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Periodic Classification Of Elements

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Introduction:

  • There are 118 elements in total known to us at present.
  • Out of 118 only 98 elements occur naturally.
  • Each and every element has a unique property which makes it difficult to organize them in a specific way.
  • To resolve this issue many scientists came forward and classified the patterns to arrange the elements and this method is termed as the Periodic Classification of Elements.

Döbereiner’s Triad:

In 1817, Johann Wolfgang Döbereiner tried to arrange the known elements in each group having 3 elements and termed this method as a ‘triad’.
  • In a triad the atomic mass of the middle element was roughly the average of the remaining two elements. When the elements were arranged in the increasing atomic masses.
  • For example: take the triad consisting of lithium (Li), sodium (Na) and potassium (K) with the respective atomic masses 6.9, 23.0 and 39.0.
    Group A Atomic mass Group B Atomic mass Group C Atomic mass
    N 14.0 Ca 40.1 Cl 35.5
    P 31.0 Sr 87.6 Br 79.9
    As 74.9 Ba 137.3 I 126.9
  • Drawback of Döbereiner’s triad: He was only able to form 3 triads from all the elements known at the time. That’s why this wasn’t the useful method.

Newland’s Law of Octave:

In 1866, John Newlands, an English scientist, came forward with the arrangement of the then-known elements in the series of increasing atomic masses.
  • He found out that every eighth element had properties similar to that of the first.
  • He compared this pattern with the octaves found in music and entitled this arrangement as the “Newlands Law of Octaves”.
  • Ex: the properties of lithium and sodium were found to be the same as the Sodium is the eighth element after lithium.
sa
(do)		 re
(re)		 ga
(mi)		 ma
(fa)		 pa
(so)		 dh
(la)		 ni
(ti)
H Li Be B C N O
F Na Mg Al Si P S
Cl K Ca Cr Ti Mn Fe
Co and Ni Cu Zn Y In As Se
Br Rb Sr Ce and La Zr - -

Limitations of Newland’s Law of Octaves:

  • After Calcium the remaining elements doesn’t possess this property.
  • At that time only 56 elements were discovered and Newland stated that no further elements will be discovered in future but this wasn’t correct.
  • In order to fit the elements Newlands adjusted more than 1 element in a single slot, he also put some unlike elements under the same note.

Mendleev’s Periodic Table:

Dmitri Ivanovich Mendeléev, a Russian chemist play a vital role in arranging the then known 63 elements on the basis of their fundamental property, the atomic mass, and also on the similarity of chemical properties.
  • He concentrated on the compounds formed by elements with oxygen and hydrogen.
  • The formulae of the hydrides and oxides formed by an element were treated as one of the basic properties of an element for its classification.
  • While arranging the elements it was observed that there occurs a periodic recurrence of elements with similar physical and chemical properties.
  • Based on this he formulated a Periodic Law, which states that ‘the properties of elements are the periodic function of their atomic masses’.
  • Mendeléev’s Periodic Table contains vertical columns called ‘groups’ and horizontal rows called ‘periods’.

Achievements of Mendeleev’s Periodic Table:

  • Mendeleev left some gaps in the table for the elements that were not discovered at that time.
  • He named them by prefixing a Sanskrit numeral, Eka (one) to the name of preceding element in the same group.
  • Scandium, gallium and germanium, discovered later, have properties similar to Eka–boron, Eka–aluminum and Eka–silicon, respectively.
  • Mendeleev made a separate column for the new elements and that’s the reason all the scientists to accept this Periodic table.
  • In future when noble gases like Helium (He), Neon (Ne) and Argon (Ar) where added in the table without disturbing the existing order of the elements.

Limitations of Mendeleev’s Classification:

  • There was no fixed position assigned to the hydrogen.
  • Isotopes of all elements was the another challenge as they were discovered long after the Periodic table.
  • The atomic masses were not regular while going from one to the next element.

Modern Periodic Table:

In 1913, Henry Moseley modified the fundamental property of the Periodic table from the atomic mass to the atomic number. He stated the Modern Periodic Table as:
“Properties of elements are a periodic function of their atomic number”.

Position of Elements in the Modern Periodic Table:

  • This table includes 18 vertical columns known as ‘groups’ and 7 horizontal rows known as ‘periods’.
  • The elements present in any one group have the same number of valence electrons.
  • The number of valence shell electrons increases by one unit, as the atomic number increases by one unit on moving from left to right in a period.
  • The position of an element in the Periodic Table tells us about its chemical reactivity.

Trends in the Modern Periodic Table:

  • Valency: the number of valence electrons present in the outermost shell of its atom.
  • Atomic size: the distance between the centre of the nucleus and the outermost shell of an isolated atom. Ex: hydrogen atom is 37 pm (picometre, 1 pm = 10–12m).
    • The atomic radius decreases in moving from left to right along a period as the nuclear charge increases which tends to pull the electrons closer to the nucleus and reduces the size of the atom.
    • The atomic size increases down the group because new shells are being added.
  • Metallic and Non-metallic Properties:
    • The metals like Na and Mg are towards the left-hand side of the Periodic Table.
    • The non-metals like sulphur and chlorine are found on the right-hand side.
    • In the middle, we have silicon, which is classified as a semi-metal or metalloid (it exhibits some properties of both metals and non-metals).
    • A zig-zag line separates metals from non-metals. The borderline elements – boron, silicon, germanium, arsenic, antimony, tellurium and polonium – are called metalloids or semi-metals.
    • The valence shell electrons increases across a period, the tendency to lose electrons will decrease.
    • Down the group, the outermost electrons are farther away from the nucleus as the nuclear charge decreases.
    • The metallic character decreases across a period and increases down a group.
    • Non-metals are electronegative they tend to form bonds by gaining electrons.
    • Non-metals are found on the right-hand side of the Periodic Table towards the top.