gases.
  •  They are all odorless, colourless and monatomic gases with very low chemical reactivity.
  •  Since their valence shell is considered to be “full”, they have little tendency to participate in chemical reactions.
  • When discovered and identified, scientists thought they are exceedingly rare, as well as chemically inert, and therefore these gases were also given the names ‘rare’ or ‘inert’ gases.
  • Number of valence electrons = 8

Characteristics of Periods and Groups

  • Valency and Valence Electrons – On moving left to right in a period, valency increases, and then it decreases.
  • But it remains the same down in a group.
  • As we move from left to right in a period, the valence electron increases and remains the same as we go down the group.

  • Atomic Size – It decreases left to right in a period as the nuclear charge increases due to large positive charges on the nucleus.

  • Atomic size increases down in a group due to a decrease in nuclear charges and the addition of a new shell.

  • Metallic Character – The ability of an atom to lose the electron is known as Metallic Character.
  • Metallic character decreases from left to right in a period.
  • This is due to an increase in nuclear charge. But non-metallic character increases left to right in a period.
  • And metallic character increases down the group as the size increases it can easily lose an electron.

  •  Ionization Energy is the energy required to remove an electron from an isolated gaseous atom.
  • Ionization energy increases as we move left to right in a period.
  • This is due to an increase in nuclear charge as we move left to right in a period.
  • But down in a group ionization energy decreases due to a decrease in nuclear charge but there are some exceptional cases.
  • Electropositive character decreases from left to right in a periodic table and increases down the group.

  • This is due to a decrease in metallic character from left to right in a period.
  •  Basic Character of Oxides increases down the group as atomic radius increases and ionization energy decreases.
  • This is due to an increase in metallic character or electro positivity of elements.
  • Acidic character of oxides decreases as the non-metallic character of elements decreases from top to bottom

different properties of element in modern periodic table
different properties of an element in the modern periodic table
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