Earlier Attempts at Classification of Elements

• In 1869, Dmitri Mendeleev and Lothar Meyer individually came up with their own periodic law "when the elements are arranged in order of increasing atomic mass, certain sets of properties recur periodically."
• Meyer based his laws on the atomic volume (the atomic mass of an element divided by the density of its solid.

The generalization made by the English chemist J.A.R. Newlands in 1865 that, if the chemical elements are arranged according to increasing atomic weight, those with similar physical and chemical properties occur after each interval of seven elements.

• Law of Triads is applicable to Chlorine, Bromine, and Iodine.
• Only four triads were mentioned – (Li,Na,K + Ca,Sr,Ba + Cl,Br,I + S,Se,Te).

Additional Information: 

• Dobereiner stated in his law of triads that the arithmetic mean of the atomic masses of the first and third element in a triad would be approximately equal to the atomic mass of the second element in that triad.
• He also suggested that this law could be extended for other quantifiable properties of elements, such as density.

• In Mendeleev's periodic table, the groups were divided into two subgroups.
• Groups from I to VII are meant for normal elements and group VIII is for transition elements.
• Groups from I to VII have been divided into two subgroups, while group VIII is meant for three elements.

• Elements are arranged on the basis of increasing order of atomic numbers.
• The horizontal rows in the periodic table are called Periods and the vertical columns are known as the Groups.
• The elements in the Modern Periodic Table are arranged in 7 Periods and 18 Groups.

Over 100 years ago, Henry Moseley carried out a systematic series of experiments that showed that the frequencies of the X-rays emitted from an elemental target under bombardment by cathode rays were characteristic of that element and could be used to identify the charge on its atomic nucleus.

• Mendeleev arranged the elements in order of their atomic masses while taking chemical properties into consideration.
• He left gaps predicting that some elements will be discovered later.
• When inert gases were discovered, they could be placed in a new group without disturbing his existing table.

Since statement A states that groups 7 and 8 were arranged on the basis of equivalent weights, it stands incorrect. 

In 1913, Henry Moseley showed that the atomic number of an element is a more fundamental property than its atomic mass.

• In 1829, Johann Dobereiner, a German scientist made some groups of three elements each and called them triads.
• He observed that the atomic mass of the middle element of a triad was nearly equal to the arithmetic mean of the atomic masses of the other two elements. 
• All three elements of a triad were similar in their properties.

• There are a total of 7 periods in the periodic table.
• The sixth period of the periodic table contains 32 elements.
• It is beginning with cesium and ends with radon.
• Lead (Pb) is currently the last stable element whereas all subsequent elements are radioactive.

• The horizontal row of the periodic table is called the Period.
• The vertical row of the periodic table is called the Group.
• There are seven periods and eighteen groups in a modern periodic table.

• In the second and third period of the modern periodic table does not involve any d orbital so we have s and p which have 8 electrons combined.
• So for the second period, we have eight elements and for the third period, we have 8 elements so we have total of 16 elements in all.

Additional Information: 

• The second and third periods contain 8 elements each and are called short periods.
  • The 2nd-period elements: lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine, and neon.
  • The 3rd-period elements: sodium, magnesium, aluminum, silicon, phosphorus, sulphur, chlorine, and argon.
• The First period contains 2 elements (Hydrogen, Helium) and is called a very short period. 

• John Newlands arranged the known elements in increasing order of their atomic masses.
• He started with the element having the lowest atomic mass ( hydrogen) and ended at thorium which was the 56th element.
• He found that every eighth element had properties similar to the first.

• According to Dobereiner's law of triads each triad contains three elements.
• He also noticed that the middle element of each of the triads had an atomic weight about halfway between the atomic weights of the other two.

• The periodic table of today owes its development to two chemists namely the Russian chemist Dmitri Mendeleev and the German chemist J. Lother Meyer.
• In 1869, they independently proposed that when the elements are arranged in the increasing order of their atomic weights, similarities in both physical and chemical properties appear at regular intervals.

• Mendeleev predicted the existence of 'eka-silicon', which would fit into a gap next to silicon.
• The element germanium was discovered later.
• Its properties were found to be similar to the predicted ones and confirmed Mendeleev's periodic table.

Eka aluminum predicted by Mendeleev is Gallium.

Eka-aluminum and gallium are two names for the same element, as Eka-Aluminium has almost identical properties to the gallium element itself.

Additional information: Mendeleev names unnamed elements as EKA- Boron EKA- Aluminium and EKA Silicon which were later replaced as Scandium, Gallium, and germanium respectively.

• Eka boron is The element Scandium.
• Eka aluminum is the element Gallium.
• Eka silicon is the element Germanium.

In the long form of the periodic table elements are arranged in increasing order of their Atomic number.

• Gallium and Germanium were the elements not discovered at that time and Mendeleev put gaps in the periodic table.
• Gallium was called Eka aluminium
• Germanium was called as Eka silicon

• There are 118 elements on the periodic table, Four with atomic numbers – 113 (Nihonium), 115 (Moskovi), 117 (Tennesin) and 118 (Oganesson) – were added in 2016.
• With the discoveries of new elements, it's difficult to ascertain how long the table is going to be in the future.