Classification of elements

May 9, 2018 Sushil Humagain Chemistry 0

 

  1. Classification of Elements:
  • Based on distinct physical properties like hardness, malleability & lustre

a.Non-Metals

b.Metals :

  • Based on Chemical Properties:
  1. Alkali Metals: Metals which react with water to form alkali and hydrogen e.g Li, Na, K

Li +H2O —–> LiOH+H2

      2. Alkaline Earth Metals: Oxides of these metals when dissolved in water are alkaline in nature e.g. Be, Mg, Ca etc.

MgO+H2O —–> Mg(OH)2

  1. Dates & contribution of different scientists:
  • 1803: John Dalton gave “Atomic Theory”
  • 1815: Prout established relationship between atomic masses of elements & the atomic mass of hydrogen
  • 1829: Dobereiner showed the relation between the chemical properties of elements and their atomic weights

i.e. Elements having similar properties can be arranged in triads(group of three) ii which the atomic weight of the middle element is average of the atomic weights of the other two.

e.g.

Element Atomic Mass(A)
Li 7
Na 23 (7+39)/2 = 23
K 39

 

       Demerit: Only limited number of elements can be arranged as Triads.

  • 1863: John Newlands arranged some elements in the ascending order of their atomic weights and observed that the properties of the every eighth element starting from an element is similar to that of the first i.e. Law of Octaves.

It was as the eight notes of an octave in musical scale. The 1st and the 8th note are the same.

 

Octaves of Music Sa Re Ga Ma Pa Dha Ni
Sa
Octaves of Elements Li Be B C N O F
Na

 

Demerit: This was true for the elements with small but failed in case of elements with larger Atomic mass.

  • 1869: Lothar Meyer and D’mitri Mendeleev independently and simultaneously classified elements according to increasing atomic masses.
  • 1913: Henery Moseley found that atomic number is the fundamental property of an element and took atomic number as the basis of classification of elements.

Classification of elements: The grouping of elements according to their similar characteristics

  • Importance of Classification of elements:
  1. Classification makes the study of properties of elements easy and fast.
  2. The study of the properties of a typical element of a particular group enables us to assess the properties of other elements of that group.

Periodic table: The table or chart which is obtained by arranging elements on the basis of their similarities and dissimilarities

  • Mendeleev’s Periodic Law: “The physical and chemical properties of elements are the periodic functions of their atomic weights.”
  • It means that when elements are arranged on the basis of increasing atomic weights, the elements having similar physical and chemical properties are repeated after a regular interval.
  • Mendeleev’s Periodic Table: Table or chart obtained by arranging elements on the basis of their increasing atomic weight

Characteristics of Mendeleev’s Periodic Table:

  1. Mendeleev arranged 63 elements on the basis of increasing atomic masses.
  2. There are six horizontal rows i.e. periods and eight vertical columns i.e. groups.
  3. Mendeleev left gaps for undiscovered elements e.g. Scandium(named as eka-Boron *1), Gallium (named as eka-Aluminium *2), Germanium(named as eka-Silicon *3), Technitium(named as eka-Manganese *4).
  4. Inert gases were not discovered, so there was no separate place for them.

 

Merits of Mendeleev’s Periodic Table:

  1. It was the first scientific as well as systematic classification of elements, which made their study easy, fast and systematic.
  2. Many elements were not discovered during the time of Mendeleev. So, he left gaps for those elements. When these elements were discovered later on, they were placed in these gaps, without disturbing the existing elements.
  3. The atomic masses of elements like beryllium, platinum, gold etc. were not confirmed before Mendeleev’s Periodic Table came into existence. With the help of this table, these were corrected by placing them in their proper position.
  4. Mendeleev’s periodic table encouraged subsequent scientists to discover new elements as he left gaps for them.

Demerits of Mendeleev’s Periodic Table:

  1. Mendeleev could not arrange hydrogen properly because it sometimes gains electrons like halogens (group VIIA) and sometimes it loses electrons like alkali metals (group IA).
  2. He couldn’t arrange isotopes properly as they have the same atomic number but different atomic masses.
  3. In some cases, elements with more atomic masses were placed before those with less atomic masses. E.g. Ar with atomic mass 39.9 was to be placed before K with mass 39.1
  4. Some dissimilar elements were placed in the same group e.g. very less reactive coinage metals( Cu, Ag & Au) and highly reactive alkali metals (Li, Na, K) were placed in the same group.
  5. Lanthanides and Actinides don’t have proper position in the periodic table.
  • Modern Periodic Law: Henery Moseley’s periodic law states, “The physical and chemical properties of elements are the periodic functions of their atomic numbers.”
  • It means that when elements are arranged on the basis of their increasing atomic number, the elements with similar properties are repeated after a regular interval.
  • Modern Periodic Table: Table or chart obtained by arranging elements on the basis of their increasing atomic numbers

Characteristics of Modern Periodic Table:

  1. The 118 elements are arranged in order of their increasing atomic number.
  2. There are 7 horizontal rows called periods. These are divided into 4 types on the basis of the number of elements present. They are:
Type Period Number of elements
Very Short 1st 2
Short 2nd & 3rd 8
Long 4th & 5th 18
Very long 6th & 7th 32

 

  1. There are 18 vertical columns called groups. There are 16 groups i.e. groups IA, IB, IIA, IIB…..VIIIA (0) and VIII B (VIII). Group VIII B (VIII) has three vertical columns.
  2. Noble/ Inert gases are placed in a separate group i.e. group VIIIA (0 or 18) at the extreme right side of the table.
  3. Metals are kept at the left hand side, non-metals at the right hand side and metalloids between metals and non-metals i.e. next to the staircase line e.g. B, Si, Ge, As, Te
  4. The lanthanides and the actinides and the actinides are placed separately at the bottom in the two rows. They belong to group III B (3) & f-block.
  • Special Groups & Periods:
  • Lanthanides: The 14 elements after Lanthanum(57La) to Lutetium(71Lu); they form complex salts
  • Actinides: The 14 elements after Actinium (89Ac) to Lawrencium (103Lr); they are radioactive in nature.
  • Alkali metals: Elements present in group IA (1) of the modern periodic table. e.g. Li, Na, K etc.
  • Metals which react with water to form alkali and hydrogen

e.g. Li +H2O —–> LiOH+H2

  • Alkaline Earth Metals: Elements present in group IIA (2) of the modern periodic table. e.g. Be, Mg, Ca
  • Oxides of these metals are abundant in the earth’s crust which when dissolved in water are alkaline in nature

e.g. MgO+H2O —–> Mg(OH)2

  • Halogens: Elements present in group VIIA(17) of the modern periodic table e.g. F, Cl, Br, I
  • They form corresponding halides (salts) when reacted with alkali metals.

2Na+Cl2 ->2NaCl

  • Inert/Noble gases: Elements present in group VIIIA (0 or 18) of the modern periodic table having stable electronic configuration and don’t take part in chemical reaction e.g. He, Ne, Ar, Kr, Xe, Rn etc.

Merits of the Modern Periodic Table:

  1. It is based on the atomic number of elements which is the most fundamental property of the atom.
  2. It has elements arranged according to their electronic configuration. So, it explains why elements in a group has similar properties.
  3. It gives proper explanation for the chemical periodicity in the properties of elements. The periodicity in properties arises due to periodicity in the electronic configuration of elements.
  4. It removes all the defects of Mendeleev’s periodic table. e.g. the position of isotopes, wrong order of some elements on basis of their atomic masses, positioning of dissimilar elements.
  5. The elements have been clearly separated as normal, transition, inner- transition (Lanthanides & Actinides) and noble gases. Metals, non-metals & metalloids are also separated.

Demerits of the Modern Periodic Table:

  1. Position of Hydrogen:

Hydrogen is an electropositive s-block element. It donates 1 valence electron like other alkali metals. Hence, it is placed in Group IA (1) but it is a non-metal and shows similar properties to halogens {Group VIIA(17)} i.e. it is a diatomic gas and has valency 1. So, positon of Hydrogen is still not well defined.

  1. Position of Helium:

On basis of electronic configuration, He should be placed in group IIA(2) but it has been placed in group 0 due to its inert nature.

Author: Sulaksha Purna Shrestha

Classification of elements