Lothar Meyer proposed that on arranging the elements in order of increasing atomic weights; similarities appear in physical and chemical properties.

Law of octaves
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.

For Dobereiner's triads, the atomic weight of a middle element is almost average of the other two. Thus, the law of triad is applicable to Cl,Br,I

Mendeleev divided his periodic table in eight groups and seven periods. 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 in two sub groups, 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.
• The elements in the Modern Periodic Table are classified into four categories viz. Representative Elements, Transition Elements, Inner Transition Elements, and Noble Gases.

The origins of Moseley's conclusion that there was a whole number relationship in the frequencies of X-rays emitted from atoms after electron bombardment, i.e., the discovery of atomic numbers.

In Mendeleev’s periodic table, elements were arranged on the basis of atomic weights.

The scientist who first of all showed that the atomic number is more fundamental property of an element than its atomic mass was Henry Moseley.

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.

The 2nd and 3rd period consists of 8 elements each. On the other hand, the 4th and 5th consists of 18 elements each while the sixth and seventh period consists of 32 elements.

There are totally 7 horizontal rows are there they are called periods
Vertical columns are called groups or families.

In 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 element so we have total 16 elements in all.

According to Newlands' law of octaves when the elements are arranged in order of increasing atomic weights then every eighth element has properties similar to that of the first element.

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

Lothar Meyer proposed that on arranging the elements in order of increasing atomic weights similarities appear in both physical and chemical properties.

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 aluminium predicted by Mendeleev is Gallium. Eka-aluminium and gallium are the two names of the same element as Eka -Aluminum has almost exactly the same properties as the actual properties of the gallium element. 

In long form of 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 as 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.